text:
Physiology and Hygiene for Secondary Schools

by Francis M. Walters, A.M.

Edition 1, (November 15, 2005)

                     D.C. Heath and Co. - Publishers

                         Original copyright 1909

    "It is quite possible to give instruction in this subject in such
    a manner as not only to confer knowledge which is useful in
    itself, but to serve the purpose of a training in accurate
    observation, and in the methods of reasoning of physical
    science."--_Huxley._

PREFACE

The aim in the preparation of this treatise on the human body has been,
first, to set forth in a _teachable_ manner the actual science of
physiology; and second, to present the facts of hygiene largely as
_applied physiology_. The view is held that "right living" consists in the
harmonious adjustment of one's habits to the nature and plan of the body,
and that the best preparation for such living is a correct understanding
of the physical self. It is further held that the emphasizing of
physiology augments in no small degree the educative value of the subject,
greater opportunity being thus afforded for exercise of the reasoning
powers and for drill in the _modus operandi_ of natural forces. In the
study of physiology the facts of anatomy have a place, but in an
elementary course these should be restricted to such as are necessary for
revealing the general structure of the body.

Although no effort has been spared to bring this work within the
comprehension of the pupil, its success in the classroom will depend
largely upon the method of handling the subject by the teacher. It is
recommended, therefore, that the _relations_ which the different organs
and processes sustain to each other, and to the body as a whole, be given
special prominence. The pupil should be impressed with the essential unity
of the body and should see in the diversity of its activities the serving
of a common purpose. In creating such an impression the introductory
paragraphs at the beginning of many of the chapters and the summaries
throughout the book, as well as the general arrangement of the
subject-matter, will be found helpful.

Since the custom largely prevails of teaching physiology in advance of the
sciences upon which it rests--biology, physics, and chemistry--care should
be exercised to develop correct ideas of the principles and processes
derived from these sciences. Too much latitude has been taken in the past
in the use of comparisons and illustrations drawn from "everyday life." To
teach that the body is a "house," "machine," or "city"; that the nerves
carry "messages"; that the purpose of oxygen is to "burn up waste"; that
breathing is to "purify the blood," etc., may give the pupil phrases which
he can readily repeat, but teaching of this kind does not give him correct
ideas of his body.

The method of teaching, however, that uses the pupil's experience as a
basis upon which to build has a value not to be overlooked. The fact that
such expressions as those quoted above are so easily remembered proves the
value of connecting new knowledge with the pupil's experience. But _the
inadequacy of this experience must be recognized_ and taken into account.
The concepts of the average pupil are entirely too indefinite and limited
to supply the necessary _foundation for a science_ such as physiology.
Herein lies the great value of experiments and observations. They
supplement the pupil's experience, and increase both the number and
definiteness of his concepts. No degree of success can be attained if this
phase of the study is omitted.

The best results in physiology teaching are of course attained where
laboratory work is carried on by the pupils, but where this cannot be
arranged, class experiments and observations must suffice. The Practical
Work described at the close of most of the chapters is mainly for class
purposes. While these serve a necessary part in the development of the
subject, it is not essential that all of the experiments and observations
be made, the intention being to provide for some choice on the part of the
teacher. A note-book should be kept by the pupil.

To adapt the book to as wide a range of usefulness as possible, more
subject-matter is introduced than is usually included in an elementary
course. Such portions, however, as are unessential to a proper
understanding of the body by the pupil are set in small type, to be used
at the discretion of the teacher.

The use of books of reference is earnestly recommended. For this purpose
the usual high school texts may be employed to good advantage. A few more
advanced works should, however, be frequently consulted. For this purpose
Martin's _Human Body_ (Advanced Course), Rettger's _Advanced Lessons in
Physiology_, Thornton's _Human Physiology_, Huxley's _Lessons in
Elementary Physiology_, Howell's _A Text-book of Physiology_, Hough and
Sedgwick's _Hygiene and Sanitation_, and Pyle's _Personal Hygiene_ will be
found serviceable.

In the preparation of this work valuable assistance has been rendered by
Dr. C.N. McAllister, Department of Psychology, and by Professor B.M.
Stigall, Department of Biology, along the lines of their respective
specialties, and in a more general way by President W.J. Hawkins and
others of the Warrensburg, Missouri, State Normal School. Expert advice
from Professor S.D. Magers, Instructor in Physiology and Bacteriology,
State Normal School, Ypsilanti, Michigan, has been especially helpful, and
many practical suggestions from the high school teachers of physiology of
Kansas City, Missouri, Professor C.H. Nowlin, Central High School, Dr.
John W. Scott, Westport High School, and Professor A.E. Shirling, Manual
Training High School, all of whom read both manuscript and proofs, have
been incorporated. Considerable material for the Practical Work, including
the respiration experiment (page 101) and the reaction time experiment
(page 323), were contributed by Dr. Scott. Professor Nowlin's suggestions
on subject-matter and methods of presentation deserve special mention. To
these and many others the author makes grateful acknowledgment.

                                                                    F.M.W.

MISSOURI STATE NORMAL SCHOOL,
SECOND DISTRICT, May 1, 1909.

CONTENTS

Preface
Contents
PART I: THE VITAL PROCESSES
   CHAPTER I - INTRODUCTION
   CHAPTER II - GENERAL VIEW OF THE BODY
   CHAPTER III - THE BODY ORGANIZATION
   CHAPTER IV - THE BLOOD
   CHAPTER V - THE CIRCULATION
   CHAPTER VI - THE LYMPH AND ITS MOVEMENT THROUGH THE BODY
   CHAPTER VII - RESPIRATION
   CHAPTER VIII - PASSAGE OF OXYGEN THROUGH THE BODY
   CHAPTER IX - FOODS AND THE THEORY OF DIGESTION
   CHAPTER X - ORGANS AND PROCESSES OF DIGESTION
   CHAPTER XI - ABSORPTION, STORAGE, AND ASSIMILATION
   CHAPTER XII - ENERGY SUPPLY OF THE BODY
   CHAPTER XIII - GLANDS AND THE WORK OF EXCRETION
PART II: MOTION, COORDINATION, AND SENSATION
   CHAPTER XIV - THE SKELETON
   CHAPTER XV - THE MUSCULAR SYSTEM
   CHAPTER XVI - THE SKIN
   CHAPTER XVII - STRUCTURE OF THE NERVOUS SYSTEM
   CHAPTER XVIII - PHYSIOLOGY OF THE NERVOUS SYSTEM
   CHAPTER XIX - HYGIENE OF THE NERVOUS SYSTEM
   CHAPTER XX - PRODUCTION OF SENSATIONS
   CHAPTER XXI - THE LARYNX AND THE EAR
   CHAPTER XXII - THE EYE
   CHAPTER XXIII - THE GENERAL PROBLEM OF KEEPING WELL
APPENDIX
INDEX

PHYSIOLOGY AND HYGIENE

PART I: THE VITAL PROCESSES

CHAPTER I - INTRODUCTION

To derive strength equal to the daily task; to experience the advantages
of health and avoid the pain, inconvenience, and danger of disease; to
live out contentedly and usefully the natural span of life: these are
problems that concern all people. They are, however, but different phases
of one great problem--the problem of properly managing or caring for the
body. To supply knowledge necessary to the solution of this problem is the
chief reason why the body is studied in our public schools.

*Divisions of the Subject.*--The body is studied from three standpoints:
structure, use of parts, and care or management. This causes the main
subject to be considered under three heads, known as anatomy, physiology,
and hygiene.

_Anatomy_ treats of the construction of the body--the parts which compose
it, what they are like, and where located. Its main divisions are known as
gross anatomy and histology. _Gross anatomy_ treats of the larger
structures of the body, while _histology_ treats of the minute structures
of which these are composed--parts too small to be seen with the naked eye
and which have to be studied with the aid of the microscope.

_Physiology_ treats of the function, or use, of the different parts of the
body--the work which the parts do and how they do it--and of their relations
to one another and to the body as a whole.

_Hygiene_ treats of the proper care or management of the body. In a
somewhat narrower sense it treats of the "laws of health." Hygiene is said
to be _personal_, when applied by the individual to his own body;
_domestic_, when applied to a small group of people, as the family; and
public, or _general_, when applied to the community as a whole or to the
race.

*The General Aim of Hygiene.*--There are many so-called laws of health, and
for these laws it is essential in the management of the body to find a
common basis. This basic law, suggested by the nature of the body and
conditions that affect its well-being, may be termed the _Law of Harmony:
The mode of living must harmonize with the plan of the body_. To live
properly one must supply the conditions which his body, on account of its
nature and plan, requires. On the other hand, he must avoid those things
and conditions which are injurious, _i.e._, out of harmony with the body
plan. To secure these results, it is necessary to determine what is and
what is not in harmony with the plan of the body, and to find the means of
applying this knowledge to the everyday problems of living. Such is the
general aim of hygiene. Stated in other words: Hygiene has for its general
aim the bringing about of an essential harmony between the body and the
things and conditions that affect it.(1)

*Relation of Anatomy and Physiology to the Study of Hygiene.*--If the chief
object in studying the body is that of learning how to manage or care for
it, and hygiene supplies this information, why must we also study anatomy
and physiology? The answer to this question has already been in part
suggested. In order to determine what things and conditions are in harmony
with the plan of the body, we must know what that plan is. This knowledge
is obtained through a study of anatomy and physiology. The knowledge
gained through these subjects also renders the study of hygiene more
interesting and valuable. One is enabled to see _why_ and _how_ obedience
to hygienic laws benefits, and disobedience to them injures, the body.
This causes the teachings of hygiene to be taken more seriously and
renders them more practical. In short, anatomy and physiology supply a
necessary basis for the study of hygiene.

*Advantages of Properly Managing the Body.*--One result following the
mismanagement of the body is loss of health. But attending the loss of
health are other results which are equally serious and far-reaching.
Without good health, people fail to accomplish their aims and ambitions in
life; they miss the joy of living; they lose their ability to work and
become burdens on their friends or society. The proper management of the
body means health, and it also means the capacity for work and for
enjoyment. Not only should one seek to preserve his health from day to
day, but he should so manage his body as to use his powers to the best
advantage and prolong as far as possible the period during which he may be
a capable and useful citizen.

CHAPTER II - GENERAL VIEW OF THE BODY

*External Divisions.*--Examined from the outside, the body presents certain
parts, or divisions, familiar to all. The main, or central, portion is
known as the _trunk_, and to this are attached the _head_, the _upper
extremities_, and the _lower extremities_. These in turn present smaller
divisions which are also familiar. The upper part of the trunk is known as
the _thorax_, or chest, and the lower part as the _abdomen_. The portions
of the trunk to which the arms are attached are the _shoulders_, and those
to which the legs are joined are the _hips_, while the central rear
portion between the neck and the hips is the _back_. The fingers, the
hand, the wrist, the forearm, the elbow, and the upper arm are the main
divisions of each of the upper extremities. The toes, the foot, the ankle,
the lower leg, the knee, and the thigh are the chief divisions of each of
the lower extremities. The head, which is joined to the trunk by the neck,
has such interesting parts as the eyes, the ears, the nose, the jaws, the
cheeks, and the mouth. The entire body is inclosed in a double covering,
called the _skin_, which protects it in various ways.

*The Tissues.*--After examining the external features of the body, we
naturally inquire about its internal structures. These are not so easily
investigated, and much which is of interest to advanced students must be
omitted from an elementary course. We may, however, as a first step in
this study, determine what kinds of materials enter into the construction
of the body. For this purpose the body of some small animal should be
dissected and studied. (See observation at close of chapter.) The
different materials found by such a dissection correspond closely to the
substances, called _tissues_, which make up the human body. The main
tissues of the body, as ordinarily named, are the _muscular_ tissue, the
_osseous_ tissue, the _connective_ tissue, the _nervous_ tissue, the
_adipose_ tissue, the _cartilaginous_ tissue, and the _epithelial_ and
_glandular_ tissue. Most of these present different varieties, making all
together some fifteen different kinds of tissues that enter into the
construction of the body.(2)

*General Purposes of the Tissues.*--The tissues, first of all, _form the
body_. As a house is constructed of wood, stone, plaster, iron, and other
building materials, so is the body made up of its various tissues. For
this reason the tissues have been called the _building materials_ of the
body.

In addition to forming the body, the tissues supply the means through
which its work is carried on. They are thus the _working materials_ of the
body. In serving this purpose the tissues play an active role. All of them
must perform the activities of growth and repair, and certain ones (the
so-called active tissues) must do work which benefits the body as a whole.

*Purposes of the Different Tissues.*--In the construction of the body and
also in the work which it carries on, the different tissues are made to
serve different purposes. The osseous tissue is the chief substance in the
bony framework, or skeleton, while the muscular tissue produces the
different movements of the body. The connective tissue, which is
everywhere abundant, serves the general purpose of connecting the
different parts together. Cartilaginous tissue forms smooth coverings over
the ends of the bones and, in addition to this, supplies the necessary
stiffness in organs like the larynx and the ear. The nervous tissue
controls the body and brings it into proper relations with its
surroundings, while the epithelial tissue (found upon the body surfaces
and in the glands) supplies it with protective coverings and secretes
liquids. The adipose tissue (fat) prevents the too rapid escape of heat
from the body, supplies it with nourishment in time of need, and forms
soft pads for delicate organs like the eyeball.

*Properties of the Tissues.*--If we inquire how the tissues are able to
serve such widely different purposes, we find this answer. The tissues
differ from one another both in composition and in structure and, on this
account, differ in their properties.(3) Their different properties enable
them to serve different purposes in the body. Somewhat as glass is adapted
by its transparency, hardness, and toughness to the use made of it in
windows, the special properties of the tissues adapt them to the kinds of
service which they perform. Properties that adapt tissues to their work in
the body are called _essential_ properties. The most important of these
essential properties are as follows:

1. Of osseous tissue, hardness, stiffness, and toughness. 2. Of muscular
tissue, contractility and irritability. 3. Of nervous tissue, irritability
and conductivity. 4. Of cartilaginous tissue, stiffness and elasticity. 5.
Of connective tissue, toughness and pliability. 6. Of epithelial tissue,
ability to resist the action of external forces and power to secrete.

                                 [Fig. 1]

 Fig. 1--Hand and forearm, showing the grouping of muscular and connective
                    tissues in the organ for grasping.

*Tissue Groups.*--In the construction of the body the tissues are grouped
together to form its various divisions or parts. A group of tissues which
serves some special purpose is known as an _organ_. The hand, for example,
is an organ for grasping (Fig. 1). While the different organs of the body
do not always contain the same tissues, and never contain them in the same
proportions, they do contain such tissues as their work requires and these
have a special arrangement--one adapted to the work which the organs
perform.

In addition to forming the organs, the tissues are also grouped in such a
manner as to provide supports for organs and to form cavities in which
organs are placed. The various cavities of the body are of particular
interest and importance. The three largest ones are the _cranial_ cavity,
containing the brain; the _thoracic_ cavity, containing the heart and the
lungs; and the _abdominal_ cavity, containing the stomach, the liver, the
intestines, and other important organs (Fig. 2). Smaller cavities serving
different purposes are also found.

                                 [Fig. 2]

   Fig. 2--Diagram of a lengthwise section of the body to show its large
               cavities and the organs which they contain.

*Organs and Systems.*--The work of the body is carried on by its various
organs. Many, in fact the majority, of these organs serve more than one
purpose. The tongue is used in talking, in masticating the food, and in
swallowing. The nose serves at least three distinct purposes. The mouth,
the arms, the hands, the feet, the legs, the liver, the lungs, and the
stomach are also organs that serve more than one purpose. This introduces
the principle of economy into the construction of the body and diminishes
the number of organs that would otherwise be required.

The various organs also _combine_ with one another in carrying on the work
of the body. An illustration of this is seen in the digestion of the
food--a process which requires the combined action of the mouth, stomach,
liver, intestines, and other organs. A number of organs working together
for the same purpose form a _system_. The chief systems of the body are
the digestive system, the circulatory system, the respiratory system, the
muscular system, and the nervous system.

*The Organ and its Work.*--A most interesting question relating to the work
of the organ is this: Does the organ work for its own benefit or for the
benefit of the body as a whole? Does the hand, for example, grasp for
itself or in order that the entire body may come into possession? Only
slight study is sufficient to reveal the fact that each organ performs a
work which benefits the body as a whole. In other words, just as the organ
itself is a part of the body, the work which it does is a part of the
necessary work which the body has to do.

But in working for the general good, or for the body as a whole, each
organ becomes a sharer in the benefits of the work done by every other
organ. While the hand receives only a little of the nourishment contained
in the food which it places in the mouth or of the heat from, fuel which
it places on the fire, it is aided and supported by the work of all the
other organs of the body--eyes,  feet, brain, heart, etc. The hand does not
and cannot work independently of the other organs. It is one of the
partners in a very close combination where, by doing a particular work,
it, shares in the profits of all. What is true of the hand is true of
every other organ of the body.

*An Organization.*--The relations which the different organs sustain to
each other and to the body as a whole suggest the possibility of
classifying the body as an organization. This term is broadly applied to a
variety of combinations. An organization is properly defined as _any group
of individuals which, in working together for a common purpose, practices
the division of labor_. This definition will be better understood by
considering a few familiar examples.

A baseball team is an organization. The team is made up of individual
players. These work together for the common purpose of winning games. They
practice the division of labor in that the different players do different
things--one catching, another pitching, and so on. A manufacturing
establishment which employs several workmen may also be an organization.
The article manufactured provides the common purpose toward which all
strive; and, in the assignment of different kinds of work to the
individual workmen, the principle of division of labor is carried out. For
the same reason a school, a railway system, an army, and a political party
are organizations.

An organization of a lower order of individuals than these human
organizations is to be found in a hive of bees. This is made up of the
individual bees, and these, in carrying on the general work of the hive,
are known to practice the division of labor.

*Is the Body an Organization*?--If the body is an organization, it must
fulfill the conditions of the definition. It must be made up of separate
or individual parts. These must work together for the same general
purpose, and, in the accomplishment of this purpose, must practice the
division of labor. That the body practices the division of labor is seen
in the related work of the different organs. That it is made up of minute,
but individual, parts will be shown in the chapter following. That it
carries on a _general work_ which is accomplished through the combined
action of its individual parts is revealed through an extended study of
its various activities. _The body is an organization._ Moreover, it is one
of the most complex and, at the same time, most perfect of the
organizations of which we have knowledge.

*Summary.*--Viewed from the outside, the body is seen to be made up of
divisions which are more or less familiar. Viewed internally, it is found
to consist of different kinds of materials, called tissues. The tissues
are adapted, by their properties, to different purposes both in the
construction of the body and in carrying on its work. The working parts of
the body are called organs and these in their work combine to form
systems. The entire body, on account of the method of its construction and
the character of its work, may be classed as an organization.

*Exercises.*--1. Name and locate the chief external divisions of the body.

2. What tissues may be found by dissecting the leg of a chicken?

3. Name the most important properties and the most important uses of
muscular tissue, osseous tissue, and connective tissue.

4. Define an organ. Define a system. Name examples of each.

5. Name the chief cavities of the body and the organs which they contain.

6. What tissues are present in the hand? How does each of these aid in the
work of the hand?

7. Define an organization. Show that a railway system, an army, and a
school are organizations.

8. What is meant by the phrase "division of labor"? In what manner is the
division of labor practiced in a shoe or watch factory? What are the
advantages?

9. What are the proofs that the body is an organization?

PRACTICAL WORK

*Observation on the Tissues.*--Examine with care the structures in the
entire leg of a chicken, squirrel, rabbit, or other small animal used for
food. Observe, first of all, the external covering, consisting of cuticle
and hair, claws, scales, or feathers, according to the specimen. These are
similar in structure, and they form the epidermis, which is one kind of
_epithelial_ tissue. With a sharp knife lay open the skin and observe that
it is attached to the parts underneath by thin, but tough, threads and
sheaths. These represent a variety of _connective_ tissue. The reddish
material which forms the greater portion of the specimen is a variety of
_muscular_ tissue, and its divisions are called muscles. With a blunt
instrument, separate the muscles, by tearing apart the connective tissue
binding them together, and find the glistening white strips of connective
tissue (tendons) which attach them to the bones. Find near the central
part of the leg a soft, white cord (a nerve) which represents one variety
of _nervous_ tissue. The bones, which may now be examined, form the
_osseous_ tissue. At the ends of the bones will be found a layer of
smooth, white material which represents one kind of _cartilaginous_
tissue. The _adipose_, or fatty, tissue, which is found under the skin and
between the other tissues, is easily recognized.

*Relation of the Tissues to the Organs.*--Observe in the specimen just
studied the relation of the different tissues to the organ as a whole
(regarding the leg as an organ), _i.e._, show how each of the tissues aids
in the work which the organ accomplishes. Show in particular how the
muscles supply the foot with motion, by tracing out the tendons that
connect them with the toes. Pull on the different tendons, noting the
effect upon the different parts of the foot.

CHAPTER III - THE BODY ORGANIZATION

What is the nature of the body organization? What are the individual
parts, or units, that make it up? What general work do these carry on and
upon what basis do they practice the division of labor? The answers to
these questions will suggest the main problems in the study of the body.

                                 [Fig. 3]

  Fig. 3--Diagram showing the relation of the cells and the intercellular
            material. _C._ Cells. _I._ Intercellular material.

*Complex Nature of the Tissues.*--To the unaided eye the tissues have the
appearance of simple structures. The microscope, however, shows just the
reverse to be true. When any one of the tissues is suitably prepared and
carefully examined with this instrument, at least two classes of materials
can be made out. One of these consists of minute particles, called
_cells_; the other is a substance lying between the cells, known as the
_intercellular material_ (Fig. 3). The cells and the intercellular
material, though varying in their relative proportions, are present in all
the tissues.

*The Body a Cell Group.*--The biologist has found that the bodies of all
living things, plants as well as animals, consist either of single cells
or of groups of cells. The single cells live independently of one another,
but the cells that form groups are attached to, and are more or less
dependent upon, one another. In the first condition are found the very
lowest forms of life. In the second, life reaches its greatest
development. The body of man, which represents the highest type of life,
is recognized as a group of cells. In this group each cell is usually
separate and distinct from the others, but is attached to them, and is
held in place by the intercellular material.

*Protoplasm, the Cell Substance.*--The cell is properly regarded as an
_organized_ bit of a peculiar material, called _protoplasm_. This is a
semi-liquid and somewhat granular substance which resembles in appearance
the white of a raw egg. Its true nature and composition are unknown,
because any attempt to analyze it kills it, and dead protoplasm is
essentially different from living protoplasm. It is known, however, to be
a highly complex substance and to undergo chemical change readily. It
appears to be the only kind of matter with which life is ever associated,
and for this reason protoplasm is called the _physical basis of life_. Its
organization into separate bits, or cells, is necessary to the life
activities that take place within it.

*Structure of the Cell.*--Though all portions of the cell are formed from
the protoplasm, this essential substance differs both in structure and in
function at different places in the cell. For this reason the cell is
looked upon as a complex body having several distinct parts. At or near
the center is a clear, rounded body, called the _nucleus_. This plays some
part in the nourishment of the cell and also in the formation of new
cells. If it be absent, as is sometimes the case, the cell is short-lived
and unable to reproduce itself. The variety of protoplasm contained in the
nucleus is called the _nucleoplasm_.

                                 [Fig. 4]

Fig. 4--Diagram of a typical cell (after Wilson). 1. Main body. 2. Nucleus.
3. Attraction sphere. 4. Food particles and waste. 5. Cell-wall. 6. Masses
       of active material found in certain cells, called plastids.

Surrounding the nucleus is the _main body_ of the cell, sometimes referred
to as the "protoplasm." Since the protoplasm forms all parts of the cell,
this substance is more properly called the _cytoplasm_, or cell plasm.
Surrounding and inclosing the cytoplasm, in many cells, is a thin outer
layer, or membrane, which affords more or less protection to the contents
of the cell. This is usually referred to as the _cell-wall_. A fourth part
of the cell is also described, being called the _attraction sphere_. This
is a small body lying near the nucleus and cooeperating with that body in
the formation of new cells. Food particles, wastes, and other substances
may also be present in the cytoplasm. The parts of a typical cell are
shown in Fig. 4.

*Importance of the Cells.*--The cells must be regarded as the living,
working parts of the body. They are the active agents in all of the
tissues, enabling them to serve their various purposes. Working through
the tissues, they build up the body and carry on its different activities.
They are recognized on this account as _the units of structure and of
function_, and are the "individuals" in the body organization. Among the
most important and interesting of the activities of the cells are those by
which they build up the body, or cause it to grow.

*How the Cells enable the Body to Grow.*--Every cell is able to take new
material into itself and to add this to the protoplasm. This tends to
increase the amount of the protoplasm, thereby causing the cells to
increase in size. A general increase in the size of the cells has the
effect of increasing the size of the entire body, and this is one way by
which they cause it to grow. There is, however, a fixed limit, varying
with different cells, to the size which they attain, and this is quite
low. (The largest cells are scarcely visible to the naked eye.) Any marked
increase in the size of the body must, therefore, be brought about by
other means. Such a means is found in the formation of new cells, or _cell
reproduction_. The new cells are always formed _by_ and _from_ the old
cells, the essential process being known as _cell-division_.

                                 [Fig. 5]

Fig. 5--Steps in cell-division (after Wilson). Note that the process begins
with the division of the attraction sphere, then involves the nucleus, and
                     finally separates the main body.

*Cell-Division.*--By dividing, a single cell will, on attaining its growth,
separate into two or more new cells. The process is quite complex and is
imperfectly understood. It is known, however, that the act of separation
is preceded by a series of changes in which the attraction sphere and the
nucleus actively participate, and that, as a result of these changes, the
contents of the old cell are rearranged to form the new cells. Some of the
different stages in the process, as they have been studied under the
microscope, are indicated in Fig. 5.

Gradually, through the formation of new cells and by the growth of these
cells after they have been formed, the body attains its full size. When
growth is complete, cell reproduction is supposed to cease except where
the tissues are injured, as in the breaking of a bone, or where cells,
like those at the surface of the skin, are subject to wear. Then new
material continues to be added to the protoplasm throughout life, but in
amount only sufficient to replace that lost from the protoplasm as waste.

                                 [Fig. 6]

Fig. 6--A tumbler partly filled with marbles covered with water, suggesting
                 the relations of the cells to the lymph.

*Cell Surroundings.*--All cells are said to be _aquatic_. This means simply
that they require water for carrying on their various activities. The
cells, in order to live, must take in and give out materials, and water is
necessary to both processes. It is also an essential part of the
protoplasm. Deprived of water, cells become inactive and usually die.
Aquatic surroundings are provided for the cells of the body through a
liquid known as the _lymph_, which is distributed throughout the
intercellular material (Fig. 6). This consists of water containing oxygen
and food substances in solution. Besides supplying these to the cells, the
lymph also receives their wastes. Through the lymph the necessary
conditions for cell life are provided in the body.

*The General Work of Cells.*--In handling the materials derived from the
lymph, the cells carry on three well-defined processes, known as
absorption, assimilation, and excretion.

_Absorption_ is the process of taking water, food, and oxygen into the
cells.

_Assimilation_ is a complex process which results in the addition of the
absorbed materials to the protoplasm. Through assimilation the protoplasm
is built up or renewed.

_Excretion_ is the throwing off of such waste materials as have been
formed in the cells. These are passed into the lymph and thence to the
surface of the body.

Absorption, assimilation, excretion, and also reproduction are performed
by all classes of cells. They are, on this account, referred to as the
_general work of cells_.

*The Special Work of Cells.*--In addition to the general work which all
cells do in common, each class of cells in the body is able to do some
particular kind of work--a work which the others cannot do or which they
can do only to a limited extent. This is spoken of as the _special work of
cells_. Examples of the special work of cells are found in the production
of motion by muscle cells and in the secretion of liquids by gland cells.
It may be noted that while the general work of cells benefits them
individually, their special work benefits the body as a whole. Another
example of the special work of cells is found in the

                                 [Fig. 7]

  Fig. 7--Cartilage cells, surrounded by the intercellular material which
                           they have deposited.

*Production of the Intercellular Material.*--Though most of the cells of
the body deposit to a slight extent this material, the greater part of it
is produced by a single class of cells found in bone, cartilage, and
connective tissue. Cartilage, bone, and connective tissue differ greatly
from the other tissues in the amount of intercellular material which they
contain, the difference being due to these cells. In the connective tissue
they deposit the fibrous material so important in holding the different
parts of the body together. In the cartilage they produce the gristly
substance which forms by far its larger portion (Fig. 7). In the bones
they deposit a material similar to that in the cartilage, except that with
it is mixed a mineral substance which gives the bones their hardness and
stiffness.(4) The intercellular material, in addition to connecting the
cells, supplies to certain tissues important properties, such as the
elasticity of cartilage and the stiffness of the bones.

*Nature of the Body Organization.*--The division of labor carried on by the
different organs, as shown in the preceding chapter, is in reality carried
on by the cells that form the organs. To see that this is true we have
only to observe the relation of cells to tissues and of tissues to organs.
The cells form the tissues and the tissues form the organs. This
arrangement enables the special work of different kinds of cells to be
combined in the work of the organ as a whole. This is seen in the hand
which, in grasping, uses motion supplied by the muscle cells, a
controlling influence supplied by the nerve cells, a framework supplied by
the bone cells, and so on. The cells supply the basis for the body
organization and, properly speaking, the body is _an organization of
cells_(5) (Recall the definition of an organization, page 10.) In this
organization there are to be observed:

1. A definite arrangement of the cells to form the tissues. A tissue is a
group of like cells.

2. A definite arrangement of the tissues in the organ. Each organ contains
the tissues needed for its work.

3. In several instances there is a definite arrangement of organs to form
systems.

4. The body as a whole is made up of organs and systems, together with the
structures necessary for their support and protection.

There now remains a further question for consideration. What is the one
supreme end, or purpose, toward which all the activities of the body
organization are directed? This purpose will naturally have some relation
to the maintenance, or preservation, of the cell group which we call the
body.

*The Maintenance of Life.*--The preservation of any cell group in its
natural condition, whether it be plant or animal, is accomplished through
keeping it alive. If life ceases, the group quickly disintegrates and its
elements become scattered, a fact which is verified through everyday
observation. Though the nature of life is unknown, it may be looked upon
as the organizer and preserver of the protoplasm. But in preserving the
protoplasm it also preserves the entire cell group, or body. Life is thus
the most essential condition of the body. _With life all portions of the
body are concerned, and toward its maintenance all the activities of the
body organization are directed_.

*The Nutrient Fluid in its Relations to the Cells.*--The maintenance of
life within the cells requires, as we have seen, that they be supplied
with water, food, and oxygen, and that they be relieved of such wastes as
they form. This double purpose is accomplished through the agency of an
internal nutrient fluid, a portion of which has already been referred to
as the lymph. Not only does this fluid supply the means for keeping the
cells alive, but, through the cells, it is also the means of preserving
the life of the body as a whole.

The cells, however, rapidly exhaust the nutrient fluid. They take from it
food and oxygen and they put into it their wastes. To prevent its becoming
unfit for supplying their needs, food and oxygen must be continually added
to this fluid, and waste materials must be continually removed. This is
not an easy task. As a matter of fact, the preparation, distribution, and
purification of the nutrient fluid requires the direct or indirect aid of
practically all parts of the body. It supplies for this reason a broad
basis for the division of labor on the part of the cells.

*Relation of the Body to its Environment.*--While life is directly
dependent upon the internal nutrient fluid, it is indirectly dependent
upon the physical surroundings of the body. Herein lies the need of the
_external_ organs--the feet and legs for moving about, the hands for
handling things, the eyes for directing movements, etc. That the great
needs of the body are supplied from its surroundings are facts of common
experience. Food, shelter, air, clothing, water, and the means of
protection are external to the body and form a part of its environment. In
making the things about him contribute to his needs, man encounters a
problem which taxes all his powers. Only by toil and hardship, "by the
sweat of his brow," has he been able to wrest from his surroundings the
means of his sustenance.

*The Main Physiological Problems.*--The study of the body is thus seen to
resolve itself naturally into the consideration of two main problems:

1. _That of maintaining in the body a nutrient fluid for the cells._

2. _That of bringing the body into such relations with its surroundings as
will enable it to secure materials for the nutrient fluid and satisfy its
other needs._

The first problem is _internal_ and includes the so-called vital
processes, known as digestion, circulation, respiration, and excretion.
The second problem is _external_, as it were, and includes the work of the
external organs--the organs of motion and of locomotion and the organs of
special sense. These problems are closely related, since they are the two
divisions of the one problem of maintaining life. Neither can be
considered independently of the other. In the chapter following is taken
up the first of these problems.

*Summary.*--The individual parts, or units, that form the body organization
are known as cells. These consist of minute but definitely arranged
portions of protoplasm and are held together by the intercellular
material. They build up the body and carry on its different activities.
The tissues are groups of like cells. By certain general activities the
cells maintain their existence in the tissues and by the exercise of
certain special activities they adapt the tissues to their purposes in the
body. The body, as a cell organization, has its activities directed under
normal conditions toward a single purpose--that of maintaining life. In the
accomplishment of this purpose a nutrient fluid is provided for the cells
and proper relations between the body and its surroundings are
established.

*Exercises.*--1. If a tissue be compared to a brick wall, to what do the
separate bricks correspond? To what the mortar between the bricks?

2. Draw an outline of a typical cell, locating and naming the main
divisions.

3. How do the cells enable the body to grow? Describe the process of
cell-division.

4. How does the general work of cells differ from their special work?
Define absorption, excretion, and assimilation as applied to the cells.

5. Compare the conditions surrounding a one-celled animal, living in
water, to the conditions surrounding the cells in the body.

6. What is meant by the term "environment"? How does man's environment
differ from that of a fish?

7. What is the necessity for a nutrient fluid in the body?

8. Why is the maintenance of life necessarily the chief aim of all the
activities of the body?

9. State the two main problems in the study of the body.

PRACTICAL WORK

*Observations.*--1. Make some scrapings from the inside of the cheek with a
dull knife and mix these with a little water on a glass slide. Place a
cover-glass on the same and examine with a compound microscope. The large
pale cells that can be seen in this way are a variety of epithelial cells.

2. Mount in water on a glass slide some thin slices of cartilage and
examine first with a low and then with a high power of microscope.
(Suitable slices may be cut, with a sharp razor, from the cartilage found
at the end of the rib of a young animal.) Note the small groups of cells
surrounded by, and imbedded in, the intercellular material.

3. Mount and examine with the microscope thin slices of elder pith,
potato, and the stems of growing plants. Make drawings of the cells thus
observed.

4. Examine with the microscope a small piece of the freshly sloughed off
epidermis of a frog's skin. Examine it first in its natural condition, and
then after soaking for an hour or two in a solution of carmine. Make
drawings.

5. Mount on a glass slide some of the scum found on stagnant water and
examine it with a compound microscope. Note the variety and relative size
of the different things moving about. The forms most frequently seen by
such an examination are one-celled plants. Many of these have the power of
motion.

6. Examine tissues of the body, such as nervous, muscular, and glandular
tissues, which have been suitably prepared and mounted for microscopic
study, using low and high powers of the microscope. Make drawings of the
cells in the different tissues thus observed.

CHAPTER IV - THE BLOOD

Two liquids of similar nature are found in the body, known as the blood
and the lymph. These are closely related in function and together they
form the nutrient fluid referred to in the preceding chapter. The blood is
the more familiar of the two liquids, and the one which can best be
considered at this time.

*The Blood: where Found.*--The blood occupies and moves through a system of
closed tubes, known as the blood vessels. By means of these vessels the
blood is made to circulate through all parts of the body, but from them it
does not escape under normal conditions. Though provisions exist whereby
liquid materials may both enter and leave the blood stream, it is only
when the blood vessels are cut or broken that the blood, as blood, is able
to escape from its inclosures.

*Physical Properties of the Blood.*--Experiments such as those described at
the close of this chapter reveal the more important physical properties of
the blood. It may be shown to be heavier and denser than water; to have a
faint odor and a slightly salty taste; to have a bright red color when it
contains oxygen and a dark red color when oxygen is absent; and to
undergo, when exposed to certain conditions, a change called coagulation.
These properties are all accounted for through the different materials
that enter into the formation of the blood.

                                 [Fig. 8]

  Fig. 8--Blood corpuscles, highly magnified. _A._ Red corpuscles as they
   appear in diluted blood. _B._ Arrangement of red corpuscles in rows
  between which are white corpuscles, as may be seen in undiluted blood.
           _C._ Red corpuscles much enlarged to show the form.

*Composition of the Blood.*--To the naked eye the blood appears as a thick
but simple liquid; but when examined with a compound microscope, it is
seen to be complex in nature, consisting of at least two distinct
portions. One of these is a clear, transparent liquid; while the other is
made up of many small, round bodies that float in the liquid. The liquid
portion of the blood is called the _plasma_; the small bodies are known as
_corpuscles_. Two varieties of corpuscles are described--the _red_
corpuscles and the _white_ corpuscles (Fig. 8). Other round particles,
smaller than the corpuscles, may also be seen under favorable conditions.
These latter are known as _blood platelets_.

*Red Corpuscles.*--The red corpuscles are classed as cells, although, as
found in the blood of man and the other mammals (Fig. 9), they have no
nuclei.(6) Each one consists of a little mass of protoplasm, called the
_stroma_, which contains a substance having a red color, known as
_hemoglobin_. The shape of the red corpuscle is that of a circular disk
with concave sides. It has a width of about 1/3200 of an inch (7.9
microns(7)) and a thickness of about 1/13000 of an inch (1.9 microns). The
red corpuscles are exceedingly numerous, there being as many as five
millions in a small drop (one cubic millimeter) of healthy blood. But the
number varies somewhat and is greatly diminished during certain forms of
disease.

                                 [Fig. 9]

Fig. 9--Red corpuscles from various animals. Those from mammals are without
   nuclei, while those from birds and cold-blooded animals have nuclei.

It is the _function_ of the red corpuscles to serve as _oxygen carriers_
for the cells. They take up oxygen at the lungs and release it at the
cells in the different tissues.(8) The performance of this function
depends upon the hemoglobin.

*Hemoglobin.*--This substance has the remarkable property of forming, under
certain conditions, a weak chemical union with oxygen and, when the
conditions are reversed, of separating from it. It forms about nine tenths
of the solid matter of the red corpuscles and to it is due the colors of
the blood. When united with the oxygen it forms a compound, called
_oxyhemoglobin_, which has a bright red color; the hemoglobin alone has a
dark red color. These colors are the same as those of the blood as it
takes on and gives off oxygen. The stroma, which forms only about one
tenth of the solid matter of the corpuscles, serves as a contrivance for
holding the hemoglobin. The conditions which cause the hemoglobin to unite
with oxygen in the lungs and to separate from it in the tissues, will be
considered later (Chapter VIII).

*Disappearance and Origin of Red Corpuscles.*--The red corpuscles, being
cells without nuclei, are necessarily short-lived. It has been estimated
that during a period of one to two months, all the red corpuscles in the
body at a given time will have disappeared and their places taken by new
ones. The origin of new corpuscles, however, and the manner of ridding the
blood of old ones are problems that are not as yet fully solved. The
removal of the products of broken down corpuscles is supposed to take
place both in the liver and in the spleen.(9)

Regarding the origin of the red corpuscles, the evidence now seems
conclusive that large numbers of them are formed in the red marrow of the
bones. The red marrow is located in what is known as the spongy substance
of the bones (Chapter XIV) and consists, to a large extent, of cells
somewhat like the red corpuscles, but differing from them in having
nuclei. These appear to be constantly in a state of reproduction. The
blood, flowing through the minute cavities containing these cells, carries
those that have been loosened out into the blood stream. Nuclei appear in
the red corpuscles at the time of their formation, but these quickly
separate and, according to some authorities, form the blood platelets.

*White Corpuscles.*--The white corpuscles, or _leucocytes_, are cells of a
general spherical shape, each containing one, two, or more nuclei. They
are much less numerous than the red, there being on the average only one
white corpuscle to about every five hundred of the red ones. On the other
hand, the white corpuscles are larger than the red, one of the former
being equal in volume to about three of the latter.

                                [Fig. 10]

Fig. 10--*Escape of white corpuscles from a small blood vessel* (Hall). At
     _A_ the conditions are normal, but at _B_ some excitation in the
 surrounding tissue leads to a migration of corpuscles. 1, 2, and 3 show
                     different stages of the passage.

The white corpuscles are found, when studied under favorable conditions,
to possess the power of changing their shape and, by this means, of moving
from place to place. This property enables them to penetrate the walls of
capillaries and to pass with the lymph in between the cells of the
tissues. The white corpuscles are, therefore, not confined to the blood
vessels, as are the red corpuscles, but migrate through the intercellular
spaces (Fig. 10). If any part of the body becomes inflamed, the white
corpuscles collect there in large numbers; and, on breaking down, they
form most of the white portion of the sore, called the _pus_.

New white corpuscles are formed from old ones, by cell-division. Their
production may occur in almost any part of the body, but usually takes
place in the lymphatic glands (Chapter VI) and in the spleen, where
conditions for their development are especially favorable. In these places
they are found in great abundance and in various stages of development.

*Functions of White Corpuscles.*--The main use of the white corpuscles
appears to be that of a destroyer of disease germs. These consist of
minute organisms that find their way into the body and, by living upon the
tissues and fluids and by depositing toxins (poisons) in them, cause
different forms of disease. Besides destroying germs that may be present
in the blood, the white corpuscles also leave the blood and attack germs
that have invaded the cells. By forming a kind of wall around any foreign
substance, such as a splinter, that has penetrated the skin, they are able
to prevent the spread of germs through the body. In a similar manner they
also prevent the germs from boils, abscesses, and sore places in general
from getting to and infecting other parts of the body.(10) Another
function ascribed to the white corpuscles is that of aiding in the
coagulation of the blood (page 31); and still another, of aiding in the
healing of wounds.

*Plasma.*--The plasma is a complex liquid, being made up of water and of
substances dissolved in the water. The dissolved substances consist mainly
of foods for the cells and wastes from the cells.

1. _The foods_ represent the same classes of materials as are taken in the
daily fare, _i.e._, proteids, carbohydrates, fats, and salts (Chapter IX).
Three kinds of proteids are found in the plasma, called _serum albumin_,
_serum globulin_, and _fibrinogen_. These resemble, in a general way, the
white of raw egg, but differ from each other in the readiness with which
they coagulate. Fibrinogen coagulates more readily than the others and is
the only one that changes in the ordinary coagulation of the blood. The
others remain dissolved during this process, but are coagulated by
chemical agents and by heat. While all of the proteids probably serve as
food for the cells, the fibrinogen, in addition, is a necessary factor in
the coagulation of the blood (page 31).

The only representative of the carbohydrates in the plasma is _dextrose_.
This is a variety of sugar, being derived from starch and the different
sugars that are eaten. The _fat_ in the plasma is in minute quantities and
appears as fine droplets--the form in which it is found in milk. While
several mineral salts are present in small quantities in the plasma,
_sodium chloride_, or common salt, is the only one found in any
considerable amount. The mineral salts serve various purposes, one of
which is to cause the proteids to dissolve in the plasma.

2. _The wastes_ are formed at the cells, whence they are passed by the
lymph into the blood plasma. They are carried by the blood until removed
by the organs of excretion. The two waste products found in greatest
abundance in the plasma are carbon dioxide and urea.

The substances dissolved in the plasma form about 10 per cent of the whole
amount. The remaining 90 per cent is water. Practically all the
constituents of the plasma, except the wastes, enter the blood from the
digestive organs.

*Purposes of Water in the Blood.*--Not only is water the most abundant
constituent of the blood; it is, in some respects, the most important. It
is the liquefying portion of the blood, holding in solution the
constituents of the plasma and floating the corpuscles. Deprived of its
water, the blood becomes a solid substance. Through the movements of the
blood the water also serves the purpose of a transporting agent in the
body. The cells in all parts of the body require water and this is
supplied to them from the blood. Water is present in the corpuscles as
well as in the plasma and forms about 80 per cent of the entire volume of
the blood.

*Coagulation of the Blood.*--If the blood is exposed to some unnatural
condition, such as occurs when it escapes from the blood vessels, it
undergoes a peculiar change known as _coagulation_.(11) In this change the
corpuscles are collected into a solid mass, known as the _clot_, thereby
separating from a liquid called the _serum_. The serum, which is similar
in appearance to the blood plasma, differs from that liquid in one
important respect as explained below.

*Causes of Coagulation.*--Although coagulation affects all parts of the
blood, only one of its constituents is found in reality to coagulate. This
is the fibrinogen. The formation of the clot and the separation of the
serum is due almost entirely to the action of this substance. Fibrinogen
is for this reason called the _coagulable constituent of the blood_. In
the plasma the fibrinogen is in a liquid form; but during coagulation it
changes into a white, stringy solid, called _fibrin_. This appears in the
clot and is the cause of its formation. Forming as a network of
exceedingly fine and very delicate threads (Fig. 11) _throughout the mass
of blood_ that is coagulating, the fibrin first entangles the corpuscles
and then, by contracting, draws them into the solid mass or clot.(12) The
contracting of the fibrin also squeezes out the serum. This liquid
contains all the constituents of the plasma except the fibrinogen.

                                [Fig. 11]

 Fig. 11--*Fibrin threads* (after Ranvier). These by contracting draw the
                  corpuscles together and form the clot.

*Fibrin Ferment and Calcium.*--Most difficult of all to answer have been
the questions: What causes the blood to coagulate outside of the blood
vessels and what prevents its coagulation inside of these vessels? The
best explanation offered as yet upon this point is as follows: Fibrinogen
does not of itself change into fibrin, but is made to undergo this change
by the presence of another substance, called _fibrin ferment_. This
substance is not a regular constituent of the blood, but is formed as
occasion requires. It is supposed to result from the breaking down of the
white corpuscles, and perhaps also from the blood platelets, when the
blood is exposed to unnatural conditions. The formation of the ferment
leads in turn to the changing of the fibrinogen into fibrin.

Another substance which is necessary to the process of coagulation is the
element calcium. If compounds of calcium are absent from the blood,
coagulation does not take place. These are, however, regular constituents
of healthy blood. Whether the presence of the calcium is necessary to the
formation of the ferment or to the action of the ferment upon the
fibrinogen is unknown.

*Purpose of Coagulation.*--The purpose of coagulation is to check the flow
of blood from wounds. The fact that the blood is contained in and kept
flowing continuously through a system of _connected_ vessels causes it to
escape rapidly from the body whenever openings in these vessels are made.
Clots form at such openings and close them up, stopping in this way the
flow that would otherwise go on indefinitely. Coagulation, however, does
not stop the flow of blood from the large vessels. From these the blood
runs with too great force for the clot to form within the wound.

*Time Required for Coagulation.*--The rate at which coagulation takes place
varies greatly under different conditions. It is influenced strongly by
temperature; heat hastens and cold <DW44>s the process. It may be
prevented entirely by lowering the temperature of the blood to near the
freezing point. The presence of a foreign substance increases the rapidity
of coagulation, and it has been observed that bleeding from small wounds
is more quickly checked by covering them with linen or cotton fibers. The
fibers in this case hasten the process of coagulation.

*Quantity of Blood.*--The quantity of blood is estimated to be about one
thirteenth of the entire weight of the body. This for the average
individual is an amount weighing nearly twelve pounds and having a volume
of nearly one and one half gallons. About 46 per cent by volume of this
amount is made up of corpuscles and 54 per cent of plasma. Of the plasma
about 10 per cent consists of solids and 90 per cent of water, as already
stated.

*Functions of the Blood.*--The blood is the great carrying, or
distributing, agent in the body. Through its movements (considered in the
next chapter) it carries food and oxygen to the cells and waste materials
from the cells. Much of the blood may, therefore, be regarded as _freight_
in the process of transportation. The blood also carries, or distributes,
heat. Taking up heat in the warm parts of the body, it gives it off at
places having a lower temperature. This enables all parts of the body to
keep at about the same temperature.

In addition to serving as a carrier, the blood has antiseptic properties,
i.e., it destroys disease germs. While  this function is mainly due to the
white corpuscles, it is due in part to the plasma.(13) Through its
coagulation, the blood also closes leaks in the small blood vessels. The
blood is thus seen to be a liquid of several functions.

                                [Fig. 12]

Fig. 12--*A balanced change* in water. The level remains constant although
the water is continually changing; suggestive of the changes in the blood.

*Changes in the Blood.*--In performing its functions in the body the blood
must of necessity undergo rapid and continuous change. The red corpuscles,
whose changes have already been noted, appear to be the most enduring
constituents of the blood. The plasma is the portion that changes most
rapidly. Yet in spite of these changes the quantity and character of the
blood remain practically constant.(14) This is because there is a
_balancing_ of the forces that bring about the changes. The addition of
various materials to the blood just equals the withdrawal of the same
materials from the blood. Somewhat as a vessel of water (Fig. 12) having
an inflow and an outflow which are equal in amount may keep always at the
same level, the balancing of the intake and outgo of the blood keeps its
composition about the same from time to time.

*Hygiene of the Blood.*--The blood, being a changeable liquid, is easily
affected through our habits of living. Since it may be affected for ill as
well as for good, one should cultivate those habits that are beneficial
and avoid those that are harmful in their effects. Most of the hygiene of
the blood, however, is properly included in the hygiene of the organs that
act upon the blood--a fact which makes it unnecessary to treat this subject
fully at this time.

From a health standpoint, the most important constituents of the blood
are, perhaps, the corpuscles. These are usually sufficient in number and
vigor in the blood of those who take plenty of physical exercise, accustom
themselves to outdoor air and sunlight, sleep sufficiently, and avoid the
use of injurious drugs. On the other hand, they are deficient in quantity
and inferior in quality in the bodies of those who pursue an opposite
course. Impurities not infrequently find their way into the blood through
the digestive organs. One should eat wholesome, well-cooked food, drink
freely of _pure_ water, and limit the quantity of food _to what can be
properly digested_. The natural purifiers of the blood are the organs of
excretion. The skin is one of these and its power to throw off impurities
depends upon its being clean and active.

*Effect of Drugs.*--Certain drugs and medicines, including alcohol and
quinine,(15) have recently been shown to destroy the white corpuscles. The
effect of such substances, if introduced in considerable amount in the
body, is to render one less able to withstand attacks of disease. Many
patent medicines are widely advertised for purifying the blood. While
these may possibly do good in particular cases, the habit of doctoring
one's self with them is open to serious objection. Instead of taking drugs
and patent medicines for purifying the blood, one should study to live
more hygienically. We may safely rely upon wholesome food, pure water,
outdoor exercise and sunlight, plenty of sleep, and a clean skin for
keeping the blood in good condition. If these natural remedies fail, a
physician should be consulted.

*Summary.*--The blood is the carrying or transporting agent of the body. It
consists in part of constituents, such as the red corpuscles, that enable
it to carry different substances; and in part of the materials that are
being carried. The latter, which include food and oxygen for the cells and
wastes from the cells, may be classed as freight. Certain constituents in
the blood destroy disease germs, and other constituents, by coagulating,
close small leaks in the blood vessels. Although subject to rapid and
continuous change, the blood is able--by reason of the balancing of
materials added to and withdrawn from it--to remain about the same in
quantity and composition.

Exercises.--1. Compare blood and water with reference to weight, density,
color, odor, and complexity of composition.

2. Show by an outline the different constituents of the blood.

3. Compare the red and white corpuscles with reference to size, shape,
number, origin, and function.

4. Name some use or purpose for each constituent of the blood.

5. What constituents of the blood may be regarded as freight and what as
agents for carrying this freight?

6. After coagulation, what portions of the blood are found in the clot?
What portions are found in the serum?

7. What purposes are served by water in the blood?

8. Show how the blood, though constantly changing, is kept about the same
in quantity, density, and composition.

9. In the lungs the blood changes from a dark to a bright red color and in
the tissues it changes back to dark red. What is the cause of these
changes?

10. If the oxygen and hemoglobin formed a strong instead of a weak
chemical union, could the hemoglobin then act as an oxygen carrier? Why?

11. What habits of living favor the development of corpuscles in the
blood?

12. Why will keeping the skin clean and active improve the quality of
one's blood?

PRACTICAL WORK

*To demonstrate the Physical Properties of Blood* (Optional).--Since blood
is needed in considerable quantity in the following experiments, it is
best obtained from the butcher. To be sure of securing the blood in the
manner desired, take to the butcher three good-sized bottles bearing
labels as follows:

*1* Fill two thirds full. While the blood is cooling, stir rapidly with
the hand or a bunch of switches to remove the clot.

*2* Fill two thirds full and set aside without shaking or stirring.

*3* Fill two thirds full and thoroughly mix with the liquid in the bottle.

Label 3 must be pasted on a bottle, having a tight-fitting stopper, which
is filled one fifth full of a saturated solution of Epsom salts. The
purpose of the salts is to prevent coagulation until the blood is diluted
with water as in the experiments which follow.

*Experiments.*--1. Let some of the defibrinated blood (bottle 1) flow (not
fall) on the surface of water in a glass vessel. Does it remain on the
surface or sink to the bottom? What does the experiment show with
reference to the relative weight of blood and water?

2. Fill a large test tube or a small bottle one fourth full of the
defibrinated blood and thin it by adding an equal amount of water. Then
place the hand over the mouth and shake until the blood is thoroughly
mixed with the air. Compare with a portion of the blood not mixed with the
air, noting any difference in color. What substance in the air has acted
on the blood to change its color?

3. Fill three tumblers each two thirds full of water and set them in a
warm place. Pour into one of the tumblers, and thoroughly mix with the
water, two tablespoonfuls of the blood containing the Epsom salts. After
an interval of half an hour add blood to the second tumbler in the same
manner, and after another half hour add blood to the third. The water
dilutes the salts so that coagulation is no longer prevented. Jar the
vessel occasionally as coagulation proceeds; and if the clot is slow in
forming, add a trace of some salt of calcium (calcium chloride). After the
blood has been added to the last tumbler make a comparative study of all.
Note that coagulation begins in all parts of the liquid at the same time
and that, as the process goes on, the clot shrinks and is drawn toward the
center.

4. Place a clot from one of the tumblers in experiment 3 in a large vessel
of water. Thoroughly wash, adding fresh water, until a white, stringy
solid remains. This substance is fibrin.

5. Examine the coagulated blood obtained from the butcher (bottle 2).
Observe the dark central mass (the clot) surrounded by a clear liquid (the
serum). Sketch the vessel and its contents, showing and naming the parts
into which the blood separates by coagulation.

*To examine the Red Corpuscles.*--Blood for this purpose is easily obtained
from the finger. With a handkerchief, wrap one of the fingers of the left
hand from the knuckle down to the first joint. Bend this joint and give it
a sharp prick with the point of a sterilized 'needle just above the root
of the nail. Pressure applied to the under side of the finger will force
plenty of blood through a very small opening. (To prevent any possibility
of blood poisoning the needle should be sterilized. This may be done by
dipping it in alcohol or by holding it for an instant in a hot flame. It
is well also to wash the finger with soap and water, or with alcohol,
before the operation.) Place a small drop of the blood in the middle of a
glass slide, protect the same with a cover glass, and examine with a
compound microscope. At least two specimens should be examined, one of
which should be diluted with a little saliva or a physiological salt
solution.(16) In the diluted specimen the red corpuscles appear as
amber-, circular, disk-shaped bodies. In the undiluted specimen
they show a decided tendency to arrange themselves in rows, resembling
rows of coins. (Singly, the corpuscles do not appear red when highly
magnified.)

A few white corpuscles may generally be found among the red ones in the
undiluted specimen. These become separated by the formation of the red
corpuscles into rows. They are easily recognized by their larger size and
by their silvery appearance, due to the light shining through them.

*To examine White Corpuscles.*--Obtain from the butcher a small piece of
the neck sweetbread of a calf. Press it between the fingers to squeeze out
a whitish, semi-liquid substance. Dilute with physiological salt solution
on a glass slide and examine with a compound microscope. Numerous white
corpuscles of different kinds and sizes will be found. Make sketches.

*To prepare Models of Red Corpuscles.*--Several models of red corpuscles
should be prepared for the use of the class. Clay and putty may be pressed
into the form of red corpuscles and allowed to harden, and small models
may be cut out of blackboard crayon. Excellent models can be molded from
plaster of Paris as follows: Coat the inside of the lid of a baking powder
can with oil or vaseline and fill it even full of a thick mixture of
plaster of Paris and water. After the plaster has set, remove it from the
lid and with a pocket-knife round off the edges and hollow out the sides
until the general form of the corpuscle is obtained. The models may be
 red if it is desired to match the color as well as the form of the
corpuscle.

CHAPTER V - THE CIRCULATION

A Carrier must move. To enable the blood to carry food and oxygen _to_ the
cells and waste materials _from_ the cells, and also to distribute heat,
it is necessary to keep it moving, or circulating, in all parts of the
body. So closely related to the welfare of the body is the circulation(17)
of the blood, that its stoppage for only a brief interval of time results
in death.

*Discovery of the Circulation.*--The discovery of the circulation of the
blood was made about 1616 by an English physician named Harvey. In 1619 he
announced it in his public lectures and in 1628 he published a treatise in
Latin on the circulation. The chief arguments advanced in support of his
views were the presence of valves in the heart and veins, the continuous
movement of the blood in the same direction through the blood vessels, and
the fact that the blood comes from a cut artery in jets, or spurts, that
correspond to the contractions of the heart.

No other single discovery with reference to the human body has proved of
such great importance. A knowledge of the nature and purpose of the
circulation was the necessary first step in understanding the plan of the
body and the method of maintaining life, and physiology as a science dates
from the time of Harvey's discovery.

*Organs of Circulation.*--The organs of circulation, or blood vessels, are
of four kinds, named the heart, the arteries, the capillaries, and the
veins. They serve as contrivances both for holding the blood and for
keeping it in motion through the body. The heart, which is the chief organ
for propelling the blood, acts as a force pump, while the arteries and
veins serve as tubes for conveying the blood from place to place.
Moreover, the blood vessels are so connected that the blood moves through
them in a regular order, performing two well-defined circuits.

                                [Fig. 13]

Fig. 13--*Heart* in position in thoracic cavity. Dotted lines show positin
                  of diaphragm and of margins of lungs.

*The Heart.*--The human heart, roughly speaking, is about the size of the
clenched fist of the individual owner. It is situated very near the center
of the thoracic cavity and is almost completely surrounded by the lungs.
It is cone-shaped and is so suspended that the small end hangs downward,
forward, and a little to the left. When from excitement, or other cause,
one becomes conscious of the movements of the heart, these appear to be in
the left portion of the chest, a fact which accounts for the erroneous
impression that the heart is on the left side. The position of the heart
in the cavity of the chest is shown in Fig. 13.

*The Pericardium.*--Surrounding the heart is a protective covering, called
the pericardium. This consists of a closed membranous sac so arranged as
to form a double covering around the heart. The heart does not lie inside
of the pericardial sac, as seems at first glance to be the case, but its
relation to this space is like that of the hand to the inside of an empty
sack which is laid around it (Fig. 14). The inner layer of the pericardium
is closely attached to the heart muscle, forming for it an outside
covering. The outer layer hangs loosely around the heart and is continuous
with the inner layer at the top. The outer layer also connects at certain
places with the membranes surrounding the lungs and is attached below to
the diaphragm. Between the two layers of the pericardium is secreted a
liquid which prevents friction from the movements of the heart.

                                [Fig. 14]

 Fig. 14--*Diagram of section of the pericardial sac*, heart removed. _A._
 Place occupied by the heart. _B._ Space inside of pericardial sac. _a._
Inner layer of pericardium and outer lining of heart. _b._ Outer layer of
           pericardium. _C._ Covering of lung. _D._ Diaphragm.

*Cavities of the Heart.*--The heart is a hollow, muscular organ which has
its interior divided by partitions into four distinct cavities. The main
partition extends from top to bottom and divides the heart into two
similar portions, named from their positions the right side and the left
side. On each side are two cavities, the one being directly above the
other. The upper cavities are called _auricles_ and the lower ones
_ventricles_. To distinguish these cavities further, they are named from
their positions the right auricle and the left auricle, and the right
ventricle and the left ventricle (Fig. 15). The auricles on each side
communicate with the ventricles below; but after birth there is no
communication between the cavities on the opposite sides of the heart. All
the cavities of the heart are lined with a smooth, delicate membrane,
called the _endocardium_.

                                [Fig. 15]

   Fig. 15--*Diagram showing plan of the heart.* 1. Semilunar valves. 2.
 Tricuspid valve. 3. Mitral valve. 4. Right auricle. 5. Left auricle. 6.
 Right ventricle. 7. Left ventricle. 8. Chordae tendineae. 9. Inferior vena
    cava. 10. Superior vena cava. 11. Pulmonary artery. 12. Aorta. 13.
                             Pulmonary veins.

*Valves of the Heart.*--Located at suitable places in the heart are four
gate-like contrivances, called valves. The purpose of these is _to give
the blood a definite direction_ in its movements. They consist of tough,
inelastic sheets of connective tissue, and are so placed that pressure on
one side causes them to come together and shut up the passageway, while
pressure on the opposite side causes them to open. A valve is found at the
opening of each auricle into the ventricle, and at the opening of each
ventricle into the artery with which it is connected.

The valve between the right auricle and the right ventricle is called the
_tricuspid_ valve. It is suspended from a thin ring of connective tissue
which surrounds the opening, and its free margins extend into the
ventricle (Fig. 16). It consists of three parts, as its name implies,
which are thrown together in closing the opening. Joined to the free edges
of this valve are many small, tendinous cords which connect at their lower
ends with muscular pillars in the walls of the ventricle. These are known
as the _chordae tendineae_, or heart tendons. Their purpose is to serve as
_valve stops_, to prevent the valve from being thrown, by the force of the
blood stream, back into the auricle.

The _mitral_, or bicuspid, valve is suspended around the opening between
the left auricle and the left ventricle, with the free margins extending
into the ventricle. It is exactly similar in structure and arrangement to
the tricuspid valve, except that it is stronger and is composed of two
parts instead of three.

                                [Fig. 16]

Fig. 16--*Right side of heart* dissected to show cavities and valves. _B._
 Right semilunar valve. The tricuspid valve and the chordae tendineae shown
                            in the ventricle.

The _right semilunar_ valve is situated around the opening of the right
ventricle into the pulmonary artery. It consists of three pocket-shaped
strips of connective tissue which hang loosely from the walls when there
is no pressure from above; but upon receiving pressure, the pockets fill
and project into the opening, closing it completely (Fig. 16). The _left
semilunar_ valve is around the opening of the left ventricle into the
aorta, and is similar in all respects to the right semilunar valve.

*Differences in the Parts of the Heart.*--Marked differences are found in
the walls surrounding the different cavities of the heart. The walls of
the ventricles are much thicker and stronger than those of the auricles,
while the walls of the left ventricle are two or three times thicker than
those of the right. A less marked but similar difference exists between
the auricles and also between the valves on the two sides of the heart.
These differences in structure are all accounted for by the work done by
the different portions of the heart. The greater the work, the heavier the
structures that perform the work.

                                [Fig. 17]

Fig. 17--*Diagram of the circulation*, showing in general the work done by
 each part of the heart. The right ventricle forces the blood through the
 lungs and into the left auricle. The left ventricle forces blood through
 all parts of the body and back to the auricle. The auricles force blood
                           into the ventricles.

*Connection with Arteries and Veins.*--Though the heart is in communication
with all parts of the circulatory system, it makes actual connection with
only a few of the blood tubes. These enter the heart at its upper portion
(Fig. 15), but connect with its different cavities as follows:

1. _With the right auricle_, the superior and the inferior venae cavae and
the coronary veins. The superior vena cava receives blood from the head
and the upper extremities; the inferior vena cava, from the trunk and the
lower extremities; and the coronary veins, from the heart itself.

2. _With the left auricle_, the four pulmonary veins. These receive blood
from the lungs and empty it into the left auricle.

3. _With the right ventricle_, the pulmonary artery. This receives blood
from the heart and by its branches distributes it to all parts of the
lungs.

4. _With the left ventricle_, the aorta. The aorta receives blood from the
heart and through its branches delivers it to all parts of the body.

*How the Heart does its Work.*--The heart is a muscular pump(18) and does
its work through the contracting and relaxing of its walls. During
contraction the cavities are closed and the blood is forced out of them.
During relaxation the cavities open and are refilled. The valves direct
the flow of the blood, being so arranged as to keep it moving always in
the same direction (Fig. 17).

The heart, however, is not a single or a simple pump, but consists in
reality of _four_ pumps which correspond to its different cavities. These
connect with each other and with the blood vessels over the body in such a
manner that each aids in the general movement of the blood.

                                [Fig. 18]

            Fig. 18--Diagram illustrating the "cardiac cycle."

*Work of Auricles and Ventricles Compared.*--In the work of the heart the
two auricles contract at the same time--their contraction being followed
immediately by the contraction of both ventricles. After the contraction
of the ventricles comes a period of rest, or relaxation, about equal in
time to the period of contraction of both the auricles and the
ventricles.(19) On account of the work which they perform, the auricles
have been called the "feed pumps" of the heart; and the ventricles, the
"force pumps."(20) It is the function of the auricles to collect the blood
from the veins, to let this run slowly into the ventricles when both the
auricles and ventricles are relaxed, and finally, by contracting, _to
force an excess of blood into the ventricles_, thereby distending their
walls. The ventricles, having in this way been fully charged by the
auricles, now contract and force their contents into the large arteries.

*Sounds of the Heart.*--Two distinct sounds are given out by the heart as
it pumps the blood. One of them is a dull and rather heavy sound, while
the other is a short, sharp sound. The short sound follows quickly after
the dull sound and the two are fairly imitated by the words "lubb, dup."
While the cause of the first sound is not fully understood, most
authorities believe it to be due to the contraction of the heart muscle
and the sudden tension on the valve flaps. The second sound is due to the
closing of the semilunar valves. These sounds are easily heard by placing
an ear against the chest wall. They are of great value to the physician in
determining the condition of the heart.

*Arteries and Veins.*--These form two systems of tubes which reach from the
heart to all parts of the body. The arteries receive blood from the heart
and distribute it to the capillaries. The veins receive the blood from the
capillaries and return it to the heart. The arteries and veins are similar
in structure, both having the form of tubes and both having three distinct
layers, or coats, in their walls. The corresponding coats in the arteries
and veins are made up of similar materials, as follows:

1. _The inner coat_ consists of a delicate lining of flat cells resting
upon a thin layer of connective tissue. The inner coat is continuous with
the lining of the heart and provides a smooth surface over which the blood
glides with little friction.

2. _The middle coat_ consists mainly of non-striated, or involuntary,
muscular fibers. This coat is quite thin in the veins, but in the arteries
it is rather thick and strong.

3. _The outer coat_ is made up of a variety of connective tissue and is
also much thicker and stronger in the arteries than in the veins.

                                [Fig. 19]

               Fig. 19--Artery dissected to show the coats.

Marked differences exist between the arteries and the veins, and these
vessels are readily distinguished from each other. The walls of the
arteries are much thicker and heavier than those of the veins (Fig. 19).
As a result these tubes stand open when empty, whereas the veins collapse.
The arteries also are highly elastic, while the veins are but slightly
elastic. On the other hand, many of the veins contain valves, formed by
folds in the inner coat (Fig. 20), while the arteries have no valves. The
blood flows more rapidly through the arteries than through the veins, the
difference being due to the fact that the system of veins has a greater
capacity than the system of arteries.

                                [Fig. 20]

               Fig. 20--Vein split open to show the valves.

*Why the Arteries are Elastic.*--The elasticity of the arteries serves a
twofold purpose. It keeps the arteries from bursting when the blood is
forced into them from the ventricles, and it is a means of _supplying
pressure to the blood while the ventricles are in a condition of
relaxation._ The latter purpose is accomplished as follows:

Contraction of the ventricles fills the arteries overfull, causing them to
swell out and make room for the excess of blood. Then while the ventricles
are resting and filling, the stretched arteries press upon the blood to
keep it flowing into the capillaries. In this way _they cause the
intermittent flow from, the heart to become a steady stream in the
capillaries_.

The swelling of the arteries at each contraction of the ventricle is
easily felt at certain places in the body, such as the wrist. This
expansion, known as the "pulse," is the chief means employed by the
physician in determining the force and rapidity of the heart's action.

*Purpose of the Valves in the Veins.*--The valves in the veins are not used
for directing the _general_ flow of the blood, the valves of the heart
being sufficient for this purpose. Their presence is necessary because of
the pressure to which the veins are subjected in different parts of the
body. The contraction of a muscle will, for example, close the small veins
in its vicinity and diminish the capacity of the larger ones. The natural
tendency of such pressure is to empty the veins in two directions--one in
the same direction as the regular movement of the blood, but the other in
the opposite direction. The valves by closing cause the contracting muscle
to push the blood in one direction only--toward the heart. The valves in
the veins are, therefore, an economical device for _enabling variable
pressure_ in different parts of the body _to assist in the circulation_.
Veins like the inferior vena cava and the veins of the brain, which are
not compressed by movements of the body, do not have valves.

*Purposes of the Muscular Coat.*--The muscular coat, which is thicker in
the arteries than in the veins and is more marked in small arteries than
in large ones, serves two important purposes. In the first place it,
together with the elastic tissue, keeps the capacity of the blood vessels
_equal to the volume of the blood_. Since the blood vessels are capable of
holding more blood than may be present at a given time in the body, there
is a liability of empty spaces occurring in these tubes. Such spaces would
seriously interfere with the circulation, since the heart pressure could
not then reach all parts of the blood stream. This is prevented by the
contracted state, or "tone," of the blood vessels, due to the muscular
coat.

In the second place, the muscular coat serves the purpose of _regulating_
the amount of blood which any given organ or part of the body receives.
This it does by varying the caliber of the arteries going to the organ in
question. To increase the blood supply, the muscular coat relaxes. The
arteries are then dilated by the blood pressure from within so as to let
through a larger quantity of blood. To diminish the supply, the muscle
contracts, making the caliber of the arteries less, so that less blood can
flow to this part of the body. Since the need of organs for blood varies
with their activity, the muscular coat serves in this way a very necessary
purpose.

                                [Fig. 21]

Fig. 21--Diagram of network of capillaries between a very small artery and
 a very small vein. Shading indicates the change of color of the blood as
    it passes through the capillaries. _S._ Places between capillaries
                          occupied by the cells.

*Capillaries.*--The capillaries consist of a network of minute blood
vessels which connect the terminations of the smallest arteries with the
beginnings of the smallest veins (Fig. 21). They have an average diameter
of less than one two-thousandth of an inch (12  mu) and an average length of
less than one twenty-fifth of an inch (1 millimeter). Their walls consist
of a single coat which is continuous with the lining of the arteries and
veins. This coat is formed of a single layer of thin, flat cells placed
edge to edge (Fig. 22). With a few exceptions, the capillaries are found
in great abundance in all parts of the body.

                                [Fig. 22]

Fig. 22--*Surface of capillary* highly magnified, showing its coat of thin
                        cells placed edge to edge.

*Functions of the Capillaries.*--On account of the thinness of their walls,
the capillaries are able to serve a twofold purpose in the body:

1. They admit materials into the blood vessels.

2. They allow materials to pass from the blood vessels to the surrounding
tissues.

When it is remembered that the blood, as blood, does not escape from the
blood vessels under normal conditions, the importance of the work of the
capillaries is apparent. To serve its purpose as a carrier, there must be
places where the blood can load up with the materials which it is to
carry, and places also where these can be unloaded. Such places are
supplied by the capillaries.

The capillaries also serve the purpose of spreading the blood out and of
bringing it very near the individual cells in all parts of the body (Fig.
21).

*Functions of Arteries and Veins.*--While the capillaries provide the means
whereby materials may both enter and leave the blood, the arteries and
veins serve the general purpose of passing the blood from one set of
capillaries to another. Since pressure is necessary for moving the blood,
these tubes must connect with the source of the pressure, which is the
heart. In the arteries and veins the blood neither receives nor gives up
material, but having received or given up material at one set of
capillaries, it is then pushed through these tubes to where it can serve a
similar purpose in another set of capillaries (Fig. 23).

*Divisions of the Circulation.*--Man, in common with all warm-blooded
animals, has a double circulation, a fact which explains the double
structure of his heart. The two divisions are known as the _pulmonary_ and
the _systemic_ circulations. By the former the blood passes from the right
ventricle through the lungs, and is then returned to the left auricle; by
the latter it passes from the left ventricle through all parts of the
body, returning to the right auricle.

The general plan of the circulation is indicated in Fig. 23. All the blood
flows continuously through both circulations and passes the various parts
in the following order: right auricle, tricuspid valve, right ventricle,
right semilunar valve, pulmonary artery and its branches, capillaries of
the lungs, pulmonary veins, left auricle, mitral valve, left ventricle,
left semilunar valve, aorta and its branches, systemic capillaries, the
smaller veins, superior and inferior venae cavae, and then again into the
right auricle.

In the pulmonary capillaries the blood gives up carbon dioxide and
receives oxygen, changing from a dark red to a bright red color. In the
systemic capillaries it gives up oxygen, receives carbon dioxide and other
impurities, and changes back to a dark red color.

In addition to the two main divisions of the circulation, special circuits
are found in various places. Such a circuit in the liver is called the
_portal_ circulation, and another in the kidneys is termed the _renal_
circulation. To some extent the blood supply to the walls of the heart is
also outside of the general movement; it is called the _coronary_
circulation.

                                [Fig. 23]

  Fig. 23--*General scheme of the circulation*, showing places where the
 blood takes on and gives off materials. 1. Body in general. 2. Lungs. 3.
 Kidneys. 4. Liver. 5. Organs of digestion. 6. Lymph ducts. 7. Pulmonary
                            artery. 8. Aorta.

*Blood Pressure and Velocity.*--The blood, in obedience to physical laws,
passes continuously through the blood vessels, moving always from a place
of greater to one of less pressure. Through the contraction of the
ventricles, a relatively high pressure is maintained in the arteries
nearest the heart.(21) This pressure diminishes rapidly in the small
arteries, becomes comparatively slight in the capillaries, and falls
practically to nothing in the veins. Near the heart in the superior and
inferior venae cavae, the pressure at intervals is said to be _negative_.
This means that the blood from these veins is actually drawn into the
right auricle by the expansion of the chest walls in breathing.(22)

The velocity of the blood is greatest in the arteries, less in the veins,
and _much_ less in the capillaries than in either the arteries or the
veins. The slower flow of the blood through the capillaries is accounted
for by the fact that their united area is many times greater than that of
the arteries which supply, or the veins which relieve, them. This allows
the same quantity of blood, flowing through them in a given time, a wider
channel and causes it to move more slowly. The time required for a
complete circulation is less than one minute.

*Summary of Causes of Circulation.*--The chief factor in the circulation of
the blood is, of course, the heart. The ventricles keep a pressure on the
blood which is sufficient to force it through all the blood tubes and back
to the auricles. The heart is aided in its work by the elasticity of the
arteries, which keeps the blood under pressure while the ventricles are in
a state of relaxation. It is also aided by the muscles and elastic tissue
in all of the blood vessels. These, by keeping the blood vessels in a
state of "tone," or so contracted that their capacity just equals the
volume of the blood, enable pressure from the heart to be transmitted to
all parts of the blood stream. A further aid to the circulation is found
in the valves in the veins, which enable muscular contraction within the
body, and variable pressure upon its surface, to drive the blood toward
the heart. The heart is also aided to some extent by the movements of the
chest walls in breathing. The organs Of circulation are under the control
of the nervous system (Chapter XVIII).

HYGIENE OF THE CIRCULATION

*Care of the Heart.*--The heart, consisting largely of muscle, is subject
to the laws of muscular exercise. It may be injured by over-exertion, but
is strengthened by a moderate increase in its usual work.(23) It may even
be subjected to great exertion without danger, if it be trained by
gradually increasing its work. Such training, by giving the heart time to
gain in size and strength, prepares it for tasks that could not at first
be accomplished.

In taking up a new exercise requiring considerable exertion, precautions
should be observed to prevent an overstrain of the heart. The heart of the
amateur athlete, bicyclist, or mountain climber is frequently injured by
attempting more than the previous training warrants. The new work should
be taken up gradually, and feats requiring a large outlay of physical
energy should be attempted only after long periods of training.

Since the heart is controlled by the nervous system, it frequently becomes
irregular in its action through conditions that exhaust the nervous
energy. Palpitations of the heart, the missing of beats, and pains in the
heart region frequently arise from this cause. It is through their effect
upon the nervous system that worry, overstudy, undue excitement, and
dissipation cause disturbances of the heart. In all such cases the remedy
lies in the removal of the cause. The nervous system should also be "toned
up" through rest, plenty of sleep, and moderate exercise in the open air.

*Effect of Drugs.*--A number of substances classed as drugs, mainly by
their action on the nervous system, produce undesirable effects upon the
organs of circulation. Unfortunately some of these are extensively used,
alcohol being one of them. If taken in any but small quantities, alcohol
is a disturbing factor in the circulation. It increases the rate of the
heart beat and dilates the capillaries. Its effect upon the capillaries is
shown by the "bloodshot" eye and the "red nose" of the hard drinker.
Another bad effect from the use of much alcohol is the weakening of the
heart through the accumulation of fat around this organ and within the
heart muscle. The use of alcohol also leads in many cases to a hardening
of the walls of the arteries, such as occurs in old age. This effect makes
the use of alcohol especially dangerous for those in advanced years.

Tobacco contains a drug, called nicotine, which has a bad effect upon the
heart in at least two ways: 1. When the use of tobacco is begun in early
life, it interferes with the growth of the heart, leading to its weakness
in the adult. 2. When used in considerable quantity, by young or old, it
causes a nervous condition both distressing and dangerous, known as
"tobacco heart."

Tea and coffee contain a drug, called caffeine, which acts upon the
nervous system and which may, on this account, interfere with the proper
control of the heart. In some individuals the taking of a very small
amount of either tea or coffee is sufficient to cause irregularities in
the action of the heart. Tea is considered the milder of the two liquids
and the one less liable to injure.

*Effect of Rheumatism.*--The disease which affects the heart more
frequently than any other is rheumatism. This attacks the lining membrane,
or endocardium, and causes, not infrequently, a shrinkage of the heart
valves. The heart is thus rendered defective and, to perform its function
in the body, must work harder than if it were in a normal condition.
Rheumatic attacks of the heart do most harm when they occur in early
life--the period when the valves are the most easily affected. Any tendency
toward rheumatism in children has, therefore, a serious significance and
should receive the attention of the physician. Any one having a defective
heart should avoid all forms of exercise that demand great exertion.

*Strengthening of the Blood Vessels.*--Disturbances of the circulation,
causing too much blood to be sent to certain parts of the body and an
insufficient amount to others, when resulting from slight causes, are
usually due to weakness of the walls of the blood vessels, particularly of
the muscular coat. Such weakness is frequently indicated by extreme
sensitiveness to heat or cold and by a tendency to "catch cold." From a
health standpoint the preservation of the normal muscular "tone" of the
blood vessels is a problem of great importance. Though the muscles of the
blood vessels cannot be exercised in the same manner as the voluntary
muscles, they may be called actively into play through all the conditions
that induce changes in the blood supply to different parts of the body.
The usual forms of physical exercise necessitate such changes and
indirectly exercise the muscular coat. The exposure of the body to cold
for short intervals, because of the changes in the circulation which this
induces, also serves the same purpose. A cold bath taken with proper
precautions is beneficial to the circulation of many and so also is a
brisk walk on a frosty morning. Both indirectly exercise and strengthen
the muscular coat of the blood vessels. On the other hand, too much time
spent indoors, especially in overheated rooms, leads to a weakening of the
muscular coat and should be avoided.

*Checking of Flow of Blood from Wounds.*--The loss of any considerable
quantity of blood is such a serious matter that every one should know the
simpler methods of checking its flow from wounds. In small wounds the flow
is easily checked by binding cotton or linen fiber over the place. The
absorbent cotton, sold in small packages at drug stores, is excellent for
this purpose and should be kept in every home. A simple method of checking
"nosebleed" is that of drawing air through the bleeding nostril, while the
other nostril is compressed with the finger.(24) Another method is to
"press with the finger (or insert a small roll of paper) under the lip
against the base of the nose." (25) Where the bleeding is persistent, the
nostril should be plugged with a small roll of clean cotton or paper. When
this is done, the plug should not be removed too soon because of the
likelihood of starting the flow afresh.

In dealing with large wounds the services of a physician are
indispensable. But in waiting for the physician to arrive temporary aid
must be rendered. The one who gives such aid should first decide whether
an artery or a vein has been injured. This is easily determined by the
nature of the blood stream, which is in jets, or spurts, from an artery,
but flows steadily from a vein. If an artery is injured, the limb should
be tightly bandaged on the side of the wound nearest the heart; if a vein,
on the side farthest from the heart. In addition to this, the edges of the
wound should be closed and covered with cotton fiber and the limb should
be placed on a support above the level of the rest of the body. A large
handkerchief makes a convenient bandage if properly applied. This should
be folded diagonally and a knot tied in the middle. Opposite ends are then
tied, making a loose-fitting loop around the limb. The knot is placed
directly over the blood vessel to be compressed and a short stick inserted
in the loop. The necessary pressure is then applied by twisting the
handkerchief with the stick. Time must not be lost, however, in the
preparation of a suitable bandage. The blood vessel should be compressed
with the fingers while the bandage is being prepared.

*Summary.*--The blood, to serve as a transporting agent, must be kept
continually moving through all parts of the body. The blood vessels hold
the blood, supply the channels and force necessary for its circulation,
and provide conditions which enable materials both to enter and to leave
the blood stream. The heart is the chief factor in propelling the blood,
although the muscles and the elastic tissue in the walls of the arteries
and the valves in the veins are necessary aids in the process. In the
capillaries the blood takes on and gives off materials, while the arteries
and veins serve chiefly as tubes for conveying the blood from one system
of capillaries to another.

*Exercises.*--1. Of what special value in the study of the body was the
discovery of the circulation of the blood?

2. State the necessity for a circulating liquid in the body.

3. Show by a drawing the general plan of the heart, locating and naming
the essential parts. Show also the connection of the large blood vessels
with the cavities of the heart.

4. Compare the purpose served by the chordae tendineae to that served by
doorstops (the strips against which the door strikes in closing).

5. Explain how the heart propels the blood. To what class of pumps does it
belong? What special work is performed by each of its divisions?

6. Define a valve. Of what use are the valves in the heart? In the veins?

7. By what means is pressure from contracting muscles in different parts
of the body made to assist in the circulation?

8. Of what advantage is the elasticity of the arteries?

9. How is blood forced from the capillaries back to the heart?

10. Why should there be a difference in structure between the two sides of
the heart?

11. Following Fig. 23, trace the blood through a complete circulation,
naming all the divisions of the system in the order of the flow of the
blood.

12. If the period of rest following the period of contraction of the heart
be as long as the period of contraction, how many hours is the heart able
to rest out of every twenty-four?

13. State the functions of the capillaries. Show how their structure
adapts them to their work.

14. What kind of physical exercise tends to strengthen the heart? What
forms of exercise tend to injure it? State the effects of alcohol and
tobacco on the heart.

15. How may rheumatism injure the heart?

16. Give directions for checking the flow of blood from small and from
large blood vessels.

PRACTICAL WORK

In showing the relations of the different parts of the heart, a large
dissectible model is of great service (Fig. 24). Indeed, where the time of
the class is limited, the practical work may be confined to the study of
the heart model, diagrams of the heart and the circulation, and a few
simple experiments. However, where the course is more extended, the
dissection of the heart of some animal as described below is strongly
advised.

*Observations on the Heart.*--Procure, by the assistance of a butcher, the
heart of a sheep, calf, or hog. To insure the specimen against mutilation,
the lungs and the diaphragm must be left attached to the heart. In
studying the different parts, good results will be obtained by observing
the following order:

1. Observe the connection of the heart to the lungs, diaphragm, and large
blood vessels. Inflate the lungs and observe the position of the heart
with reference to them.

2. Examine the sac surrounding the heart, called the _pericardium_. Pierce
its lower portion and collect the pericardial fluid. Increase the opening
thus made until it is large enough to slip the heart out through it. Then
slide back the pericardium until its connection with the large blood
vessels above the heart is found. Observe that a thin layer of it
continues down from this attachment, forming the outer covering of the
heart.

3. Trace out for a short distance and study the veins and arteries
connected with the heart. The arteries are to be distinguished by their
thick walls. The heart may now be severed from the lungs by cutting the
large blood vessels, care being taken to leave a considerable length of
each one attached to the heart.

                                [Fig. 24]

                Fig. 24--Model for demonstrating the heart.

4. Observe the outside of the heart. The thick, lower portion contains the
cavities called _ventricles_; the thin, upper, ear-shaped portions are the
_auricles_. The thicker and denser side lies toward the left of the
animal's body and is called the _left_ side of the heart; the other is the
_right_ side. Locate the right auricle and the right ventricle; the left
auricle and the left ventricle.

5. Lay the heart on the table with the front side up and the apex pointing
from the operator. This places the left side of the heart to his left and
the right side to his right. Notice the groove between the ventricles,
called the inter-ventricular groove. Make an incision half an inch to the
right of this groove and cut toward the base of the heart until the
pulmonary artery is laid open. Then, following within half an inch of the
groove, cut down and around the right side of the heart. The wall of the
right ventricle may now be raised and the cavity exposed. Observe the
extent of the cavity, its shape, its lining, its columns of muscles, its
half columns of muscles, its tendons (chordae tendineae), the tricuspid
valve from the under side, etc. Also notice the valve at the beginning of
the pulmonary artery (the right semilunar) and the sinuses, or
depressions, in the artery immediately behind its divisions.

6. Now cut through the middle of the loosened ventricular wall from the
apex to the middle of the right auricle, laying it open for observation.
Observe the openings into the auricle, there being one each for the vena
cava superior, the vena cava inferior, and the coronary vein. Compare the
walls, lining, shape, size, etc., with the ventricle below.

7. Cut off the end of the left ventricle about an inch above the apex.
This will show the extension of the cavity to the apex; it will also show
the thickness of the walls and the shape of the cavity. Split up the
ventricular wall far enough to examine the mitral valve and the chordae
tendineae from the lower side.

8. Make an incision in the left auricle. Examine its inner surface and
find the places of entrance of the pulmonary veins. Examine the mitral
valve from above. Compare the two sides of the heart, part for part.

9. Separate the aorta from the other blood vessels and cut it entirely
free from the heart, care being taken to leave enough of the heart
attached to the artery to insure the semilunar valve's being left in good
condition. After tying or plugging up the holes in the sides of the
artery, pour water into the small end and observe the closing of the
semilunar valve. Repeat the experiment until the action of the valve is
understood. Sketch the artery, showing the valve in a closed condition.

*To illustrate the Action of a Ventricle.*--Procure a syringe bulb with an
opening at each end. Connect a rubber tube with each opening, letting the
tubes reach into two tumblers containing water. By alternately compressing
and releasing the bulb, water is pumped from one vessel into the other.
The bulb may be taken to represent one of the ventricles. What action of
the ventricle is represented by compressing the bulb? By releasing the
pressure? Show by a sectional drawing the arrangement of the valves in the
syringe bulb.

                                [Fig. 25]

               Fig. 25--Illustrating elasticity of arteries.

*To show the Advantage of the Elasticity of Arteries.*--Connect the syringe
bulb used in the last experiment with a rubber tube three or four feet in
length and having rather thin walls. In the opposite end of the rubber
tube insert a short glass tube which has been drawn (by heating) to a fine
point (Fig. 25). Pump water into the rubber tube, observing:

1. The swelling of the tube (pulse) as the water is forced into it. (This
is best observed by placing the fingers on the tube.)

2. The forcing of water from the pointed tubs during the interval when no
pressure is being applied from the bulb. Compare with the action of the
arteries when blood is forced into them from the ventricles.

Repeat the experiment, using a long glass tube terminating in a point
instead of the rubber tube. (In fitting the glass tube to the bulb use a
very short rubber tube.) Observe and account for the differences in the
flow of water through the inelastic tube.

*To show the Advantage of Valves in the Veins.*--Attach an open glass tube
one foot in length to each end of the rubber tube used in the preceding
experiment and fill with water (by sucking) to within about six inches of
the end. Lay on the table with the glass tubes secured in an upright
position (Fig. 26). Now compress the tube with the hand, noting that the
water rises in both tubes, being pushed in both directions. This effect is
similar to that produced on the blood when a vein having no valves is
compressed.

                                [Fig. 26]

  Fig. 26.--*Simple apparatus* for showing advantage of valves in veins.

Now imitate the action of a valve by clamping the tube at one point, or by
closing it by pressure from the finger, and then compressing with the hand
some portion of the tube on the table. Observe in this instance that the
water is *all* pushed in the same direction. The movement of the water is
now like the effect produced on the blood in veins having valves when the
veins are compressed.

*To show the Position of the Valves in the Veins.*--Exercise the arm and
hand for a moment to increase the blood supply. Expose the forearm and
examine the veins on its surface. With a finger, stroke one of the veins
toward the heart, noting that, as the blood is pushed along on one side of
the finger the blood follows on the other side. Now stroke the vein toward
the hand. Places are found beyond which the blood does not follow the
finger. These mark the positions of valves.

*To show Effect of Exercise upon the Circulation.*--1. With a finger on the
"pulse" at the wrist or temple, count the number of heart beats during a
period of one minute under the following conditions: (_a_) when sitting;
(_b_) when standing; (_c_) after active exercise, as running. What
relation, if any, do these observations indicate between the general
activity of the body and the work of the heart?

2. Compare the size of the veins on the backs of the hands when they are
placed side by side on a table. Then exercise briskly the right hand and
arm, clenching and unclenching the fist and flexing the arm at the elbow.
Place the hands again side by side and, after waiting a minute, observe
the increase in the size of the veins in the hand exercised. How is this
accounted for?

*To Show the Effect of Gravity on the Circulation.*--Hold one hand high
above the head, at the same time letting the other hand hang loosely by
the side. Observe the difference in the color of the hands and the degree
to which the large veins are filled. Repeat the experiment, reversing the
position of the hands. What results are observed? In what parts of the
body does gravity aid in the return of the blood to the heart? In what
parts does it hinder? Where fainting is caused by lack of blood in the
brain (the usual cause), is it better to let the patient lie down flat or
to force him into a sitting posture?

*To study the Circulation in a Frog's Foot* (Optional).--A compound
microscope is needed in this study and for extended examination it is best
to destroy the frog's brain. This is done by inserting some blunt-pointed
instrument into the skull cavity from the neck and moving it about. A
small frog, on account of the thinness of its webs, gives the best
results. It should be attached to a thin board which has an opening in one
end over which the web of the foot may be stretched. Threads should extend
from two of the toes to pins driven into the board to secure the necessary
tension of the web, and the foot and lower leg should be kept moist. Using
a two-thirds-inch objective, observe the branching of the small arteries
into the capillaries and the union of the capillaries to form the small
veins. The appearance is truly wonderful, but allowance must be made for
the fact that the _motion_ of the blood is magnified, as well as the
different structures, and that it appears to move much faster than it
really does. With a still higher power, the movements of the corpuscles
through the capillaries may be studied.

NOTE.--To perform this experiment without destroying the brain, the frog is
first carefully wrapped with strips of wet cloth and securely tied to the
board. The wrapping, while preventing movements of the frog, must not
interfere with the circulation.

CHAPTER VI - THE LYMPH AND ITS MOVEMENT THROUGH THE BODY

                                [Fig. 27]

  Fig. 27--*Diagram showing position of the lymph* with reference to the
blood and the cells. The central tube is a capillary. The arrows indicate
             the direction of slight movements in the lymph.

The blood, it will be remembered, moves everywhere through the body in a
system of _closed_ tubes. These keep it from coming in contact with any of
the cells of the body except those lining the tubes themselves. The
capillaries, to be sure, bring the blood very near the cells of the
different tissues; still, there is need of a liquid to fill the space
between the capillaries and the cells and to transfer materials from one
to the other. The lymph occupies this position and does this work. The
position of the lymph with reference to the capillaries and the cells is
shown in Fig. 27.

*Origin of the Lymph.*--The chief source of the lymph is the plasma of the
blood. As before described, the walls of the capillaries consist of a
single layer of flat cells placed edge to edge. Partly on account of the
pressure upon the blood and partly on account of the natural tendency of
liquids to pass through animal membranes, a considerable portion of the
plasma penetrates the thin walls and enters the spaces occupied by the
lymph.

The cells themselves also help to form the lymph, since the water and
wastes leaving the cells add to its bulk. These mix with the plasma from
the blood, forming the resultant liquid which is the lymph. A considerable
amount of the material absorbed from the food canal also enters the lymph
tubes, but this passes into the blood before reaching the cells.

*Composition and Physical Properties of the Lymph.*(26)--As would naturally
be expected, the composition of the lymph is similar to that of the blood.
In fact, nearly all the important constituents of the blood are found in
the lymph, but in different proportions. Food materials for the cells are
present in smaller amounts than in the blood, while impurities from the
cells are in larger amounts. As a rule the red corpuscles are absent from
the lymph, but the white corpuscles are present and in about the same
numbers as in the blood.

The physical properties of the lymph are also similar to those of the
blood. Like the blood, the lymph is denser than water and also coagulates,
but it coagulates more slowly than does the blood. The most noticeable
difference between these liquids is that of color, the lymph being
colorless. This is due to the absence of red corpuscles. The quantity of
lymph is estimated to be considerably greater than that of the blood.

*Lymph Vessels.*--Most of the lymph lies in minute cavities surrounding the
cells and in close relations with the capillaries (Figs. 27 and 30). These
are called _lymph spaces_. Connecting with the lymph spaces on the one
hand, and with certain blood vessels on the other, is a system of tubes
that return the lymph to the blood stream. The smallest of these, and the
ones in greatest abundance, are called _lymphatics_. They consist of
slender, thin-walled tubes, which resemble veins in structure, and, like
the veins, have valves. They differ from veins, however, in being more
uniform in size and in having thinner walls.

                                [Fig. 28]

 Fig. 28--*Diagram of drainage system for the lymph.* 1. Thoracic duct. 2.
 Right lymphatic duct. 3. Left subclavian vein. 4. Right subclavian vein.
 5. Superior vena cava. 6. Lacteals. 7. Lymphatic glands. The small tubes
    connecting with the lymph spaces in all parts of the body are the
                               lymphatics.

The lymphatics in different places gradually converge toward, and empty
into, the two main lymph tubes of the body. The smaller of these tubes,
called the _right lymphatic duct_, receives the lymph from the lymphatics
in the right arm, the right side of the head, and the region of the right
shoulder. It connects with, and empties its contents into, the right
subclavian vein at the place where it is joined by the right jugular vein
(Fig. 28).

The larger of the lymph tubes is called the _thoracic duct_. This receives
lymph from all parts of the body not drained by the right lymphatic duct,
and empties it into the left subclavian vein. Connection is made with the
subclavian vein on the upper side at the place where it is joined by the
left jugular vein. The thoracic duct has a length of from sixteen to
eighteen inches, and is about as large around as a goose quill. The lower
end terminates in an enlargement in the abdominal cavity, called the
_receptacle of the chyle_. It is provided with valves throughout its
course, in addition to one of considerable size which guards the opening
into the blood vessel.

The lymphatics which join the thoracic duct from the small intestine are
called the _lacteals_ (Fig. 28). These do not differ in structure from the
lymphatics in other parts of the body, but they perform a special work in
absorbing the digested fat (Chapter XI).

*Lymphatic Glands.*--The lymphatic glands, sometimes called lymph nodes,
are small and somewhat rounded bodies situated along the course of the
lymphatic tubes. They vary in size, some of them being an inch or more in
length. The lymph vessels generally open into them on one side and leave
them on the other (Figs. 28 and 30). They are not glands in function, but
are so called because of their having the general form of glands. They
provide favorable conditions for the development of white corpuscles (page
29). They also separate harmful germs and poisonous wastes from the lymph,
thereby preventing their entrance into the blood.

*Relations of the Lymph, the Blood, and the Cells.*--While the blood is
necessary as a carrying, or transporting, agent in the body, the lymph is
necessary for transferring materials from the blood to the cells and _vice
versa_. Serving as a physiological "go between," or medium of exchange,
the lymph enables the blood to minister to the needs of the cells. But the
lymph and the blood, everything considered, can hardly be looked upon as
two separate and distinct liquids. Not only do they supplement each other
in their work and possess striking similarities, but each is made in its
movements to pass into the vessels occupied by the other, so that they are
constantly mixing and mingling. For these and other reasons, they are more
properly regarded as two divisions of a single liquid--one which, by
adapting itself to different purposes,(27) supplies all the conditions of
a nutrient fluid for the cells.

*Movements of the Lymph.*--As compared with the blood, the lymph must be
classed as a quiet liquid. But, as already suggested, it has certain
movements which are necessary to the purposes which it serves. A careful
study shows it to have three well-defined movements as follows:

1. A movement from the capillaries toward the cells.

2. A movement from the cells toward the capillaries.

3. A movement of the entire body of lymph from the lymph spaces into the
lymphatics and along these channels to the ducts through which it enters
the blood.

By the first movement the cells receive their nourishment. By the second
and third movements the lymph, more or less laden with impurities, is
returned to the blood stream. (See Figs. 28 and 30.)

*Causes of the Lymph Movements.*--Let us consider first the movement
through the lymph tubes. No pump, like the heart, is known to be connected
with these tubes and to supply the pressure necessary for moving the
lymph. There are, however, several forces that indirectly aid in its flow.
The most important of these are as follows:

1. _Blood Pressure at the Capillaries._--The plasma which is forced through
the capillary walls by pressure from the heart makes room for itself by
pushing a portion of the lymph out of the lymph spaces. This in turn
presses upon the lymph in the tubes which it enters. In this way pressure
from the heart is transmitted to the lymph, forcing it to move.

2. _Variable Pressure on the Walls of the Lymph Vessels._--Pressure exerted
on the sides of the lymph tubes by contracting muscles tends to close them
up and to push the lymph past the valves, which, by closing, prevent its
return (Fig. 29). Pressure at the surface of the body, provided that it is
variable, also forces the lymph along. The valves in the lymph vessels
serve the same purpose as those in the veins.

                                [Fig. 29]

Fig. 29--*Diagram* to show how the muscles pump lymph. _A._ Relaxed muscle
      beside which is a lymphatic tube. _B._ Same muscle in state of
                               contraction.

3. _The Inspiratory Force._--When the thoracic cavity is enlarged in
breathing, the unbalanced atmospheric pressure is exerted from all
directions towards the thoracic space. This not only causes the air to
flow into the lungs (Chapter VII), but also causes a movement of the blood
and lymph in such of their tubes as enter this cavity. It will be noted
that both of the large lymph ducts terminate where their contents may be
influenced by the respiratory movements. (See Practical Work.)

*Where the Lymph enters the Blood.*--The fact that the lymph is poured into
the blood at but two places, and these very close to each other, requires
a word of explanation. As a matter of fact, it is impossible for the lymph
to flow into blood vessels at most places on account of the blood
pressure. This would force the blood into the lymph vessels, instead of
allowing the lymph to enter the blood. The lymph can enter only at some
place where the blood pressure is less than the pressure that moves the
lymph. Such a place is found in the thoracic cavity. As already pointed
out (page 54), the blood pressure in the veins entering this cavity
becomes, with each expansion of the chest, negative, i.e., less than the
pressure of the atmosphere on the outside of the body. This, as we have
seen, aids in the flow of the blood into the right auricle. It also aids
in the passage of lymph into the blood vessels. The lymph is said to be
"sucked in," which means that it is forced in by the unbalanced pressure
of the atmosphere.(28) Some advantage is also gained by the lymph duct's
entering the subclavian vein on the upper side and at its union with the
jugular vein. Everything considered, it is found that the lymph flows into
the blood vessels where it can be "drawn in" by the movements of breathing
and where it meets with no opposition from the blood stream itself (Fig.
30).

                                [Fig. 30]

  Fig. 30--*Diagram* showing general movement of lymph from the place of
 relatively high pressure at the lymph spaces to the place of relatively
                   low pressure in the thoracic cavity.

*Lymph Movements at the Cells.*--The double movement of the lymph from the
capillaries toward the cells and from the cells toward the capillaries is
not entirely understood. Blood pressure in the capillaries undoubtedly has
much to do in forcing the plasma through the capillary walls, but this
tends to prevent the movement of the lymph in the opposite direction.
Movements between the blood and the lymph are known to take place in part
according to a general principle, known as _osmosis_, or dialysis.

                                [Fig. 31]

  Fig. 31--*Vessel* with an upright membranous partition for illustrating
                                 osmosis.

*Osmosis.*--The term "osmosis" is used to designate the passage of liquids
through some partition which separates them. Thus, if a vessel with an
upright membranous partition be filled on the one side with pure water and
on the other with water containing salt, an exchange of materials will
take place through the membrane until the same proportion of salt exists
on the two sides (Fig. 31). The cause of osmosis is the motion of the
molecules, or minute particles, that make up the liquid substance. If the
partition were not present, this motion would simply cause a mixing of the
liquids.

*Conditions under which Osmosis occurs.*--Osmosis may be shown by suitable
experiments (see Practical Work) to take place under the following
conditions:

1. The liquids on the two sides of the partition must be _unlike_ either
in density or in composition. Since the effect of the movement is to
reduce the liquids to the same condition, _a difference in density causes
the flow to be greater from the less dense toward the denser liquid_, than
in the opposite direction; while _a difference in composition causes the
substances in solution to move from the place of greater abundance toward
places of less abundance_.

2. The liquids must be capable of wetting, or penetrating, the partition.
If but one of the liquids penetrates the partition, the flow will be in
but one direction.

3. The liquids on the two sides of the partition must readily mix with
each other.

*Osmosis at the Cells.*--In the body osmosis takes place between the blood
and the lymph and between the lymph and the cells, the movements being
through the capillary walls and the membranes inclosing the cells (Fig.
27). Oxygen and food materials, which are found in great abundance in the
blood, are less abundant in the lymph and still less abundant in the
cells. According to the principle of osmosis, the main flow of oxygen and
food is from the capillaries toward the cells. On the other hand, the
wastes are most abundant in the cells where they are formed, less abundant
in the lymph, and least abundant in the blood. Hence the wastes flow from
the cells toward the capillaries.

*Solutions.*--Neither the blood plasma nor the lymph, as already shown, are
simple liquids; but they consist of water and different substances
dissolved in the water. They belong to a class of substances called
_solutions_. The chief point of interest about substances in solution is
that they are very finely divided and that their little particles are free
to move about in the liquid that contains them. Both the motion and the
finely divided condition of the dissolved substances are necessary to the
process of osmosis. All substances, however, that appear to be in solution
are not able to penetrate membranes, or take part in osmosis.

*Kinds of Solutions in the Body.*--The substances in solution in the body
liquids are of two general kinds known as _colloids_ and _crystalloids_.
The crystalloids are able to pass through membranous partitions, while the
colloids are not. An example of a colloid is found in the albumin of an
egg, which is unable to penetrate the membrane which surrounds it.
Examples of crystalloids are found in solutions of salt and sugar in
water. The inability of a colloid to penetrate a membrane is due to the
fact that it does not form a true solution. Its particles (molecules),
instead of being completely separated, still cling together, forming
little masses that are too large to penetrate the membrane. Since,
however, it has the appearance, on being mixed with water, of being
dissolved, it is called a _colloidal solution_. The crystalloid substance,
on the other hand, completely separates in the water and forms a _true
solution_--one which is able to penetrate the partition or membrane.

*Osmosis not a Sufficient Cause.*--The passage of materials through animal
membranes, according to the principle of osmosis, is limited to
crystalloid substances. But colloid substances are also known to pass
through the various partitions of the body. An example of such is found in
the proteids of the blood which, as a colloidal solution, pass through the
capillary walls to become a part of the lymph. Perhaps the best
explanation offered as yet for this passage is that the colloidal
substances are changed by the cells lining the capillaries into substances
that form true solutions and that after the passage they are changed back
again to the colloidal condition.

*Summary.*--Between the cells and the capillaries is a liquid, known as the
lymph, which is similar in composition and physical properties to the
blood. It consists chiefly of escaped plasma. The vessels that contain it
are connected with the system for the circulation of the blood. By adding
new material to the lymph and withdrawing waste material from it, the
blood keeps this liquid in a suitable condition for supplying the needs of
the cells. Supplementing each other in all respects, the blood and the
lymph together form the nutrient cell fluid of the body. The interchange
of material between the blood and the lymph, and the lymph and the cells,
takes place in part according to the principle of osmosis.

*Exercises.*--1. Explain the necessity for the lymph in the body.

2. Compare lymph and water with reference to density, color, and
complexity of composition.

3. Compare lymph and blood with reference to color, composition, and
movement through the body.

4. Show how blood pressure in the capillaries causes a flow of the lymph.

5. Show how contracting muscles cause the lymph to move. Compare with the
effect of muscular contraction upon the blood in the veins.

6. Trace the lymph in its flow from the right hand to where it enters the
blood; from the feet to where it enters the blood.

7. What conditions prevail at the cells to cause a movement of food and
oxygen in one direction and of waste materials in the opposite direction?

8. What part does water play in the exchanges at the cells?

9. Show that the blood and the lymph together fulfill all the requirements
of a nutrient cell fluid in the body.

PRACTICAL WORK

*To illustrate the Effect of Breathing upon the Flow of Lymph.*--Tightly
holding one end of a glass tube between the lips, let the other end extend
into water in a tumbler on a table. In this position quickly inhale air
through the nostrils, noting that with each inhalation there is a slight
movement of the water up the tube. (No sucking action should be exerted by
the mouth.) Apply to the movements in the large blood and lymph vessels
entering the thoracic cavity.

*To illustrate Osmosis.*--1. Separate the shell from the lining membrane at
one end of an egg, over an area about one inch in diameter. To do this
without injuring the membrane, the shell must first be broken into small
pieces and then picked off with a pair of forceps, or a small knife blade.
Fit a small glass tube, eight or ten inches long, into the other end so
that it will penetrate the membrane and pass down into the yolk. Securely
fasten the tube to the shell by melting beeswax around it, and set the egg
in a small tumbler partly filled with water. Examine in the course of half
an hour. What evidence now exists that the water has passed through the
membrane?

2. Tie over the large end of a "thistle tube" (used by chemists) a thin
animal membrane, such as a piece of the pericardium or a strip of the
membrane from around a sausage. Then fill the bulb and the lower end of
the tube with a concentrated solution of some solid, such as sugar, salt,
or copper sulphate. Suspend in a vessel of water so that the liquid which
it contains is just on a level with the water in the vessel. Examine from
time to time, looking for evidence of a movement in each direction through
the membrane. Why should the movement of the water into the tube be
greater than the movement in the opposite direction? (If the thistle tube
has a very slender stem, it is better to fill the bulb before tying on the
membrane. The opening in the stem may be plugged during the process of
filling.)

                                [Fig. 32]

                         Fig. 32--An osmosometer.

NOTE.--With a special piece of apparatus, known as an _osmosometer_, the
principle of osmosis may be more easily illustrated than by the method in
either of the above experiments (Fig. 32). This apparatus may be obtained
from supply houses.

CHAPTER VII - RESPIRATION

Through the movements of the blood and the lymph, materials entering the
body are transported to the cells, and wastes formed at the cells are
carried to the organs which remove them from the body. We are now to
consider the passage of materials from outside the body to the cells and
_vice versa_. One substance which the body constantly needs is oxygen, and
one which it is constantly throwing off is carbon dioxide. Both of these
are constituents of

*The Atmosphere.*--The atmosphere, or air, completely surrounds the earth
as a kind of envelope, and comes in contact with everything upon its
surface. It is composed chiefly of oxygen and nitrogen,(29) but it also
contains a small per cent of other substances, such as water-vapor, carbon
dioxide, and argon. All of the regular constituents of the atmosphere are
gases, and these, as compared with liquids and solids, are very light.
Nevertheless the atmosphere has weight and, on this account, exerts
pressure upon everything on the earth. At the sea level, its pressure is
nearly fifteen pounds to the square inch. The atmosphere forms an
essential part of one's physical environment and serves various purposes.
The process by which gaseous materials are made to pass between the body
and the atmosphere is known as

*Respiration.*--As usually defined, respiration, or breathing, consists of
two simple processes--that of taking air into special contrivances in the
body, called the lungs, and that of expelling air from the lungs. The
first process is known as _inspiration_; the second as _expiration_. We
must, however, distinguish between respiration by the lungs, called
_external respiration_, and respiration by the cells, called _internal
respiration_.

_The purpose of respiration_ is indicated by the changes that take place
in the air while it is in the lungs. Air entering the lungs in ordinary
breathing parts with about five per cent of itself in the form of oxygen
and receives about four and one half per cent of carbon dioxide,
considerable water-vapor, and a small amount of other impurities. These
changes suggest a twofold purpose for respiration:

1. To obtain from the atmosphere the supply of oxygen needed by the body.

2. To transfer to the atmosphere certain materials (wastes) which must be
removed from the body.

The chief organs concerned in the work of respiration are

*The Lungs.*--The lungs consist of two sac-like bodies suspended in the
thoracic cavity, and occupying all the space not taken up by the heart.
They are not simple sacs, however, but are separated into numerous
divisions, as follows:

1. The lung on the right side of the thorax, called the right lung, is
made up of three divisions, or _lobes_, and the left lung is made up of
two lobes.

2. The lobes on either side are separated into smaller divisions, called
_lobules_ (Fig. 33). Each lobule receives a distinct division of an air
tube and has in itself the structure of a miniature lung.

                                [Fig. 33]

Fig. 33--*Lungs and air passages* seen from the front. The right lung shows
 the lobes and their divisions, the lobules. The tissue of the left lung
              has been dissected away to show the air tubes.

3. In the lobule the air tube divides into a number of smaller tubes, each
ending in a thin-walled sac, called an _infundibulum_. The interior of the
infundibulum is separated into many small spaces, known as the _alveoli_,
or air cells.

The lungs are remarkable for their lightness and delicacy of
structure.(30) They consist chiefly of the tissues that form their sacs,
air tubes, and blood vessels; the membranes that line their inner and
outer surfaces; and the connective tissue that binds these parts together.
All these tissues are more or less elastic. The relation of the different
parts of the lungs to each other and to the outside atmosphere will be
seen through a study of the

*Air Passages.*--The air passages consist of a system of tubes which form a
continuous passageway between the outside atmosphere and the different
divisions of the lungs. The air passes through them as it enters and
leaves the lungs, a fact which accounts for the name.

                                [Fig. 34]

 Fig. 34--*Model of section through the head*, showing upper air passages
  and other parts. 1. Left nostril. 2. Pharynx. 3. Tongue and cavity of
               mouth. 4. Larynx. 5. Trachea. 6. Esophagus.

The incoming air first enters the _nostrils_. These consist of two narrow
passages lying side by side in the nose, and connecting with the pharynx
behind. The lining of the nostrils, called _mucous membrane_ is quite
thick, and has its surface much extended by reason of being spread over
some thin, scroll-shaped bones that project into the passage. This
membrane is well supplied with blood vessels and secretes a considerable
quantity of liquid. Because of the nature and arrangement of the membrane,
the nostrils are able to _warm_ and _moisten_ the incoming air, and to
_free it from dust particles_, preparing it, in this way, for entrance
into the lungs (Fig. 34).

The nostrils are separated from the mouth by a thin layer of bone, and
back of both the mouth and the nostrils is the pharynx. The _pharynx_ and
the _mouth_ serve as parts of the food canal, as well as air passages, and
are described in connection with the organs of digestion (Chapter X). Air
entering the pharynx, either by the nostrils or by the mouth, passes
through it into the _larynx_. The larynx, being the special organ for the
production of the voice, is described later (Chapter XXI). The entrance
into the larynx is guarded by a movable lid of cartilage, called the
_epiglottis_, which prevents food particles and liquids, on being
swallowed, from passing into the lower air tubes. The relations of the
nostrils, mouth, pharynx, and larynx are shown in Fig. 34.

From the larynx the air enters the _trachea_, or windpipe. This is a
straight and nearly round tube, slightly less than an inch in diameter and
about four and one half inches in length. Its walls contain from sixteen
to twenty C-shaped, cartilaginous rings, one above the other and
encircling the tube. These incomplete rings, with their openings directed
backward, are held in place by thin layers of connective and muscular
tissue. At the lower end the trachea divides into two branches, called the
bronchi, each of which closely resembles it in structure. Each _bronchus_
separates into a number of smaller divisions, called the _bronchial
tubes_, and these in turn divide into still smaller branches, known as the
_lesser bronchial tubes_ (Fig. 33). The lesser bronchial tubes, and the
branches into which they separate, are the smallest of the air tubes. One
of these joins, or expands into, each of the minute lung sacs, or
infundibula. Mucous membrane lines all of the air passages.

*General Condition of the Air Passages.*--One necessary condition for the
movement of the air into and from the lungs is an unobstructed
passageway.(31) The air passages must be kept open and free from
obstructions. They are _kept open_ by special contrivances found in their
walls, which, by supplying a degree of stiffness, cause the tubes to keep
their form. In the trachea, bronchi, and larger bronchial tubes, the
stiffness is supplied by rings of cartilage, while in the smaller tubes
this is replaced by connective and muscular tissue. The walls of the
larynx contain strips and plates of cartilage; while the nostrils and the
pharynx are kept open by their bony surroundings.

                                [Fig. 35]

Fig. 35--*Ciliated epithelial cells.* _A._ Two cells highly magnified. _c._
 Cilia, _n._ Nucleus. _B._ Diagram of a small air tube showing the lining
                                of cilia.

The air passages are _kept clean_ by cells especially adapted to this
purpose, known as the _ciliated epithelial cells_. These are slender,
wedge-shaped cells which have projecting from a free end many small,
hair-like bodies, called _cilia_ (Fig. 35). They line the mucous membrane
in most of the air passages, and are so placed that the cilia project into
the tubes. Here they keep up an inward and outward wave-like movement,
which is quicker and has greater force in the _outward_ direction. By this
means the cilia are able to move small pieces of foreign matter, such as
dust particles and bits of partly dried mucus, called phlegm, to places
where they can be easily expelled from the lungs.(32)

                                [Fig. 36]

Fig. 36--*Terminal air sacs.* The two large sacs are infundibula; the small
                    divisions are alveoli. (Enlarged.)

*The Alveoli.*--The alveoli, or air cells, are the small divisions of the
infundibula (Fig. 36). They are each about one one-hundredth of an inch
(1/4 mm.) in diameter, being formed by the infolding of the infundibular
wall. This wall, which has for its framework a thin layer of elastic
connective tissue, supports a dense network of capillaries (Fig. 37), and
is lined by a single layer of cells placed edge to edge. By this
arrangement the air within the alveoli is brought very near a large
surface of blood, and the exchange of gases between the air and the blood
is made possible. It is at the alveoli that the oxygen passes from the air
into the blood, and the carbon dioxide passes from the blood into the air.
At no place in the lungs, however, do the air and the blood come in direct
contact. Their exchanges must in all cases take place through the
capillary walls and the layer of cells lining the alveoli.

                                [Fig. 37]

Fig. 37--*Inner lung surface (magnified)*, the blood vessels injected with
  coloring matter. The small pits are alveoli, and the vessels in their
                      walls are chiefly capillaries.

                                [Fig. 38]

Fig. 38.--*Diagram to show the double movement of air and blood through the
lungs.* The blood leaves the heart by the pulmonary artery and returns by
   the pulmonary veins. The air enters and leaves the lungs by the same
                             system of tubes.

                                [Fig. 39]

 Fig. 39--*Diagram to show air and blood movements in a terminal air sac.*
  While the air moves into and from the space within the sac, the blood
                    circulates through the sac walls.

*Blood Supply to the Lungs.*--To accomplish the purposes of respiration,
not only the air, but the blood also, must be passed into and from the
lungs. The chief artery conveying blood to the lungs is the _pulmonary
artery_. This starts at the right ventricle and by its branches conveys
blood to the capillaries surrounding the alveoli in all parts of the
lungs. The branches of the pulmonary artery lie alongside of, and divide
similarly to, the bronchial tubes. At the places where the finest
divisions of the air tubes enter the infundibula, the little arteries
branch into the capillaries that penetrate the infundibular walls (Figs.
38 and 39). From these capillaries the blood is conveyed by the pulmonary
veins to the left auricle.

The lungs also receive blood from two (in some individuals three) small
arteries branching from the aorta, known as the _bronchial arteries_.
These convey to the lungs blood that has already been supplied with
oxygen, passing it into the capillaries in the walls of the bronchi,
bronchial tubes, and large blood vessels, as well as the connective tissue
between the lobes of the lungs. This blood leaves the lungs partly by the
bronchial veins and partly by the pulmonary veins. No part of the body is
so well supplied with blood as the lungs.

                                [Fig. 40]

Fig. 40--*The pleurae.* Diagram showing the general form of the pleural sacs
as they surround the lungs and line the inner surfaces of the chest (other
  parts removed). _A, A'._ Places occupied by the lungs. _B, B'._ Slight
 space within the pleural sacs containing the pleural secretion, _a, a'._
   Outer layer of pleura and lining of chest walls and upper surface of
diaphragm. _b, b'._ Inner layer of pleura and outer lining of lungs. _C._
               Space occupied by the heart. _D._ Diaphragm.

*The Pleura.*--The pleura is a thin, smooth, elastic, and tough membrane
which covers the outside of the lungs and lines the inside of the chest
walls. The covering of each lung is continuous with the lining of the
chest wall on its respective side and forms with it a closed sac by which
the lung is surrounded, the arrangement being similar to that of the
pericardium. Properly speaking, there are two pleurae, one for each lung,
and these, besides inclosing the lungs, partition off a middle space which
is occupied by the heart (Fig. 40). They also cover the upper surface of
the diaphragm, from which they deflect upward, blending with the
pericardium. A small amount of liquid is secreted by the pleura, which
prevents friction as the surfaces glide over each other in breathing.

*The Thorax.*--The force required for breathing is supplied by the box-like
portion of the body in which the lungs are placed. This is known as the
thorax, or chest, and includes that part of the trunk between the neck and
the abdomen. The space which it incloses, known as the thoracic cavity, is
a _variable_ space and the walls surrounding this space are _air-tight._ A
framework for the thorax is supplied by the ribs which connect with the
spinal column behind and with the sternum, or breast-bone, in front. They
form joints with the spinal column, but connect with the sternum by strips
of cartilage. The ribs do not encircle the cavity in a horizontal
direction, but <DW72> downward from the spinal column both toward the front
and toward the sides, this being necessary to the service which they
render in breathing.

*How Air is Brought into and Expelled from the Lungs.*--The principle
involved in breathing is that air flows from a place of _greater_ to a
place of _less_ pressure. The construction of the thorax and the
arrangement of the lungs within it provide for the application of this
principle in a most practical manner. The lungs are suspended from the
upper portion of the thoracic cavity, and the trachea and the upper air
passages provide the only opening to the outside atmosphere. Air entering
the thorax must on this account pass into the lungs. As the thorax is
enlarged the air in the lungs expands, and there is produced within them a
place of _slightly less_ air pressure than that of the atmosphere on the
outside of the body. This difference causes the air to flow into the
lungs.

                                [Fig. 41]

 Fig. 41--*Diagram illustrating the bellows principle in breathing.* _A._
     The human bellows. _B._ The hand bellows. Compare part for part.

When the thorax is diminished in size, the air within the lungs is
slightly compressed. This causes it to become denser and to exert on this
account a pressure _slightly greater_ than that of the atmosphere on the
outside. The air now flows out until the equality of the pressure is again
restored. Thus the thorax, by making the pressure within the lungs first
slightly less and then slightly greater than the atmospheric pressure,
causes the air to move into and out of the lungs.

Breathing is well illustrated by means of the common hand bellows, its
action being similar to that of the thorax. It will be observed that when
the sides are spread apart air flows into the bellows. When they are
pressed together the air flows out. If an air-tight sack were hung in the
bellows with its mouth attached to the projecting tube, the arrangement
would resemble closely the general plan of the breathing organs (Fig. 41).
One respect, however, in which the bellows differs from the thorax should
be noted. The thorax is never sufficiently compressed to drive out all the
air. Air is always present in the lungs. This keeps them more or less
distended and pressed against the thoracic walls.

*How the Thoracic Space is Varied.*--One means of varying the size of the
thoracic cavity is through the movements of the ribs and their resultant
effect upon the walls of the thorax. In bringing about these movements the
following muscles are employed:

1. The _scaleni_ muscles, three in number on each side, which connect at
one end with the vertebrae of the neck and at the other with the first and
second ribs. Their contraction slightly raises the upper portion of the
thorax.

2. The _elevators of the ribs_, twelve in number on each side, which are
so distributed that each single muscle is attached, at one end, to the
back portion of a rib and, at the other, to a projection of the vertebra a
few inches above. The effect of their contraction is to' elevate the
middle portion of the ribs and to turn them outward or spread them apart.

3. The _intercostal_ muscles, which form two thin layers between the ribs,
known as the _internal_ and the _external_ intercostal muscles. The
external intercostals are attached between the outer lower margin of the
rib above and the outer upper margin of the rib below, and extend
obliquely downward and forward. The internal intercostals are attached
between the inner margins of adjacent ribs, and they extend obliquely
downward and backward from the front. The contraction of the external
intercostal muscles raises the ribs, and the contraction of the internal
intercostals tends to lower them.

                                [Fig. 42]

  Fig. 42--*Simple apparatus* for illustrating effect of movements of the
 ribs upon the thoracic space; strips of cardboard held together by pins,
     the front part being raised or lowered by threads moving through
   attachments at 1 and 2. As the front is raised the space between the
 uprights is increased. The front upright corresponds to the breastbone,
the back one to the spinal column, the connecting strips to the ribs, and
                 the threads to the intercostal muscles.

By slightly raising and spreading apart the ribs the thoracic space is
increased in two directions--from front to back and from side to side.
Lowering and converging the ribs has, of course, the opposite effect (Fig.
42). Except in forced expirations the ribs are lowered and converged by
their own weight and by the elastic reaction of the surrounding parts.

*The Diaphragm.*--Another means of varying the thoracic space is found in
an organ known as the diaphragm. This is the dome-shaped, _movable
partition_ which separates the thoracic cavity from the cavity of the
abdomen. The edges of the diaphragm are firmly attached to the walls of
the trunk, and the center is supported by the pericardium and the pleura.
The outer margin is muscular, but the central portion consists of a strong
sheet of connective tissue. By the contraction of its muscles the
diaphragm is pulled down, thereby increasing the thoracic cavity. By
raising the diaphragm the thoracic cavity is diminished.

The diaphragm, however, is not raised by the contraction of its own
muscles, but _is pushed up_ by the organs beneath. By the elastic reaction
of the abdominal walls (after their having been pushed out by the lowering
of the diaphragm), pressure is exerted on the organs of the abdomen and
these in turn press against the diaphragm. This crowds it into the
thoracic space. In forced expirations the muscles in the abdominal walls
contract to push up the diaphragm.

*Interchange of Gases in the Lungs.*--During each inspiration the air from
the outside fills the entire system of bronchial tubes, but the alveoli
are largely filled, at the same time, by the air which the last expiratory
effort has left in the passages. By the action of currents and eddies and
by the rapid diffusion of gas particles, the air from the outside mixes
with that in the alveoli and comes in contact with the membranous walls.
Here the oxygen, after being dissolved by the moisture in the membrane,
diffuses into the blood. The carbon dioxide, on the other hand, being in
excess in the blood, diffuses toward the air in the alveoli. The
interchange of gases at the lungs, however, is not fully understood, and
it is possible that other forces than osmosis play a part.

                                [Fig. 43]

              Fig. 43--*Diagram* illustrating lung capacity.

*Capacity of the Lungs.*--The air which passes into and from the lungs in
ordinary breathing, called the _tidal_ air, is but a small part of the
whole amount of air which the lungs contain. Even after a forced
expiration the lungs are almost half full; the air which remains is called
the _residual_ air. The air which is expelled from the lungs by a forced
expiration, less the tidal air, is called the _reserve_, or supplemental,
air. These several quantities are easily estimated. (See Practical Work.)
In the average individual the total capacity of the lungs (with the chest
in repose) is about one gallon. In forced inspirations this capacity may
be increased about one third, the excess being known as the _complemental_
air (Fig. 43).

                                [Fig. 44]

Fig. 44--*Diagram* illustrating internal respiration and its dependence on
         external respiration. (Modified from Hall.) (See text.)

*Internal, or Cell, Respiration.*--The oxygen which enters the blood in the
lungs leaves it in the tissues, passing through the lymph into the cells
(Fig. 44). At the same time the carbon dioxide which is being formed at
the cells passes into the blood. An exchange of gases is thus taking place
between the cells and the blood, similar to that taking place between the
blood and the air. This exchange is known as _internal_, or cell,
respiration. By internal respiration the oxygen reaches the place where it
is to serve its purpose, and the carbon dioxide begins its movement toward
the exterior of the body. This "breathing by the cells" is, therefore,
_the final and essential act of respiration_. Breathing by the lungs is
simply the means by which the taking up of oxygen and *the* giving off of
carbon dioxide by the cells is made possible.

HYGIENE OF RESPIRATORY ORGANS

The liability of the lungs to attacks from such dread diseases as
consumption and pneumonia makes questions touching their hygiene of first
importance. Consumption does not as a rule attack sound lung tissue, but
usually has its beginning in some weak or enfeebled spot in the lungs
which has lost its "power of resistance." Though consumption is not
inherited, as some suppose, lung weaknesses may be transmitted from
parents to children. This, together with the fact, now generally
recognized, that consumption is contagious, accounts for the frequent
appearance of this disease in the same family. Consumption as well as
other respiratory affections can in the majority of cases be _prevented_,
and in many cases cured, by an intelligent observation of well-known laws
of health.

*Breathe through the Nostrils.*--Pure air and plenty of it is the main
condition in the hygiene of the lungs. One necessary provision for
obtaining _pure air_ is that of breathing through the nostrils. Air is the
carrier of dust particles and not infrequently of disease germs.(33)
Partly through the small hairs in the nose, but mainly through the moist
membrane that lines the passages, the nostrils serve as filters for
removing the minute solid particles (Fig. 45). While it is important that
nose breathing be observed at all times, it is especially important when
one is surrounded by a dusty or smoky atmosphere. Otherwise the small
particles that are breathed in through the mouth may find a lodging place
in the lungs.

                                [Fig. 45]

  Fig. 45--*Human air filter.* Diagram of a section through the nostrils;
 shows projecting bones covered with moist membrane against which the air
is made to strike by the narrow passages. 1. Air passages. 2. Cavities in
         the bones. 3. Front lower portion of the cranial cavity.

In addition to removing dust particles and germs, other purposes are
served by breathing through the nostrils. The warmth and moisture which
the air receives in this way, prepare it for entering the lungs. Mouth
breathing, on the other hand, looks bad and during sleep causes snoring.
The habit of nose breathing should be established early in life.(34)

*Cultivate Full Breathing.*--Many people, while apparently taking in
sufficient air to supply their need for oxygen, do not breathe deeply
enough to "freely ventilate the lungs." "Shallow breathing," as this is
called, is objectionable because it fails to keep up a healthy condition
of the entire lung surface. Portions of the lungs to which air does not
easily penetrate fail to get the fresh air and exercise which they need.
As a consequence, they become weak and, by losing their "power of
resistance," become points of attack in diseases of the lungs.(35) The
breathing of each individual should receive attention, and where from some
cause it is not sufficiently full and deep, the means should be found for
remedying the defect.

*Causes of Shallow Breathing.*--Anything that impedes the free movement of
air into the lungs tends to cause shallow breathing A drooping of the back
or shoulders and a curved condition of the spinal column, such as is
caused by an improper position in sitting, interfere with the free
movements of the ribs and are recognized causes. Clothing also may impede
the respiratory movements and lead to shallow breathing. If too tight
around the chest, clothing interferes with the elevation of the ribs; and
if too tight around the waist, it prevents the depression of the
diaphragm. Other causes of shallow breathing are found in the absence of
vigorous exercise, in the leading of an indoor and inactive life, in
obstructions in the nostrils and upper pharynx, and in the lack of
attention to proper methods of breathing.

To prevent shallow breathing one should have the habit of sitting and
standing erect. The clothing must not be allowed to interfere with the
respiratory movements. The taking of exercise sufficiently vigorous to
cause deep and rapid breathing should be a common practice and one should
spend considerable time out of doors. If one has a flat chest or round
shoulders, he should strive by suitable exercises to overcome these
defects. Obstructions in the nostrils or pharynx should be removed.

*Breathing Exercises.*--In overcoming the habit of shallow breathing and in
strengthening the lungs generally, the practicing of occasional deep
breathing has been found most valuable and is widely recommended. With the
hands on the hips, the shoulders drawn back and _down_, the chest pushed
upward and forward, and the chin slightly depressed, draw the air slowly
through the nostrils until the lungs are _completely_ full. After holding
this long enough to count three slowly, expel it quickly from the lungs.
Avoid straining. To get the benefit of pure air, it is generally better to
practice deep breathing out of doors or before an open window.

By combining deep breathing with simple exercises of the arms, shoulders,
and trunk much may be done towards straightening the spine, squaring the
shoulders, and overcoming flatness of the chest. Though such movements are
best carried on by the aid of a physical director, one can do much to help
himself. One may safely proceed on the principle that slight deformities
of the chest, spine, and shoulders are corrected by gaining and keeping
the natural positions, and may employ any movements which will loosen up
the parts and bring them where they naturally belong.(36)

*Serious Nature of Colds.*--That many cases of consumption have their
beginning in severe colds (on the lungs) is not only a matter of popular
belief, but the judgment also of physicians. Though the cold is a
different affection from that of consumption, it may so lower the vitality
of the body and weaken the lung surfaces that the germs of consumption
find it easy to get a start. On this account a cold on the chest which
does not disappear in a few days, but which persists, causing more or less
coughing and pain in the lungs, must be given serious consideration.(37)
The usual home remedies failing to give relief, a physician should be
consulted. It should also be noted that certain diseases of a serious
nature (pneumonia, diphtheria, measles, etc.) have in their beginning the
appearance of colds. On this account it is wise not only to call a
physician, but to call him early, in severe attacks of the lungs.
Especially if the attack be attended by difficult breathing, fever, and a
rapid pulse is the case serious and medical advice necessary.

*Ventilation.*--The process by which the air in a room is kept fresh and
pure is known as ventilation. It is a double process--that of bringing
fresh air into the room and that of getting rid of air that has been
rendered impure by breathing (38) or by lamps. Outdoor air is usually of a
different temperature (colder in winter, warmer in summer) from that
indoors, and as a consequence differs from it slightly in weight. On
account of this difference, suitable openings in the walls of buildings
induce currents which pass between the rooms and the outside atmosphere
even when there is no wind. In winter care must be taken to prevent drafts
and to avoid too great a loss of heat from the room. A cold draft may even
cause more harm to one in delicate health than the breathing of air which
is impure. To ventilate a room successfully the problem of preventing
drafts must be considered along with that of admitting the fresh air.

                                [Fig. 46]

         Fig. 46--Window adjusted for ventilation without drafts.

The method of ventilation must also be adapted to the construction of the
building, the plan of heating, and the condition of the weather. Specific
directions cannot be given, but the following suggestions will be found
helpful in ventilating rooms where the air is not warmed before being
admitted:

1. _Introduce, the air through many small openings_ rather than a few
large ones. If the windows are used for this purpose, raise the lower sash
and drop the upper one _slightly_ for _several_ windows, varying the width
to suit the conditions (Fig. 46). By this means sufficient air may be
introduced without causing drafts.

2. _Introduce the air at the warmest portions of the room._ The air
should, if possible, be warmed before reaching the occupants.

3. If the wind is blowing, _ventilate principally on the sheltered side of
the house_.

Ample provision should be made for fresh air in sleeping rooms, and here
again drafts must be avoided. Especially should the bed be so placed that
strong air currents do not pass over the sleeper. In schoolhouses and
halls for public gatherings the means for efficient ventilation should, if
possible, be provided in the general plan of construction and method of
heating.

                                [Fig. 47]

 Fig. 47--*Artificial respiration* as a laboratory experiment. Expiration.
                    Prone-posture method of Schaffer.

*Artificial Respiration.*--When natural breathing is temporarily suspended,
as in partial drowning, or when one has been overcome by breathing some
poisonous gas, the saving of life often depends upon the prompt
application of artificial respiration. This is accomplished by alternately
compressing and enlarging the thorax by means of variable pressure on the
outside, imitating the natural process as nearly as possible. Following is
the method proposed by Professor E.A. Schaffer of England, and called by
him "the _prone-posture_ method of artificial respiration":

The patient is laid face downward with an arm bent under the head, and
_intermittent_ pressure applied vertically over the shortest ribs. The
pressure drives the air from the lungs, both by compressing the lower
portions of the chest and by forcing the abdominal contents against the
diaphragm, while the elastic reaction of the parts causes fresh air to
enter (Figs. 47 and 48). "The operator kneels or squats by the side of, or
across the patient, places his hands over the lowest ribs and swings his
body backward and forward so as to allow his weight to fall vertically on
the wrists and then to be removed; in this way hardly any muscular
exertion is required.... The pressure is applied gradually and slowly,
occupying some three seconds; it is then withdrawn during two seconds and
again applied; and so on some twelve times per minute."(39)

                                [Fig. 48]

               Fig. 48--Artificial respiration. Inspiration.

The special advantages of the prone-posture method over others that have
been employed are: I. It may be applied by a single individual and fora
long period of time without exhaustion. 2. It allows the mucus and water
(in case of drowning) to run out of the mouth, and causes the tongue to
fall forward so as not to obstruct the passageway. 3. It brings a
sufficient amount of air into the lungs.(40)

While applying artificial respiration, the heat of the body should not be
allowed to escape any more than can possibly be helped. In case of
drowning, the patient should be wrapped in dry blankets or clothing, while
bottles of hot water may be placed in contact with the body. The
circulation should be stimulated, as may be done by rubbing the hands,
feet, or limbs in the direction of the flow of the blood in the veins.

*Tobacco Smoke and the Air* Passages.--Smoke consists of minute particles
of unburnt carbon, or soot, such as collect in the chimneys of fireplaces
and furnaces. If much smoke is taken into the lungs, it irritates the
delicate linings and tends to clog them up. Tobacco smoke also contains
the poison nicotine, which is absorbed into the blood. For these reasons
the cigarette user who inhales the smoke does himself great harm, injuring
his nervous system and laying the foundation for diseases of the air
passages. The practice of smoking indoors is likewise objectionable, since
every one in a room containing the smoke is compelled to breathe it.

*Alcohol and Diseases of the Lungs.*--Pneumonia is a serious disease of the
lungs caused by germs. The attacks occur as a result of exposure,
especially when the body is in a weakened condition. A noted authority
states that "alcoholism is perhaps the most potent predisposing cause" of
pneumonia.(41) A person addicted to the use of alcohol is also less likely
to recover from the disease than one who has avoided its use, a result due
in part to the weakening effect of alcohol upon the heart. The congestion
of the lungs in pneumonia makes it very difficult for the heart to force
the blood through them. The weakened heart of the drunkard gives way under
the task.

The statement sometimes made that alcohol is beneficial in pulmonary
tuberculosis is without foundation in fact. On the other hand, alcoholism
is a recognized cause of consumption. Some authorities claim that this
disease is more frequent in heavy drinkers than in those of temperate
habits, in the proportion of about three to one, and that possibly half of
the cases of tuberculosis are traceable to alcoholism.(42)

*The Outdoor Cure for Lung Diseases*--Among the many remedies proposed for
consumption and kindred diseases, none have proved more beneficial,
according to reports, than the so-called "outdoor" cure. The person having
consumption is fed plentifully upon the most nourishing food, and is made
to spend practically his entire time, including the sleeping hours, _out
of doors_. Not only is this done during the pleasant months of summer, but
also during the winter when the temperature is below freezing. Severe
exposure is prevented by overhead protection at night and by sufficient
clothing to keep the body warm. The abundant supply of pure, cold air
toughens the lungs and invigorates the entire body, thereby enabling it to
throw off the disease.

The success attending this method of treating consumptives suggests the
proper mode of strengthening lungs that are not diseased, but simply weak.
The person having weak lungs should spend as much time as he conveniently
can out of doors. He should provide the most ample ventilation at night
and have a sleeping room to himself. He should practice deep breathing
exercises and partake of a nourishing diet. While avoiding prolonged
chilling and other conditions liable to induce colds, he should take
advantage of every opportunity of exposing himself fully and freely to the
outside atmosphere.

*Summary.*--The purpose of respiration is to bring about an exchange of
gases between the body and the atmosphere. The organs employed for this
purpose, called the respiratory organs, are adapted to handling materials
in the _gaseous_ state, and are operated in accordance with principles
governing the movements of the atmosphere. By alternately increasing and
diminishing the thoracic space, air is made to pass between the outside
atmosphere and the interior of the lungs. Finding its way into the
smallest divisions of the lungs, called the alveoli, the air comes very
near a large surface of blood. By this means the carbon dioxide diffuses
out of the blood, and the free oxygen enters. Through the combined action
of the organs of respiration and the organs that move the blood and the
lymph, the cells in all parts of the body are enabled to exchange certain
gaseous materials with the outside atmosphere.

                                [Fig. 49]

                Fig. 49--Model for demonstrating the lungs.

*Exercises.--*1. How does air entering the lungs differ in composition from
air leaving the lungs? What purposes of respiration are indicated by these
differences?

2. Name the divisions of the lungs.

3. Trace air from the outside atmosphere into the alveoli. Trace the blood
from the right ventricle to the alveoli and back again to the left
auricle.

4. How does the movement of air into and from the lungs differ from that
of the blood through the lungs with respect to (_a_) the direction of the
motion. (_b_) the causes of the motion, and (_c_) the tubes through which
the motion takes place?

5. How are the air passages kept clean and open?

6. Describe the pleura. Into what divisions does it separate the thoracic
cavity?

7. Describe and name uses of the diaphragm.

8. If 30 cubic inches of air are passed into the lungs at each inspiration
and .05 of this is retained as oxygen, calculate the number of cubic feet
of oxygen consumed each day, if the number of inspirations be 18 per
minute.

9. Find the _weight_ of a day's supply of oxygen, as found in the above
problem, allowing 1.3 ounces as the weight of a cubic foot.

10. Make a study of the hygienic ventilation of the schoolroom.

11. Give advantages of full breathing over shallow breathing.

12. How may a flat chest and round shoulders be a cause of consumption?
How may these deformities be corrected?

13. Give general directions for applying artificial respiration.

PRACTICAL WORK

Examine a dissectible model of the chest and its contents (Fig. 49). Note
the relative size of the two lungs and their position with reference to
the heart and diaphragm. Compare the side to side and vertical diameters
of the cavity. Trace the air tubes from the trachea to their smallest
divisions.

*Observation of Lungs* (Optional).--Secure from a butcher the lungs of a
sheep, calf, or hog. The windpipe and heart should be left attached and
the specimen kept in a moist condition until used. Demonstrate the
trachea, bronchi, and the bronchial tubes, and the general arrangement of
pulmonary arteries and veins. Examine the pleura and show lightness of
lung tissue by floating a piece on water.

*To show the Changes that Air undergoes in the Lungs.*--1. Fill a quart jar
even full of water. Place a piece of cardboard over its mouth and invert,
without spilling, in a pan of water. Inserting a tube under the jar, blow
into it air that has been held as long as possible in the lungs. When
filled with air, remove the jar from the pan, keeping the top well
covered. Slipping the cover slightly to one side, insert a burning
splinter and observe that the flame is extinguished. This proves the
absence of sufficient oxygen to support combustion. Pour in a little
limewater(43) and shake to mix with the air. The change of the limewater
to a milky white color proves the presence of carbon dioxide.

                                [Fig. 50]

 Fig. 50--*Apparatus* for showing changes which air undergoes while in the
                                  lungs.

2. The effects illustrated in experiment 1 may be shown in a somewhat more
striking manner as follows: Fill two bottles of the same size each one
fourth full of limewater and fit each with a two-holed rubber stopper
(Fig. 50). Fit into each stopper one short and one long glass tube, the
long tube extending below the limewater. Connect the short tube of one
bottle and the long tube of the other bottle with a Y-tube. Now breathe
slowly three or four times through the Y-tube. It will be found that the
inspired air passes through one bottle and the expired air through the
other. Compare the effect upon the limewater in the two bottles. Insert a
small burning splinter into the top of each bottle and note result. What
differences between inspired and expired air are thus shown?

3. Blow the breath against a cold window pane. Note and account for the
collection of moisture.

4. Note the temperature of the room as shown by a thermometer. Now breathe
several times upon the bulb, noting the rise in the mercury. What does
this experiment show the body to be losing through the breath?

*To show Changes in the Thoracic Cavity.*--1. To a yard- or meter-stick,
attach two vertical strips, each about eight inches long, as shown in Fig.
51. The piece at the end should be secured firmly in place by screws or
nails. The other should be movable. With this contrivance measure the
sideward and forward expansion of a boy's thorax. Take the diameter first
during a complete inspiration and then during a complete expiration,
reading the difference. Compare the forward with the sideward expansion.

                                [Fig. 51]

            Fig. 51--*Apparatus* for measuring chest expansion.

2. With a tape-line take the circumference of the chest when all the air
possible has been expelled from the lungs. Take it again when the lungs
have been fully inflated. The difference is now read as the chest
expansion.

                                [Fig. 52]

 Fig. 52--*Simple apparatus* for illustrating the action of the diaphragm.

*To illustrate the Action of the Diaphragm.*--Remove the bottom from a
large bottle having a small neck. (Scratch a deep mark with a file and
hold on the end of this mark a hot poker. When the glass cracks, lead the
crack around the bottle by heating about one half inch in advance of it.)
Place the bottle in a large glass jar filled two thirds full of water
(Fig. 52). Let the space above the water represent the chest cavity and
the water surface represent the diaphragm. Raise the bottle, noting that
the water falls, thereby increasing the space and causing air to enter.
Then lower the bottle, noting the opposite effect. To show the movement of
the air in and out of the bottle, hold with the hand (or arrange a support
for) a burning splinter over the mouth of the bottle.

*To estimate the Capacity of the Lungs.*--Breathing as naturally as
possible, expel the air into a spirometer (lung tester) during a period,
say of ten respirations (Fig. 53). Note the total amount of air exhaled
and the number of "breaths" and calculate the amount of air exhaled at
each breath. This is called the _tidal_ air.

                                [Fig. 53]

Fig. 53--*Apparatus* (spirometer) for measuring the capacity of the lungs.

2. After an ordinary inspiration empty the lungs as completely as possible
into the spirometer, noting the quantity exhaled. This amount, less the
tidal air, is known as the _reserve_ air. The air which is now left in the
lungs is called the _residual_ air. On the theory that this is equal in
amount to the reserve air, calculate the capacity of the lungs in an
ordinary inspiration.

3. Now fill the lungs to the full expansion of the chest and empty them as
completely as possible into the spirometer, noting the amount expelled.
This, less the tidal air and the reserve air, is called the _complemental_
air. Now calculate the total capacity of the lungs.

CHAPTER VIII - PASSAGE OF OXYGEN THROUGH THE BODY

What is the nature of oxygen? What is its purpose in the body and how does
it serve this purpose? How is the blood able to take it up at the lungs
and give it off at the cells? What becomes of it after being used? These
are questions touching the maintenance of life and they deserve careful
consideration.

*Nature of Oxygen.*--To understand the relation which oxygen sustains to
the body we must acquaint ourselves with certain of its chemical
properties. It is an element(44) of intense affinity, or combining power,
and is one of the most active of all chemical agents. It is able to
combine with most of the other elements to form chemical compounds. A
familiar example of its combining action is found in ordinary combustion,
or burning. On account of the part it plays in this process, oxygen is
called the _supporter of combustion_; but it supports combustion by the
simple method of uniting. The ashes that are left and the invisible gases
that escape into the atmosphere are the compounds formed by the uniting
process. It thus appears that oxygen, in common with the other elements,
may exist in either of two forms:

1. That in which it is in a _free_, or uncombined, condition--the form in
which it exists in the atmosphere.

2. That in which it is a part of compounds, such as the compounds formed
in combustion.

Oxygen manifests its activity to the best advantage when it is in a free
state, or, more accurately speaking, when it is passing from the free
state into one of combination. It is separated from its compounds and
brought again into a free state by overcoming with heat, or some other
force, the affinity which causes it to unite.

*How Oxygen unites.*--The chemist believes oxygen, as well as all other
substances, to be made up of exceedingly small particles, called _atoms_.
The atoms do not exist singly in either elements or compounds, but are
united with each other to form groups of atoms that are called
_molecules_. In an element the molecules are made up of one kind of atoms,
but in a compound the molecules are made up of as many kinds of atoms as
there are elements in the compound. Changes in the composition of
substances (called chemical changes) are due to rearrangements of the
atoms and the formation of new molecules. The atoms, therefore, are the
units of chemical combination. In the formation of new compounds they
unite, and in the breaking up of existing compounds they separate.

The uniting of oxygen is no exception to this general law. All of its
combinations are brought about by the uniting of its atoms. In the burning
of carbon, for example, the atoms of oxygen and the atoms of carbon unite,
forming molecules of the compound known as carbon dioxide. The chemical
formula of this compound, which is CO_2, shows the proportion in which the
atoms unite--one atom of carbon uniting with two atoms of oxygen in each of
the molecules. The affinity of oxygen for other elements, and the affinity
of other elements for oxygen, and for each other, resides in their atoms.

*Oxidation.*--The uniting of oxygen with other elements is termed
_oxidation_. This may take place slowly or rapidly, the two rates being
designated as _slow_ oxidation and _rapid_ oxidation. Examples of slow
oxidation are found in certain kinds of decay and in the rusting of iron.
Combustion is an example of rapid oxidation. Slow and rapid oxidation,
while differing widely in their effects upon surrounding objects, are
alike in that both produce heat and form compounds of oxygen. In slow
oxidation, however, the heat may come off so gradually that it is not
observed.

*Movement of Oxygen through the Body.*--Oxygen has been shown in the
preceding chapters to pass from the lungs into the blood and later to
leave the blood and, passing through the lymph, to enter the cells. That
oxygen does not become a permanent constituent of the cells is shown by
the constancy of the body weight. Nearly two pounds of oxygen per day are
known to enter the cells of the average-sized person. If this became a
permanent part of the cells, the body would increase in weight from day to
day. Since the body weight remains constant, or nearly so, we must
conclude that oxygen leaves the body about as fast as it enters. Oxygen
enters the body as a _free_ element. The form in which it leaves the body
will be understood when we realize the purpose which it serves and the
method by which it serves this purpose.

*Purpose of Oxygen in the Body.*--The question may be raised: Is it
possible for oxygen to serve a purpose in the body without remaining in
it? This, of course, depends upon what the purpose is. That it is possible
for oxygen to serve a purpose and at the same time pass on through the
place where it serves that purpose, is seen by studying the combustion in
an ordinary stove (Fig. 54). Oxygen enters at the draft and for the most
part passes out at the flue, but in passing through the stove it unites
with, or oxidizes, the fuel, causing the combustion which produces the
heat.

                                [Fig. 54]

            Fig. 54--*Coal stove* illustrating rapid oxidation.

Now it is found that certain chemical processes, mainly oxidations, are
taking place in the body. These produce the heat for keeping it warm and
also supply other forms of energy,(45) including motion. It is the purpose
of oxygen to keep up these oxidations and, by so doing, to aid in
supplying the body with energy. It serves this purpose in much the same
way that it supports combustion, _i.e._, by uniting with, or oxidizing,
materials derived from foods that are present in the cells.

*Does Oxygen serve Other Purposes?*--It has been suggested that oxygen may
serve the purpose of oxidizing, or destroying, substances that are
injurious and of acting, in this way, as a purifying agent in the body. In
support of this view is the natural tendency of oxygen to unite with
substances and the well-known fact that oxygen is an important natural
agent in purifying water. It seems probable, therefore, that it may to a
slight extent serve this purpose in the body. It is probable also that
oxygen aids through its chemical activity in the formation of compounds
which are to become a part of the cells. Both of these uses, however, are
of minor importance when compared with _the main use of oxygen_, which _is
that of an aid in supplying energy to the body_.

*Oxygen and the Maintenance of Life.*--In the supplying of energy to the
body, one of the conditions necessary to the maintenance of life is
provided. Because oxygen is necessary to this process, and because death
quickly results when the supply of it is cut off, oxygen is frequently
called the supporter of life. This idea is misleading, for oxygen has no
more to do with the maintenance of life than have the food materials with
which it unites. Life appears to be more dependent upon oxygen than upon
food, simply because the supply of it in the body at any time is
exceedingly small. Being continually surrounded by an atmosphere
containing free oxygen, the body depends upon this as a constant source of
supply, and does not store it up. Food, on the other hand, is taken in
excess of the body's needs and stored in the various tissues, the supply
being sufficient to last for several days. When the supply of either
oxygen or food is exhausted in the body, life must cease.

*The Oxygen Movement a Necessity.*--Since _free_ oxygen is required for
keeping up the chemical changes in the cells, and since it ceases to be
free as soon as it goes into combination, its continuous movement through
the body is a necessity. The oxygen compounds must be removed as fast as
formed in order to make room for more free oxygen. This movement has
already been studied in connection with the blood and the organs of
respiration, but the consideration of certain details has been deferred
till now. By what means and in what form is the oxygen passed _to_ and
_from_ the cells?

*Passage of Oxygen through the Blood.*--In serving its purpose at the
cells, the oxygen passes twice through the blood--once as it goes toward
the cells and again as it passes from the cells to the exterior of the
body:

_Passage toward the Cells._--This is effected mainly through the hemoglobin
of the red corpuscles. At the lungs the oxygen and the hemoglobin form a
weak chemical compound that breaks up and liberates the oxygen when it
reaches the capillaries in the tissues. The separation of the oxygen from
the hemoglobin at the tissues appears to be due to two causes: first, to
the weakness of the chemical attraction between the atoms of oxygen and
the atoms that make up the hemoglobin molecule; and second, to a
difference in the so-called _oxygen pressure_ at the lungs and at the
tissues.(46)

The attraction of the oxygen and the hemoglobin is sufficient to cause
them to unite where the oxygen pressure is more than one half pound to the
square inch, but it is not sufficiently strong to cause them to unite or
to prevent their separation, if already united, where the oxygen pressure
is less than one half pound to the square inch. The oxygen pressure at the
lungs, which amounts to nearly three pounds to the square inch, easily
causes the oxygen and the hemoglobin to unite, while the almost complete
absence of any oxygen pressure at the tissues, permits their separation.
The blood in its circulation constantly flows from the place of high
oxygen pressure at the lungs to the place of low oxygen pressure at the
tissues and, in so doing, loads up with oxygen at one place and unloads it
at the other (Fig. 55).

_Passage from the Cells._--Since oxygen leaves the free state at the cells
and becomes a part of compounds, we are able to trace it from the body
only by following the course of these compounds. Three waste compounds of
importance are formed at the cells--carbon dioxide (CO2), water (H2O), and
urea (N2H4CO). The first is formed by the union of oxygen with carbon, the
second by its union with hydrogen, and the third by its union with
nitrogen, hydrogen, and carbon. These compounds are carried by the blood
to the organs of excretion, where they are removed from the body. The
water leaves the body chiefly as a liquid, the urea as a solid dissolved
in water, and the carbon dioxide as a gas. The passage of carbon dioxide
through the blood requires special consideration.

                                [Fig. 55]

   Fig. 55--*Diagram illustrating movement, of oxygen and carbon dioxide
  through the body* (S.D. Magers). Each moves from a place of relatively
         high to a place of relatively low pressure. (See text.)

*Passage of Carbon Dioxide through the Blood.*--Part of the carbon dioxide
is dissolved in the plasma of the blood, and part of it is in weak
chemical combination with substances found in the plasma and in the
corpuscles. Its passage through the blood is accounted for in the same way
as the passage of the oxygen. Its ability to dissolve in liquids and to
enter into chemical combination varies as the _carbon dioxide
pressure_(47) This in turn varies with the amount of the carbon dioxide,
which is greatest at the cells (where it is formed), less in the blood,
and still less in the lungs. Because of these differences, the blood is
able to take it up at the cells and release it at the lungs (Fig. 55).

                                [Fig. 56]

Fig. 56--*Soap bubble* floating in a vessel of carbon dioxide, illustrating
       the difference in weight between air and carbon dioxide gas.

*Properties of Carbon Dioxide.*--Carbon dioxide is a colorless gas with
little or no odor. It is classed as a heavy gas, being about one third
heavier than air(48) (Fig. 56). It does not support combustion, but on the
contrary is used to some extent to extinguish fires. It is formed by the
oxidation of carbon in the body, and by the combustion of carbon outside
of the body. It is also formed by the decay of animal and vegetable
matter. From these sources it is continually finding its way into the
atmosphere. Although not a poisonous gas, carbon dioxide may, if it
surround the body, shut out the supply of oxygen and cause death.(49)

*Final Disposition of Carbon Dioxide.*--It is readily seen that the union
of carbon and oxygen, which is continually removing oxygen from the air
and replacing it with carbon dioxide, tends to make the whole atmosphere
deficient in the one and to have an excess of the other. This tendency is
counteracted through the agency of vegetation. Green plants absorb the
carbon dioxide from the air, decompose it, build the carbon into compounds
(starch, etc.) that become a part of the plant, and return the free oxygen
to the air (Fig. 57). In doing this, they not only preserve the necessary
proportion of oxygen and carbon dioxide in the atmosphere, but also put
the carbon and oxygen in such a condition that they can again unite. The
force which enables the plant cells to decompose the carbon dioxide is
supplied by the sunlight (Chapter XII).

                                [Fig. 57]

Fig. 57--*Under surface* of a geranium leaf showing breathing pores, highly
                            magnified (O.H.).

*Summary.*--Oxygen, by uniting with materials at the cells, keeps up a
condition of chemical activity (oxidation) in the body. This supplies heat
and the other forms of bodily energy. Entering as a free element, oxygen
leaves the body as a part of the waste compounds which it helps to form.
The free oxygen is transported from the lungs to the cells by means of the
hemoglobin of the red corpuscles, while the combined oxygen in carbon
dioxide and other compounds from the cells is carried mainly by the
plasma. The limited supply of free oxygen in the body at any time makes
necessary its continuous introduction into the body.

*Exercises.*--1. Describe the properties of oxygen. How does it unite with
other elements? How does it support combustion?

2. State the purpose of oxygen in the body. What properties enable it to
fulfill this purpose?

3. What is the proof that oxygen does not remain permanently in the body?
How does the oxygen entering the body differ from the same oxygen as it
leaves the body?

4. What is the necessity for the _continuous_ introduction of oxygen into
the body, while food is introduced only at intervals?

5. How are the red corpuscles able to take up and give off oxygen? How is
the plasma able to take up and give off carbon dioxide?

6. If thirty cubic inches of air pass from the lungs at each expiration
and 4.5 per cent of this is carbon dioxide, calculate the number of cubic
feet of the gas expelled in twenty-four hours, estimating the number of
respirations at eighteen per minute.

7. What is the weight of this volume of carbon dioxide, if one cubic foot
weigh 1.79 ounces?

8. What portion of this weight is oxygen and what carbon, the ratio by
weight of carbon to oxygen in carbon dioxide being twelve to thirty-two?

9. What is the final disposition of carbon dioxide in the atmosphere?

PRACTICAL WORK

*To show the Difference between Free Oxygen and Oxygen in
Combination.*--Examine some crystals of potassium chlorate (KClO3). They
contain oxygen _in combination_ with potassium and chlorine. Place a few
of these in a small test tube and heat strongly in a gas or alcohol flame.
The crystals first melt, and the liquid which they form soon appears to
boil. If a splinter, having a spark on the end, is now inserted in the
tube, it is kindled into a flame. This shows the presence of _free_
oxygen, the heat having caused the potassium chlorate to decompose. The
difference between free and combined oxygen may also be shown by
decomposing other compounds of oxygen, such as water and mercuric oxide.

*Preparation and Properties of Oxygen.*--Intimately mix 3 grams (1/2
teaspoonful) of potassium chlorate with half its bulk of manganese
dioxide, and place the mixture in a large test tube. Close the test tube
with a tight-fitting stopper which bears a glass tube of sufficient length
and of the right shape to convey the escaping gas to a small trough or pan
partly filled with water, on the table. Fill four large-mouthed bottles
with water and, by covering with cardboard, invert each in the trough of
water. Arrange the test tube conveniently for heating, letting the end of
the glass tube terminate under the mouth of one of the bottles (Fig. 58).
Using an alcohol lamp or a Bunsen burner, heat over the greater portion of
the tube at first, but gradually concentrate the flame upon the mixture.
Do not heat too strongly, and when the gas is coming off rapidly, remove
the flame entirely, putting it back as the action slows down. After all
the bottles have been filled, remove the end of the glass tube from the
water, but leave the bottles of oxygen inverted in the trough until they
are to be used. On removing the bottles from the trough, keep the tops
covered with wet cardboard.

                                [Fig. 58]

                Fig. 58--*Apparatus* for generating oxygen.

1. Examine a bottle of oxygen, noting its lack of color. Insert a small
burning splinter in the upper part of the bottle and observe the change in
the rate of burning. The air contains free oxygen, but it is diluted with
nitrogen. Compare this with the undiluted oxygen in the bottle as to
effect in causing the splinter to burn.

2. In a second bottle of oxygen insert a splinter without the flame, but
having a small spark on the end. As soon as the oxygen kindles the spark
into a flame, withdraw from the bottle and blow out the flame, but again
insert the spark. Repeat the experiment as long as the spark is kindled by
the oxygen into a flame. This experiment is usually performed as a test
for undiluted oxygen.

3. Make a hollow cavity in the end of a short piece of crayon. Fasten a
wire to the crayon, and fill the cavity with powdered sulphur. Ignite the
sulphur in the flame of an alcohol lamp or Bunsen burner, and lower it
into a bottle of oxygen. Observe the change in the rate of burning, the
color of the flame, and the material formed in the bottle by the burning.
The gas remaining in the bottle is sulphur dioxide (SO2), formed by the
_uniting_ of the sulphur and the oxygen.

4. Bend a small loop on the end of a piece of picture wire. Heat the loop
in a flame and insert it in some powdered sulphur. Ignite the melted
sulphur which adheres, and insert it quickly in a bottle of oxygen.
Observe the dark, brittle material which is formed by the burning of the
iron. It is a compound of the iron with oxygen, similar to iron rust, and
formed by their uniting.

*Preparation and Properties of Carbon Dioxide.*--1. (_a_) Attach a piece of
carbon (charcoal) no larger than the end of the thumb to a piece of wire.
Ignite the charcoal in a hot flame and lower it into a vessel of oxygen.
Observe its combustion, letting it remain in the bottle until it ceases to
burn. Note that the burning has consumed a part of the carbon and has used
up the free oxygen. Has anything been formed in their stead?

(_b_) Remove the charcoal and add a little limewater. Cover the bottle
with a piece of cardboard, and bring the gas and the limewater in contact
by shaking. Note any change in the color of the limewater. If it turns
white, the presence of carbon dioxide is proved.

2. Burn a splinter in a large vessel of air, keeping the top covered. Add
limewater and shake. Note and account for the result.

3. Place several pieces of marble (limestone) in a jar holding at least
half a gallon. Barely cover the marble with water, and then add
hydrochloric acid until a gas is rapidly evolved. This gas is carbon
dioxide.

(_a_) Does it possess color?

(_b_) Insert a burning splinter to see if it supports combustion.

(_c_) Place a bottle of oxygen by the side of the vessel of carbon
dioxide. Light a splinter and extinguish the flame by lowering it into the
vessel of carbon dioxide. Withdraw immediately, and if a spark remains on
the splinter, thrust it into the bottle of oxygen. Then insert the
relighted splinter into the carbon dioxide. Repeat several times, kindling
the flame in one gas and extinguishing it in the other. Finally show that
the spark also may be extinguished by holding the splinter a little longer
in the carbon dioxide.

(_d_) Tip the jar containing the carbon dioxide over the mouth of a
tumbler, as in pouring water, though not far enough to spill the acid, and
then insert a burning splinter in the tumbler. Account for the result.
Inference as to the weight of carbon dioxide.

                                [Fig. 59]

Fig. 59--*Simple apparatus* for illustrating passage of oxygen through the
                                  body.

(_e_) Review experiments (page 101) showing the presence of carbon dioxide
in the breath.

*To illustrate the General Movement of Oxygen through the Body.*--Into a
glass tube, six inches in length and open at both ends, place several
small lumps of charcoal (Fig. 59). Fit into one end of this tube, by means
of a stopper, a smaller glass tube which is bent at right angles and which
is made to pass through a close-fitting stopper to the bottom of a small
bottle. Another small tube is fitted into a second hole in this stopper,
but terminating near the top of the bottle, and to this is connected a
rubber tube about eighteen inches in length. The arrangement is now such
that by sucking air from the top of the bottle, it is made to enter at the
distant end of the tube containing the charcoal. After filling the bottle
one third full of limewater, heat the tube containing the charcoal until
it begins to glow. Then suck the air through the apparatus (as in smoking,
without drawing it into the lungs), observing what happens both in the
tube and in the bottle. What are the proofs that the oxygen, in passing
through the tube, unites with the carbon, forms carbon dioxide, and
liberates energy? Compare the changes which the oxygen undergoes while
passing through the tube with the changes which it undergoes in passing
through the body.

CHAPTER IX - FOODS AND THE THEORY OF DIGESTION

The body is constantly in need of new material. Oxidation, as shown in the
preceding chapter, rapidly destroys substances at the cells, and these
have to be replaced. Upon this renewal depends the supply of energy.
Moreover, there is found to be an actual breaking down of the living
material, or protoplasm, in the body. While this does not destroy the
cells, as is sometimes erroneously stated, it reduces the quantity of the
protoplasm and makes necessary a process of repair, or rebuilding, of the
tissues. This also requires new material. Finally, substances, such as
water and common salt, are required for the aid which they render in the
general work of the body. Since these are constantly being lost in one way
or another, they also must be replaced. These different needs of the body
for new materials are supplied through

*The Foods.*--Foods are substances that, on being taken into the healthy
body, are of assistance in carrying on its work. This definition properly
includes oxygen, but the term is usually limited to substances introduced
through the digestive organs. As suggested above, foods serve at least
three purposes:

1. They, with oxygen, supply the body with energy.

2. They provide materials for rebuilding the tissues.

3. They supply materials that aid directly or indirectly in the general
work of the body.

*The Simple Foods, or Nutrients.*--From the great variety of things that
are eaten, it might appear that many different kinds of substances are
suitable for food. When our various animal and vegetable foods are
analyzed, however, they are found to be similar in composition and to
contain only some five or six kinds of materials that are essentially
different. While certain foods may contain only a single one of these,
most of the foods are mixtures of two or more. These few common materials
which, in different proportions, form the different things that are eaten,
are variously referred to as simple foods, food-stuffs, and _nutrients_,
the last name being the one generally preferred. The different classes of
nutrients are as follows:

             Nutrients:
              Proteids
            (Albuminoids)
            Carbohydrates
                Fats
            Mineral salts
                Water

It is now necessary to become somewhat familiar with the different
nutrients and the purposes which they serve in the body.

*Proteids.*--The proteids are obtained in part from the animal and in part
from the plant kingdom, there being several varieties. A well-known
variety, called _albumin_, is found in the white of eggs and in the plasma
of the blood, while the muscles contain an abundance of another variety,
known as _myosin_. Cheese consists largely of a kind of proteid, called
_casein_, which is also present in milk, but in a more diluted form. If a
mouthful of wheat is chewed for some time, most of it is dissolved and
swallowed, but there remains in the mouth a sticky, gum-like substance.
This is _gluten_, a form of proteid which occurs in different grains.
Again, certain vegetables, as beans, peas, and peanuts, are rich in a kind
of proteid which is called _legumen_.

Proteids are compounds of carbon, hydrogen, oxygen, nitrogen, and a small
per cent of sulphur. Certain ones (the nucleo-proteids from grains) also
contain phosphorus. All of the proteids are highly complex compounds and
form a most important class of nutrients.

*Purposes of Proteids.*--The chief purpose of proteids in the body is to
rebuild the tissues. Not only do they supply all of the main elements in
the tissues, but they are of such a nature chemically that they are
readily built into the protoplasm. They are absolutely essential to life,
no other nutrients being able to take their place. An animal deprived of
them exhausts the proteids in its body and then dies. In addition to
rebuilding the tissues, proteids may also be oxidized to supply the body
with energy.

*Albuminoids* form a small class of foods, of minor importance, which are
similar to proteids in composition, but differ from them in being unable
to rebuild the tissues. Gelatin, a constituent of soup and obtained from
bones and connective tissue by boiling, is the best known of the
albuminoid foods. On account of the nitrogen which they contain, proteids
and albuminoids are often classed together as _nitrogenous foods_.

*Carbohydrates.*--While the carbohydrates are not so essential to life as
are the proteids, they are of very great value in the body. They are
composed of carbon, hydrogen, and oxygen, and are obtained mainly from
plants. There are several varieties of carbohydrates, but they are similar
in composition. All of those used as food to any great extent are starch
and certain kinds of sugar.

*Starch* is the carbohydrate of greatest importance as a food, and it is
also the one found in the greatest abundance. All green plants form more
or less starch, and many of them store it in their leaves, seeds, or roots
(Fig. 60). From these sources it is obtained as food. _Glycogen_, a
substance closely resembling starch, is found in the body of the oyster.
It is also formed in the liver and muscles of the higher animals, being
prepared from the sugar of the blood, and is stored by them as reserve
food (Chapter XI). Glycogen is, on this account, called _animal starch_.
Starch on being eaten is first changed to sugar, after which it may be
converted into glycogen in the liver and in the muscles.

                                [Fig. 60]

   Fig. 60--*Starch grains* in cells of potato as they appear under the
                    microscope. (See practical work.)

*Sugars.*--There are several varieties of sugar, but the important ones
used as foods fall into one or the other of two classes, known as _double
sugars_ (disaccharides) and _single sugars_ (monosaccharides). To the
first class belong _cane sugar_, found in sugar cane and beets, _milk
sugar_, found in sweet milk, and _maltose_, a kind of sugar which is made
from starch by the action of malt. The important members of the second
class are _grape sugar_, or dextrose, and _fruit sugar_, or levulose, both
of which are found in fruits and in honey.

The most important of all sugars, so far as its use in the body is
concerned, is _dextrose_. To this form all the other sugars, and starch
also, are converted before they are finally used in the body. The close
chemical relation between the different carbohydrates makes such a
conversion easily possible.

*Fats.*--The fats used as foods belong to one or the other of two classes,
known as solid fats and oils. The solid fats are derived chiefly from
animals, and the oils are obtained mostly from plants. Butter, the fat of
meats, olive oil, and the oil of nuts are the fats of greatest importance
as foods. Fats, like the carbohydrates, are composed of carbon, hydrogen,
and oxygen. They are rather complex chemical compounds, though not so
complex as proteids. Since neither fats nor carbohydrates contain
nitrogen, they are frequently classed together as _non-nitrogenous_ foods.

*Purpose Served by Carbohydrates, Fats, and Albuminoids.*--These classes of
nutrients all serve the common purpose of supplying energy. By uniting
with oxygen at the cells, they supply heat and the other forms of bodily
force. This is perhaps their only purpose.(50) Proteids also serve this
purpose, but they are not so well adapted to supplying energy as are the
carbohydrates and the fats. In the first place they do not completely
oxidize and therefore do not supply so much energy; and, in the second
place, they form waste products that are removed with difficulty from the
body.

*Mineral Salts and their Uses.*--Mineral salts are found in small
quantities in all of the more common food materials, and, as a rule, find
their way into the body unnoticed. They supply the elements which are
found in the body in small quantities and serve a variety of purposes.(51)
Calcium phosphate and calcium carbonate are important constituents of the
bones and teeth; and the salts containing iron renew the hemoglobin of the
blood. Others perform important functions in the vital processes. The
mineral compound of greatest importance perhaps is sodium chloride, or
common salt.(52) This is a natural constituent of most of our foods, and
is also added to food in its preparation for the table. When it is
withheld from animals for a considerable length of time, they suffer
intensely and finally die. It is necessary in the blood and lymph to keep
their constituents in solution, and is thought to play an important role
in the chemical changes of the cells. It is constantly leaving the body as
a waste product and must be constantly supplied in small quantities in the
foods.

*Importance of Water.*--Water finds its way into the body as a pure liquid,
as a part of such mixtures as coffee, chocolate, and milk, and as a
constituent of all our solid foods. (See table of foods, page 126.) It is
also formed in the body by the oxidation of hydrogen. It passes through
the body unchanged, and is constantly being removed by all the organs of
excretion. Though water does not liberate energy in the body nor build up
the tissues in the sense that other foods do, it is as necessary to the
maintenance of life as oxygen or proteids. It occurs in all the tissues,
and forms about 70 per cent of the entire weight of the body. Its presence
is necessary for the interchange of materials at the cells and for keeping
the tissues soft and pliable. As it enters the body, it carries digested
food substances with it, and as it leaves it is loaded with wastes. Its
chief physiological work, which is that of a _transporter of material_,
depends upon its ability to dissolve substances and to flow readily from
place to place.

*Relative Quantity of Nutrients Needed.*--Proteids, carbohydrates, and fats
are the nutrients that supply most of the body's nourishment. The most
hygienic diet is the one which supplies the proteids in sufficient
quantity to rebuild the tissues and the carbohydrates and fats in the
right amounts to supply the body with energy. Much experimenting has been
done with a view to determining these proportions, but the results so far
are not entirely satisfactory. According to some of the older estimates, a
person of average size requires for his daily use five ounces of proteid,
two and one half ounces of fat, and fifteen ounces of carbohydrate. Recent
investigations of this problem seem to show that the body is as well, if
not better, nourished by a much smaller amount of proteid--not more than
two and one half ounces (60 grams) daily.(53)

While there is probably no necessity for the healthy individual's taking
his proteid, fat, and carbohydrate in _exact_ proportions (if the
proportions best suited to his body were known), the fact needs to be
emphasized that proteids, although absolutely necessary, should form but a
small part (not over one fifth) of the daily bill of fare. In recognition
of this fact is involved a principle of health and also one of economy.
The proteids, especially those in meats, are the most expensive of the
nutrients, whereas the carbohydrates, which should form the greater bulk
of one's food, are the least expensive.

*Effects of a One-sided Diet.*--The plan of the body is such as to require
a _mixed diet_, and all of the great classes of nutrients are necessary.
If one could subsist on any single class, it would be proteids, for
proteids are able both to rebuild tissue and to supply energy. But if
proteids are eaten much in excess of the body's need for rebuilding the
tissues, and this excess is oxidized for supplying energy, a strain is
thrown upon the organs of excretion, because of the increase in the
wastes. Not only is there danger of overworking certain of these organs
(the liver and kidneys), but the wastes may linger too long in the body,
causing disorder and laying the foundation for disease. On the other hand,
if an insufficient amount of proteid is taken, the tissues are improperly
nourished, and one is unable to exert his usual strength. What is true of
the proteids is true, though in a different way, of the other great
classes of foods. A diet which is lacking in proteid, carbohydrate, or
fat, or which has any one of them in excess, is not adapted to the
requirements of the body.

*Composition of the Food Materials.*--One who intelligently provides the
daily bill of fare must have some knowledge of the nature and quantity of
the nutrients present in the different materials used as food. This
information is supplied by the chemist, who has made extensive analyses
for this purpose. Results of such analyses are shown in Table 1 (page
126), which gives the percentage of proteids, fats, carbohydrates, water,
and mineral salts in the edible portions of the more common of our foods.

                                [Fig. 61]

 Fig. 61--Relative proportions of different nutrients in well-known foods.

*Food Supply to the Table.*--The main problem in supplying the daily bill
of fare is that of securing through the different food materials the
requisite amounts of proteids, carbohydrates, and fats. In this matter a
table showing the composition of foods can be used to great advantage.
Consulting the table on page 126, it is seen that large per cents of
proteids are supplied by lean meat, eggs, cheese, beans, peas, peanuts,
and oatmeal, while fat is in excess in fat meat, butter, and nuts (Fig.
61). Carbohydrates are supplied in abundance by potatoes, rice, corn,
sugar, and molasses. The different cereals also contain a large percentage
of carbohydrates in the form of starch.

                                  TABLE I. THE COMPOSITION OF
                                       FOOD MATERIALS(54)
Food         Water      Solids     Proteid    Fat        Carbohydrates   Mineral    Heat
Materials                                                                Matter     Value of
                                                                                One
                                                                                Pound
Animal       Per cent   Per cent   Per cent   Per cent   Per cent        Per cent   Calories(55)
foods,
edible
portion
Beef:        63.9       36.1       19.5       15.6       ...             1          1020
Shoulder
       Rib   48.1       51.9       15.4       35.6       ...             .9         1790
   Sirloin   60         40         18.5       20.5       ...             1          1210
     Round   68.2       31.8       20.5       10.1       ...             1.2        805
Veal:        68.8       31.2       20.2       9.8        ...             ...        790
Shoulder
Mutton:      61.8       38.2       18.3       19         ...             .9         1140
Leg
      Loin   49.3       50.7       15         35         ...             .7         1755
Pork:        50.3       49.7       16         32.8       ...             .9         1680
Shoulder
   Ham,      41.5       58.5       16.7       39.1       ...             2.7        1960
   salted,
   smoked
    Fat,     12.1       87.9       .9         82.8       ...             4.2        3510
    salted
Sausage:     41.5       58.8       13.8       42.8       ...             2.2        2065
Pork
Bologna      62.4       37.6       18.8       42.8       ...             3          1015
Chicken      72.2       27.8       24.4       1          ...             1.4        540
Eggs         73.8       26.2       14.9       10.5       ...             .8         721
Milk         87         13         3.6        4          4.7             .7         325
Butter       10.5       89         .6         85         .5              .3         3515
Cheese:      30.2       69.8       28.3       35.5       1.8             4.2        2070
Full
cream
 Skim milk   41.3       58.7       38.4       6.8        6.9             4.6        1165
Fish:        82.6       17.4       15.8       .5         ...             1.2        310
Codfish
    Salmon   63.6       36.4       21.6       13.4       ...             1.4        965
   Oysters   87.1       12.9       6          1.2        3.7             2          230
Vegetable
foods
Wheat        12.5       87.5       11         1.1        74.9            .5         1645
flour
Graham       13.1       86.9       11.7       1.7        71.7            1.8        1635
flour
(wheat)
Rye flour    13.1       86.9       6.7        .8         78.7            .7         1625
Buckwheat    14.6       85.4       6.9        1.4        76.1            1          1605
flour
Oatmeal      7.6        92.4       15.1       7.1        68.2            2          1850
Cornmeal     15         85         9.2        3.8        70.6            1.4        1645
Rice         12.4       87.6       7.4        .4         79.4            .4         1630
Peas         12.3       87.7       26.7       1.7        56.4            2.9        1565
Beans        12.6       87.4       23.1       2          59.2            3.1        1615
Potatoes     78.9       21.1       2.1        .1         17.9            1          375
Tomatoes     95.3       4.7        .8         .4         3.2             .3         80
Apples       83.2       16.8       .2         .4         15.9            .3         315
Sugar,       2          98         ...        ...        97.8            .3         1820
granulated
White        32.3       67.7       8.2        1.7        56.3            .0         1280
bread
(wheat)
Peanuts      9.2        90.8       25.8       24.4       38.6            2          2560
Almonds      4.8        95.2       21         17.3       54.9            2          3030
Walnuts      2.5        97.5       16.6       16.1       63.4            1.4        3285
(English)

_Variety_ in the selection of foods for the table is an essential feature,
but this should not increase either the work or the expense of supplying
the meals. Each single meal can, and should, be simple in itself and, at
the same time, differ sufficiently from the meal preceding and the one
following to give the necessary variety in the course of the day. The bill
of fare should, of course, include fruits (for their tonic effects) and
very small amounts perhaps of substances which stimulate the appetite,
such as pepper, mustard, etc., known as condiments.

*Purity of Food.*--The fact that many of the food substances are perishable
makes it possible for them to be eaten in a slightly decayed condition.
Such substances are decidedly unwholesome (some containing poisons) and
should be promptly rejected. Not only do fresh meats, fruits, and
vegetables need careful inspection, but canned and preserved goods as
well. If canned foods are imperfectly sealed or if not thoroughly cooked
in the canning process, they decay and the acids which they generate act
on the metals lining the cans, forming poisonous compounds. The contents
of "tin" cans should for this reason be transferred to other vessels as
soon as opened.

Foods are also rendered impure or weakened through adulteration, the
watering of milk being a familiar example. The manufacture of jellies,
preserves, sirups, and various kinds of pickles and condiments has perhaps
afforded the largest field for adulterations, although it is possible to
adulterate nearly all of the leading articles of food. A long step in the
prevention of food and drug adulteration was taken in this country by the
passage of the _Pure Food Law_. By forcing manufacturers of foods and
medicines to state on printed labels the composition of their products,
this law has made it possible for the consumer to know what he is
purchasing and putting into his body.

*Alcohol not a Food.*--Many people in this and other countries drink in
different beverages, such as whisky, beer, wine, etc., a varying amount of
alcohol. This substance has a temporary stimulating or exciting effect,
and the claim has been made that it serves as a food. Recently it has been
shown that alcohol when introduced into the body in small quantities and
in a greatly diluted form, is nearly all oxidized, yielding energy as does
fat or sugar. If no harmful effects attended the use of alcohol, it might
on this account be classed as a food. But alcohol is known to be harmful
to the body. When used in large quantities, it injures nearly all of the
tissues, and when taken habitually, even in small doses, it leads to the
formation of the alcohol habit which is now recognized and treated as a
disease. This and other facts show that alcohol is not adapted to the body
plan of taking on and using new material (Chapter XI), and no substance
lacking in this respect can properly be classed as a food.(56) Instead of
classing alcohol as a food, it should be placed in that long list of
substances which are introduced into the body for special purposes and
which are known by the general name of

*Drugs.*--Drugs act strongly upon the body and tend to bring about unusual
and unnatural results. Their use should in no way be confused with that of
foods. If taken in health, they tend to disturb the physiological balance
of the body by unduly increasing or diminishing the action of the
different organs. In disease where this balance is already disturbed, they
may be administered for their counteractive effects, but always under the
advice and direction of a physician. Knowing the nature of the disturbance
which the drug produces, the physician can administer it to advantage,
should the body be out of physiological balance, or diseased. Not only are
drugs of no value in health, but their use is liable to do much harm.

NATURE OF DIGESTION

Before the nutrients can be oxidized at the cells, or built into the
protoplasm, they undergo a number of changes. These are necessary for
their entrance into the body, for their distribution by the blood and the
lymph, and for the purposes which they finally serve. The first of these
changes is preparatory to the entrance of the nutrients and is known as
_digestion_. The organs which bring about this change, called digestive
organs, have a special construction which adapts them to their work. It
will assist materially in understanding these organs if we first learn
something of the nature of the work which they have to perform.

*How the Nutrients get into the Body.*--The nature of digestion is
determined by the conditions affecting the entrance of nutrients into the
body. Food in the stomach and air in the lungs, although surrounded by the
body, are still outside of what is called the _body proper_. To gain
entrance into the body proper, a substance must pass through the body
wall. This consists of the skin on the outside and of the mucous linings
of the air passages and other tubes and cavities which are connected with
the external surface.

To get from the digestive organs into the blood, the nutrients must pass
through the mucous membrane lining these organs and also the walls of
blood or lymph vessels. Only _liquid materials_ can make this passage. It
is necessary, therefore, to reduce to the liquid state all nutrients not
already in that condition. _This reduction to the liquid state constitutes
the digestive process_.

*How Substances are Liquefied.*--While the reduction of solids to the
liquid state is accomplished in some instances by heating them until they
melt, they are more frequently reduced to this state by subjecting them to
the action of certain liquids, called _solvents_. Through the action of
the solvent the minute particles of the solid separate from each other and
disappear from view. (Shown in dropping salt in water.) At the same time
they mix with the solvent, forming a _solution_, from which they separate
only with great difficulty. For this reason solids in solution can diffuse
through porous partitions along with the solvents in which they are
dissolved (page 73).

By digestion the nutrients are reduced to the form of a solution. _The
process is_, simply speaking, _one of dissolving_. The liquid employed as
_the digestive solvent is water_. The different nutrients dissolve in
water, mixing with it to form a solution which is then passed into the
body proper.

*Digestion not a Simple Process.*--Digestion is by no means a simple
process, such, for instance, as the dissolving of salt or sugar in water.
These, being soluble in water, dissolve at once on being mixed with a
sufficient amount of this liquid. The majority of the nutrients, however,
are insoluble in water and are unaffected by it when acting alone. Fats,
starch, and most of the proteids do not dissolve in water. Before these
can be dissolved they have to be changed chemically and converted into
substances that are _soluble in water_. This complicates the process and
_prevents the use of water alone_ as the digestive solvent.

*A Similar Case.*--If a piece of limestone be placed in water, it does not
dissolve, because it is insoluble in water. If hydrochloric acid is now
added to the water, the limestone is soon dissolved (Fig. 62). (See
Practical Work.) It seems at first thought that the acid dissolves the
limestone, but this is not the case. The acid produces a chemical change
in the limestone (calcium carbonate) and converts it into a compound
(calcium chloride) that is soluble in water. As fast as this is formed it
is dissolved by the water, which is the real solvent in the case. The acid
simply plays the part of a chemical converter.

                                [Fig. 62]

 Fig. 62--The dissolving of limestone in water containing acid, suggesting
            the double action in the digestion of most foods.

*The Digestive Fluids.*--Several fluids--saliva, gastric juice, pancreatic
juice, bile, and intestinal juice--are employed in the digestion of the
food. The composition of these fluids is in keeping with the nature of the
digestive process. While all of them have water for their most abundant
constituent, there are dissolved in the water small amounts of active
chemical agents. It is the work of these agents to convert the insoluble
nutrients into substances that are soluble in water. The digestive fluids
are thus able to act in a _double_ manner on the nutrients--to change them
chemically and to dissolve them. The chemical agents which bring about the
changes in the nutrients are called _enzymes_, or digestive ferments.

*Foods Classed with Reference to Digestive Changes.*--With reference to the
changes which they undergo during digestion, foods may be divided into
three classes as follows:

1. Substances already in the liquid state and requiring no digestive
action. Water and solutions of simple foods in water belong to this class.
Milk and liquid fats, or oils, do not belong to this class.

2. Solid foods soluble in water. This class includes common salt and
sugar. These require no digestive action other than dissolving in water.

3. Foods that are insoluble in water. These have first to be changed into
soluble substances, after which they are dissolved.

*Summary.*--Materials called foods are introduced into the body for
rebuilding the tissues, supplying energy, and aiding in its general work.
Only a few classes of substances, viz., proteids, carbohydrates, fats,
water, and some mineral compounds have all the qualities of foods and are
suitable for introduction into the body. Substances known as drugs, which
may be used as medicines in disease, should be avoided in health. Before
foods can be passed into the body proper, they must be converted into the
liquid form, or dissolved. In this process, known as digestion, water is
the solvent; and certain chemical agents, called enzymes, convert the
insoluble nutrients into substances that are soluble in water.

*Exercises.*--1. How does oxidation at the cells make necessary the
introduction of new materials into the body?

2. What different purposes are served by the foods?

3. What is a nutrient? Name the important classes.

4. What are food materials? From what sources are they obtained?

5. Name the different kinds of proteids; the different kinds of
carbohydrates. Why are proteids called nitrogenous foods and fats and
carbohydrates non-nitrogenous foods?

6. Show why life cannot be carried on without proteids; without water.

7. What per cents of proteid, fat, and carbohydrate are found in wheat
flour, oatmeal, rice, butter, potatoes, round beef, eggs, and peanuts?

8. State the objection to a meal consisting of beef, eggs, beans, bread,
and butter; to one consisting of potatoes, rice, bread, and butter. Which
is the more objectionable of these meals and why?

9. State the general plan of digestion.

10. Show that digestion is not a simple process like that of dissolving
salt in water.

PRACTICAL WORK

*Elements supplied by the Foods.*--The following brief study will enable
the pupil to identify most of the elements present in the body and which
have, therefore, to be supplied by the foods.

_Carbon._--Examine pieces of charred wood, coke, or coal, and also the
"lead" in lead pencils. Show that the charred wood and the coal will burn.
Recall experiment (page 114) showing that carbon in burning forms carbon
dioxide.

_Hydrogen._--Fill a test tube one third full of strong hydrochloric acid
and drop into it several small scraps of zinc. The gas which is evolved is
hydrogen. When the hydrogen is coming off rapidly, bring a lighted
splinter to the mouth of the tube. The gas should burn. Hold a cold piece
of glass over the flame and observe the deposit of moisture. Hydrogen in
burning forms water. Extinguish the flame by covering the top of the tube
with a piece of cardboard. Now let the escaping gas collect in a tumbler
inverted over the tube. After holding the tumbler in this position for two
or three minutes, remove and, keeping inverted, thrust a lighted splinter
into it. (The gas should either burn or explode.) What does this
experiment show relative to the weight of hydrogen as compared with that
of air?

_Nitrogen._--Nitrogen forms about four fifths of the atmosphere, where,
like oxygen, it exists in a free state. It may be separated from the
oxygen of an inclosed portion of air by causing that gas to unite with
phosphorus. Place a piece of phosphorus the size of a pea in a depression
in a flat piece of cork. (Handle phosphorus with wet fingers or with
forceps.) Place the cork on water and have ready a glass fruit jar holding
not more than a quart. Ignite the phosphorus with a hot wire and invert
the jar over it, pushing the mouth below the surface of the water. The
phosphorus uniting with the oxygen fills the jar with white fumes of
phosphoric oxide. These soon dissolve in the water, leaving a clear gas
above. This is nitrogen. Place a cardboard under the mouth of the jar and
turn it right side up, leaving in the water and keeping the top covered.
Light a splinter and, slipping the cover to one side, thrust the flame
into the jar of nitrogen, noting the effect. (Flame is extinguished.)
Compare nitrogen with oxygen in its relation to combustion. What purpose
is served by each in the atmosphere?

_Oxygen._--Review experiments (page 114) showing the properties of oxygen.

_Phosphorus._--Examine a small piece of phosphorus, noting that it has to
be kept under water. Lay a small piece on the table and observe the tiny
stream of white smoke rising from it, formed by slow oxidation. Dissolve a
piece as large as a pea in a teaspoonful of carbon disulphide in a test
tube, pour this on a piece of porous paper, and lay the paper on an iron
support. When the carbon disulphide evaporates the phosphorus takes fire
spontaneously. (The heat from the slow oxidation is sufficient to ignite
the phosphorus in the finely divided condition.) What is the most striking
property of phosphorus? What purpose does it serve in the match?

_Sulphur._--Examine some sulphur, noting its color and the absence of odor
or taste. (Impure sulphur may have an odor and a taste.) Burn a little
sulphur in an iron spoon, noting that the compound which it forms with
oxygen by burning has a decided odor.

_Other Elements._--_Magnesium._ Examine and burn a piece of magnesium
ribbon, noting the white compound of magnesium oxide which is formed.
_Iron._ Examine pieces of the metal and also some of its compounds, as
ferrous sulphate, ferric chloride, and ferric oxide or iron rust.
_Sodium._ Drop a piece of the metal on water and observe results. Sodium
decomposes water. It has to be kept under some liquid, such as kerosene,
which contains no oxygen. (It should not be touched except with the
fingers wet with kerosene.) _Chlorine._ Pour strong hydrochloric acid on a
little manganese dioxide in a test tube, and warm gently over a low flame.
The escaping gas is chlorine. Avoid breathing much of it.

*Composition of the Nutrients.*--The simplest way of determining what
elements make up the different nutrients is by heating them and studying
the products of decomposition, as follows:

_To show that Carbohydrates contain Carbon, Hydrogen, and Oxygen._--Place
one half teaspoonful of powdered starch in a test tube and heat strongly.
Observe that _water_ condenses on the sides of the tube and that a black,
charred mass remains behind. The black mass consists mainly of _carbon_.
The water is composed of hydrogen and oxygen. These three elements are
thus shown to be present in the starch. The experiment may be repeated,
using sugar instead of starch.

_To show that Proteids contain Carbon, Hydrogen, Oxygen, Nitrogen, and
Sulphur._--Place in a test tube some finely divided proteid which has been
thoroughly dried (dried beef or the lean of hard cured bacon). Heat
strongly in the hood of a chemical laboratory or some other place where
the odors do not get into the room. First hold in the escaping gases a wet
strip of red litmus paper. This will be turned blue, showing _ammonia_
(NH3) to be escaping. Next hold in the mouth of the tube a strip of a
paper wet with a solution of lead nitrate. This is turned black or brown
on account of _hydrogen sulphide_(H2S) which is being driven off. Observe
also that _water_ condenses in the upper part of the tube and that a
black, charred mass remains behind. Since the products of decomposition
(H2O, NH3, H2S, and the charred mass) contain hydrogen, oxygen, nitrogen,
sulphur, and carbon, these elements are of course present in the proteid
tested.

_To show the Presence of Mineral Matter._--Burn a piece of dry bread by
holding it in a clear, hot flame, and observe the ash that is left behind.
This is the mineral matter present in the bread.

*Tests for Nutrients.* _Proteids._--Cover the substance to be tested with
strong nitric acid and heat gradually to boiling. If proteid is present it
turns yellow and partly dissolves in the acid, forming a yellow solution.
Let cool and then add ammonia. The yellow solid and the solution are
turned a deep orange color. Apply this test to foods containing proteid
such as white of egg, cheese, lean meat, etc.

_Starch._--_(a)_ Place a small lump of starch in one fourth of a pint of
water and heat gradually to boiling, stirring well. Then add enough water
to form a thin liquid and fill a test tube half full. Add to this a few
drops of a solution of iodine. (Prepare by dissolving a crystal of iodine
in 25 cubic centimeters (1/20 pint) of a solution of potassium iodide in
water and add water to this until it is a light amber color.) The starch
solution is turned blue, _(b)_ Cut with a razor a thin slice from a
potato. Place this in a weak solution of iodine for a few minutes and then
examine with the microscope, using first a low and then a high power.
Numerous starch grains inclosed in cellulose walls will be seen (Fig. 60).

_Dextrose, or Grape Sugar._--Place a solution of the substance supposed to
contain grape sugar in a test tube and add a few drops of a dilute
solution of copper sulphate. Then add sodium hydroxide solution until the
precipitate which first forms is redissolved and a clear blue liquid
obtained. Heat the upper portion of the liquid slowly to near the boiling
point. A little below the boiling point the blue color disappears and a
yellow-red precipitate is formed. If the upper layer of the liquid is now
boiled, the color deepens and this may be contrasted with the blue color
below. Apply this test to the sugar in raisins and in honey.

_Fat._--Fat is recognized by its effect on paper, making a greasy stain
which does not disappear on heating and which renders the paper
translucent. Try butter, lard, or olive oil. Also show the presence of fat
in peanuts by crushing them in a mortar and rubbing the powder on thin
paper. If the substance to be tested contains but little fat, this may be
dissolved out with ether. If a drop of ether containing the fat is placed
on paper, it evaporates, leaving the fat, which then forms the stain.

*To show the Effect of Alcohol upon Proteid.*--Place some of the white of a
raw egg in a glass vessel and cover it with a small amount of alcohol. As
the albumin (proteid) hardens, or coagulates, observe that the quantity of
clear liquid increases. This is due to the _withdrawal_ of water from the
albumin by the alcohol. Since the tissues are made up chiefly of proteids,
a piece of muscle or of liver may be used in the experiment, instead of
the egg, with similar results.

*To illustrate the Digestive Process.*--To a tumbler two thirds full of
water add a little salt. Stir and observe that the salt is dissolved.
Taste the solution to see that the salt has not been changed chemically.
Now add a little powdered limestone to the water and stir as before.
Observe that the limestone does not dissolve. Then add some hydrochloric
acid and observe the result. State the part played by the acid and by the
water in dissolving the limestone. Apply to the digestion of the different
classes of foods.

CHAPTER X - ORGANS AND PROCESSES OF DIGESTION

The organs of digestion are adapted to the work of dissolving the foods by
both their structure and arrangement. Most of them consist either of tubes
or cavities and these are so connected, one with the other, as to form a
continuous passageway entirely through the body. This passageway is known
as

*The Alimentary Canal. *--The alimentary canal has a length of about thirty
feet and, while it begins at the mouth, all but about eighteen inches of
it is found in the abdominal cavity. On account of its length it lies for
the most part in coils, the two largest ones being known as the small
intestine and the large intestine. Connected with the alimentary canal are
the glands that supply the liquids for acting on the food. The divisions
of the canal and most of the glands that empty liquids into it are shown
in Fig. 63 and named in the table below:

                                 [Table]

*Coats of the Alimentary Canal.*--The walls of the alimentary canal, except
at the mouth, are distinct from the surrounding tissues and consist in
most places of at least three layers, or coats, as follows:

                                [Fig. 63]

 Fig. 63--*Diagram of the digestive system.* 1. Mouth. 2. Soft palate. 3.
Pharynx. 4. Parotid gland. 5. Sublingual gland. 6. Submaxillary gland. 7.
Esophagus. 8. Stomach. 9. Pancreas. 10. Vermiform appendix. 11. Caecum. 12.
 Ascending colon. 13. Transverse colon. 14. Descending colon. 15. Sigmoid
    flexure. 16. Rectum. 17. Ileo-caecal valve. 18. Duct from liver and
                          pancreas. 19. Liver.

     Diagram does not show comparative length of the small intestine.

1. An _inner coat_, or lining, known as the mucous membrane. This membrane
is not confined to the alimentary canal, but lines, as we have seen, the
different air passages. It covers, in fact, all those internal surfaces of
the body that connect with the external surface. It derives its name from
the substance which it secretes, called _mucus_. In structure it resembles
the skin, being continuous with the skin where cavities open to the
surface. It is made up of two layers--a thick underlayer which contains
blood vessels, nerves, and glands, and a thin surface layer, called the
_epithelium._ The epithelium, like the cuticle, is without blood vessels,
nerves, or glands.

2. A _middle coat_, which is muscular and which forms a continuous layer
throughout the canal, except at the mouth. (Here its place is taken by the
strong muscles of mastication which are separate and distinct from each
other.) As a rule the muscles of this coat are involuntary. They surround
the canal as thin sheets and at most places form two distinct layers. In
the inner layer the fibers encircle the canal, but in the outer layer they
run longitudinally, or lengthwise, along the canal.(57)

3. An _outer_ or _serous coat_, which is limited to those portions of the
canal that occupy the abdominal cavity. This coat is not found above the
diaphragm. It is a part of the lining membrane of the cavity of the
abdomen, called

                                [Fig. 64]

Fig. 64--*Diagram of the peritoneum.* 1. Transverse colon. 2. Duodenum. 3.
                      Small intestine. 4. Pancreas.

*The Peritoneum.*--The peritoneum is to the abdominal cavity what the
pleura is to the thoracic cavity. It forms the outer covering for the
alimentary canal and other abdominal organs and supplies the inner lining
of the cavity itself. It is also the means of holding these organs in
place, some of them being suspended by it from the abdominal walls (Fig.
64). By the secretion of a small amount of liquid, it prevents friction of
the parts upon one another.

*Digestive Glands.*--The glands which provide the different fluids for
acting on the foods derive their constituents from the blood. They are
situated either in the mucous membrane or at convenient places outside of
the canal and pass their liquids into it by means of small tubes, called
ducts. In the canal the food and the digestive fluids come in direct
contact--a condition which the dissolving processes require. Each kind of
fluid is secreted by a special kind of gland and is emptied into the canal
at the place where it is needed.

*The Digestive Processes.*--Digestion is accomplished by acting upon the
food in different ways, as it is passed along the canal, with the final
result of reducing it to the form of a solution. Several distinct
processes are necessary and they occur in such an order that those
preceding are preparatory to those that follow. These processes are known
as _mastication, insalivation, deglutition, stomach digestion_, and
_intestinal_ digestion. As the different materials become liquefied they
are transferred to the blood, and substances not reduced to the liquid
state are passed on through the canal as waste. The first two of the
digestive processes occur in

*The Mouth.*--This is an oval-shaped cavity situated at the very beginning
of the canal. It is surrounded by the lips in front, by the cheeks on the
sides, by the hard palate above and the soft palate behind, and by the
tissues of the lower jaw below. The mucous membrane lining the mouth is,
soft and smooth, being covered with flat epithelial cells. The external
opening of the mouth is guarded by the lips, and the soft palate forms a
_movable_ partition between the mouth and the pharynx. In a condition of
repose the mouth space is practically filled by the teeth and the tongue,
but the cavity may be enlarged and room provided for food by depressing
the lower jaw.

The mouth by its construction is well adapted to carrying on the processes
of mastication and insalivation. By the first process the solid food is
reduced, by the cutting and grinding action of the teeth, to a finely
divided condition. By the second, the saliva becomes mixed with the food
and is made to act upon it.

                                [Fig. 65]

Fig. 65--*The teeth.* _A._ Section of a single molar. 1. Pulp. 2. Dentine.
  3. Enamel. 4. Crown. 5. Neck. 6. Root. _B._ Teeth in position in lower
  jaw. 1. Incisors. 2. Canine. 3. Biscuspids. 4. Molars. _C._ Upper and
   lower teeth on one side. 1. Incisors. 2. Canines. 3. Biscuspids. 4.
Molars. 5. Wisdom. _D._ Upper and lower incisor, to show gliding contact.

*Accessory Organs of the Mouth.*--The work of mastication and insalivation
is accomplished through organs situated in and around the mouth cavity.
These comprise:

1. _The Teeth._--The teeth are set in the upper and lower jaws, one row
directly over the other, with their hardened surfaces facing. In reducing
the food, the teeth of the lower jaw move against those of the upper,
while the food is held by the tongue and cheeks between the grinding
surfaces. The front teeth are thin and chisel-shaped. They do not meet so
squarely as do the back ones, but their edges glide over each other, like
the blades of scissors--a condition that adapts them to cutting off and
separating the food (_D_, Fig. 65). The back teeth are broad and
irregular, having surfaces that are adapted to crushing and grinding.

Each tooth is composed mainly of a bone-like substance, called _dentine_,
which surrounds a central space, containing blood vessels and nerves,
known as the _pulp cavity_. It is set in a depression in the jaw where it
is held firmly in place by a bony substance, known as _cement_. The part
of the tooth exposed above the gum is the _crown_, the part surrounded by
the gum is the _neck_, and the part which penetrates into the jaw is the
_root_ (_A_, Fig. 65). A hard, protective material, called _enamel_,
covers the exposed surface of the tooth.

The teeth which first appear are known as the _temporary_, or milk, teeth
and are twenty in number, ten in each jaw. They usually begin to appear
about the sixth month, and they disappear from the mouth at intervals from
the sixth to the thirteenth year. As they leave, teeth of the second, or
_permanent_, set take their place. This set has thirty-two teeth of four
different kinds arranged in the two jaws as follows:

In front, above and below, are four chisel-shaped teeth, known as the
_incisors_. Next to these on either side is a tooth longer and thicker
than the incisors, called the _canine_. Back of these are two short,
rounded and double pointed teeth, the _bicuspids_, and back of the
bicuspids are three heavy teeth with irregular grinding surfaces, called
the _molars_ (_B_ and _C_, Fig. 65). Since the molar farthest back in each
jaw is usually not cut until maturity, it is called a _wisdom_ tooth. The
molars are known as the superadded permanent teeth because they do not
take the place of milk teeth, but form farther back as the jaw grows in
length.

                                [Fig. 66]

    Fig. 66--*Diagram* showing directions of muscular fibers in tongue.

2. _The Tongue._--The tongue is a muscular organ whose fibers extend
through it in several directions (Fig. 66). Its structure adapts it to a
variety of movements. During mastication the tongue transfers the food
from one part of the mouth to another, and, with the aid of the cheeks,
holds the food between the rows of teeth. (By an outward pressure from the
tongue and an inward pressure from the cheek the food is kept between the
grinding surfaces.) The tongue has functions in addition to these and is a
most useful organ.

3. _The Muscles of Mastication._--These are attached to the lower jaw and
bring about its different movements. The _masseter_ muscles, which are the
heavy muscles in the cheeks, and the _temporal_ muscles, located in the
region of the temples, raise the lower jaw and supply the force for
grinding the food. Small muscles situated below the chin depress the jaw
and open the mouth.

                                [Fig. 67]

 Fig. 67--*Salivary glands* and the ducts connecting them with the mouth.

4. _The Salivary Glands._--These glands are situated in the tissues
surrounding the mouth, and communicate with it by means of ducts (Fig.
67). They secrete the saliva. The salivary glands are six in number and
are arranged in three pairs. The largest, called the _parotid_ glands,
lie, one on either side, in front of and below the ears. A duct from each
gland passes forward along the cheek until it opens in the interior of the
mouth, opposite the second molar tooth in the upper jaw. Next in size to
the parotids are the _submaxillary_ glands. These are located, one on
either side, just below and in front of the triangular bend in the lower
jaw. The smallest of the salivary glands are the _sublingual_. They are
situated in the floor of the mouth, on either side, at the front and base
of the tongue. Ducts from the submaxillary and sublingual glands open into
the mouth below the tip of the tongue.

*The Saliva and its Uses.*--The saliva is a transparent and somewhat slimy
liquid which is slightly alkaline. It consists chiefly of water (about 99
per cent), but in this are dissolved certain salts and an active chemical
agent, or enzyme, called _ptyalin_, which acts on the starch. The ptyalin
changes starch into a form of sugar (maltose), while the water in the
saliva dissolves the soluble portions of the food. In addition to this the
saliva moistens and lubricates the food which it does not dissolve, and
prepares it in this way for its passage to the stomach. The last is
considered the most important use of the saliva, and dry substances, such
as crackers, which require a considerable amount of this liquid, cannot be
eaten rapidly without choking. Slow mastication favors the secretion and
action of the saliva.

*Deglutition.*--Deglutition, or swallowing, is the process by which food is
transferred from the mouth to the stomach. Though this is not, strictly
speaking, a digestive process, it is, nevertheless, necessary for the
further digestion of the food. Mastication and insalivation, which are
largely mechanical, prepare the food for certain chemical processes by
which it is dissolved. The first of these occurs in the stomach and to
this organ the food is transferred from the mouth. The chief organs
concerned in deglutition are the tongue, the pharynx, and the esophagus.

*The Pharynx* is a round and somewhat cone-shaped cavity, about four and
one half inches in length, which lies just back of the nostrils, mouth,
and larynx. It is remarkable for its openings, seven in number, by means
of which it communicates with other cavities and tubes of the body. One of
these openings is into the mouth, one into the esophagus, one into the
larynx, and one into each of the nostrils, while two small tubes (the
eustachian) pass from the upper part of the pharynx to the middle ears.

The pharynx is the part of the food canal that is crossed by the
passageway for the air. To keep the food from passing out of its natural
channel, the openings into the air passages have to be carefully guarded.
This is accomplished through the soft palate and epiglottis, which are
operated somewhat as valves. The muscular coat of the pharynx is made up
of a series of overlapping muscles which, by their contractions, draw the
sides together and diminish the cavity. The mucous membrane lining the
pharynx is smooth, like that of the mouth, being covered with a layer of
flat epithelial cells.

*The Esophagus*, or gullet, is a tube eight or nine inches long,
connecting the pharynx with the stomach. It lies for the most part in the
thoracic cavity and consists chiefly of a thick mucous lining surrounded
by a heavy coat of muscle. The muscular coat is composed of two layers--an
inner layer whose fibers encircle the tube and an outer layer whose fibers
run lengthwise.

*Steps in Deglutition.*--The process of deglutition varies with the kind of
food. With bulky food it consists of three steps, or stages, as follows:
1. By the contraction of the muscles of the cheeks, the food ball, or
bolus, is pressed into the center of the mouth and upon the upper surface
of the tongue. Then the tongue, by an upward and backward movement, pushes
the food under the soft palate and into the pharynx.

2. As the food passes from the mouth, the pharynx is drawn up to receive
it. At the same time the soft palate is pushed upward and backward,
closing the opening into the upper pharynx, while the epiglottis is made
to close the opening into the larynx. By this means all communication
between the food canal and the air passages is temporarily closed. The
upper muscles of the pharynx now contract upon the food, forcing it
downward and into the esophagus.

3. In the esophagus the food is forced along by the successive
contractions of muscles, starting at the upper end of the tube, until the
stomach is reached.

Swallowing is doubtless aided to some extent by the force of gravity. That
it is independent of this force, however, is shown by the fact that one
may swallow with the esophagus in a horizontal position, as in lying down.

                                [Fig. 68]

   Fig. 68--*Gastric Glands.* _A._ Single gland showing the two kinds of
secreting cells and the duct where the gland opens on to the surface. _B._
 Inner surface of stomach magnified. The small pits are the openings from
                               the glands.

*The Stomach.*--The stomach is the largest dilatation of the alimentary
canal. It is situated in the abdominal cavity, immediately below the
diaphragm, with the larger portion toward the left side. Its connection
with the esophagus is known as the _cardiac orifice_ and its opening into
the small intestine is called the _pyloric orifice_. It varies greatly in
size in different individuals, being on the average from ten to twelve
inches at its greatest length, from four to five inches at its greatest
width, and holding from three to five pints. It has the coats common to
the canal, but these are modified somewhat to adapt them to its work.

_The mucous membrane_ of the stomach is thick and highly developed. It
contains great numbers of minute tube-shaped bodies, known as the _gastric
glands_ (Fig. 68). These are of two general kinds and secrete large
quantities of a liquid called the gastric juice. When the stomach is
empty, the mucous membrane is thrown into folds which run lengthwise over
the inner surface. These disappear, however, when the walls of the stomach
are distended with food.

_The muscular coat_ consists of _three_ separate layers which are named,
from the direction of the fibers, the circular layer, the longitudinal
layer, and the oblique layer (Fig. 69). The circular layer becomes quite
thick at the pyloric orifice, forming a distinct band which serves as a
valve.

                                [Fig. 69]

Fig. 69--*Muscles of the stomach* (from Morris' _Human Anatomy_). The layer
                     of Longitudinal fibers removed.

The outer coat of the stomach, called the _serous coat_, is a continuation
of the peritoneum, the membrane lining the abdominal cavity.

*Stomach Digestion.*--In the stomach begins the definite work of dissolving
those foods which are insoluble in water. This, as already stated, is a
double process. There is first a chemical action in which the insoluble
are changed into soluble substances, and this is followed immediately by
the dissolving action of water. The chief substances digested in the
stomach are the proteids. These, in dissolving, are changed into two
soluble substances, known as _peptones_ and _proteoses_. The digestion of
the proteids is, of course, due to the

*Gastric Juice.*--The gastric juice is a thin, colorless liquid composed of
about 99 per cent of water and about 1 per cent of other substances. The
latter are dissolved in the water and include, besides several salts,
three active chemical agents--hydrochloric acid, pepsin, and rennin.
_Pepsin_ is the enzyme which acts upon proteids, but it is able to act
only in an acid medium--a condition which is supplied by the _hydrochloric
acid_. Mixed with the hydrochloric acid it converts the proteids into
peptones and proteoses.

*Other Effects of the Gastric Juice.*--In addition to digesting proteids,
the gastric juice brings about several minor effects, as follows:

1. It checks, after a time, the digestion of the starch which was begun in
the mouth by the saliva.(58) This is due to the presence of the
hydrochloric acid, the ptyalin being unable to act in an acid medium.

2. While there is no appreciable action on the fat itself, the proteid
layers that inclose the fat particles are dissolved away (Fig. 79), and
the fat is set free. By this means the fat is broken up and prepared for a
special digestive action in the small intestine.

3. Dissolved albumin, like that in milk, is curded, or coagulated, in the
stomach. This action is due to the _rennin_. The curded mass is then acted
upon by the pepsin and hydrochloric acid in the same manner as the other
proteids.

4. The hydrochloric acid acts on certain of the insoluble mineral salts
found in the foods and reduces them to a soluble condition.

5. It is also the opinion of certain physiologists that cane sugar and
maltose (double sugars) are converted by the hydrochloric acid into
dextrose and levulose (single sugars).

After a variable length of time, the contents of the stomach is reduced to
a rather uniform and pulpy mass which is called _chyme_. Portions of this
are now passed at intervals into the small intestine.

*Muscular Action of the Stomach.*--The muscles in the walls of the stomach
have for one of their functions the mixing of the food with the gastric
juice. By _alternately_ contracting and relaxing, the different layers of
muscle keep the form of the stomach changing--a result which agitates and
mixes its contents. This action varies in different parts of the organ,
being slight or entirely absent at the cardiac end, but quite marked at
the pyloric end.

Another purpose of the muscular coat is to empty the stomach into the
small intestine. During the greater part of the digestive period the
muscular band at the pyloric orifice is contracted. At intervals, however,
this band relaxes, permitting a part of the contents of the stomach to be
forced into the small intestine. After the discharge the pyloric muscle
again contracts, and so remains until the time arrives for another
discharge.

In addition to emptying the stomach into the small intestine, these
muscles also aid in emptying the organ upward and through the esophagus
and mouth, should occasion require. Vomiting in case of poisoning, or if
the food for some reason fails to digest, is a necessary though unpleasant
operation. It is accomplished by the contraction of all the muscles of the
stomach, together with the contraction of the walls of the abdomen. During
these contractions the pyloric valve is closed, and the muscles of the
esophagus and pharynx are in a relaxed condition.(59)

                                [Fig. 70]

  Fig. 70--*Passage from stomach* into small intestine. Illustration also
 shows arrangement of mucous membrane in the two organs. _D._ Bile duct.

*The Small Intestine.*--This division of the alimentary canal consists of a
coiled tube, about twenty-two feet in length, which occupies the central,
lower portion of the abdominal cavity (Fig. 71). At its upper extremity it
connects with the pyloric end of the stomach (Fig. 70), and at its lower
end it joins the large intestine. It averages a little over an inch in
diameter, and gradually diminishes in size from the stomach to the large
intestine. The first eight or ten inches form a short curve, known as the
_duodenum_. The upper two fifths of the remainder is called the _jejunum_,
and the lower three fifths is known as the _ileum_. The ileum joins that
part of the large intestine known as the caecum, and at their place of
union is a marked constriction which prevents material from passing from
the large into the small intestine (Fig. 73). This is known as the
_ileo-caecal valve_.

_The mucous membrane_ of the small intestine is richly supplied with blood
vessels and contains glands that secrete a digestive fluid known as the
_intestinal juice_. The membrane is thrown into many transverse, or
circular, folds which increase its surface and also prevent materials from
passing too rapidly through the intestine. One important respect in which
the small intestine differs from all other portions of the food canal is
that its surface is covered with great numbers of minute elevations known
as the villi. The purpose of these is to aid in the absorption of the
nutrients as they become dissolved (Chapter XI).

_The muscular coat_ of the small intestine is made up of two distinct
layers--the inner layer consisting of circular fibers and the outer of
longitudinal fibers. These muscles keep the food materials mixed with the
juices of the small intestine, but their main purpose is to force the
materials undergoing digestion through this long and much-coiled tube.

The outer, or _serous_, coat of the small intestine, like that of the
stomach, is an extension from the general lining of the abdominal cavity,
or peritoneum. In fact, the intestine lies in a fold of the peritoneum,
somewhat as an arm in a sling, while the peritoneum, by connecting with
the back wall of the abdominal cavity, holds this great coil of digestive
tubing in place (Fig. 64). The portion of the peritoneum which attaches
the intestine to the wall of the abdomen is called the _mesentery_.

Most of the liquid acting on the food in the small intestine is supplied
by two large glands, the liver and the pancreas, that connect with it by
ducts.

                                [Fig. 71]

     Fig. 71--*Abdominal cavity* with organs of digestion in position.

*The Liver* is situated immediately below the diaphragm, on the right side
(Figs. 71 and 72), and is the largest gland in the body. It weighs about
four pounds and is separated into two main divisions, or lobes. It is
complex in structure and differs from the other glands in several
particulars. It receives blood from two distinct sources--the portal vein
and the hepatic artery. _The portal vein_ collects the blood from the
stomach, intestines, and spleen, and passes it to the liver. This blood is
loaded with food materials, but contains little or no oxygen. The _hepatic
artery_, which branches from the aorta, carries to the liver blood rich in
oxygen. In the liver the portal vein and the hepatic artery divide and
subdivide, and finally empty their blood into a single system of
capillaries surrounding the liver cells. These capillaries in turn empty
into a single system of veins which, uniting to form the _hepatic veins_
(two or three in number), pass the blood into the inferior vena cava (Fig.
72).

                                [Fig. 72]

 Fig. 72--*Relations of the liver.* Diagram showing the connection of the
          liver with the large blood vessels and the food canal.

The liver secretes daily from one to two pounds of a liquid called _bile_.
A reservoir for the bile is provided by a small, membranous sack, called
the _gall bladder_, located on the underside of the liver. The bile passes
from the gall bladder, and from the right and left lobes of the liver, by
three separate ducts. These unite to form a common tube which, uniting
with the duct from the pancreas, empties into the duodenum. Though usually
described as a digestive gland, the liver has other functions of equal or
greater importance (Chapter XIII).

*The Bile* is a golden yellow liquid, having a slightly alkaline reaction
and a very bitter taste. It consists, on the average, of about 97 per cent
of water and 3 per cent of solids.(60) The solids include bile pigments,
bile salts, a substance called cholesterine, and mineral salts. The
pigments (coloring matter) of the bile are derived from the hemoglobin of
broken-down red corpuscles (page 27).

Much about the composition of the bile is not understood. It is known,
however, to be necessary to digestion, its chief use being to aid in the
digestion and absorption of fats. It is claimed also that the bile aids
the digestive processes in some general ways--counteracting the acid of the
gastric juice, preventing the decomposition of food in the intestines, and
stimulating muscular action in the intestinal walls. No enzymes have been
discovered in the bile.

*The Pancreas* is a tapering and somewhat wedge-shaped gland, and is so
situated that its larger extremity, or head, is encircled by the duodenum.
From here the more slender portion extends across the abdominal cavity
nearly parallel to and behind the lower part of the stomach. It has a
length of six or eight inches and weighs from two to three and one half
ounces. Its secretion, the pancreatic juice, is emptied into the duodenum
by a duct which, as a rule, unites with the duct from the liver.

*The Pancreatic Juice* is a colorless and rather viscid liquid, having an
alkaline reaction. It consists of about 97.6 per cent of water and 2.4 per
cent of solids. The solids include mineral salts (the chief of which is
sodium carbonate) and four different chemical agents, or enzymes,--trypsin,
amylopsin, steapsin, and a milk-curding enzyme. These active constituents
make of the pancreatic juice the most important of the digestive fluids.
It acts with vigor on all of the nutrients insoluble in water, producing
the following changes:

1. It converts the starch into maltose, completing the work begun by the
saliva. This action is due to the _amylopsin_,(61) which is similar to
ptyalin but is more vigorous.

2. It changes proteids into peptones and proteoses, completing the work
begun by the gastric juice. This is accomplished by the _trypsin_, which
is similar to, but more active than, the pepsin.

3. It digests fat. In this work the active agent is the _steapsin_.

The necessity of a milk-curding enzyme, somewhat similar to the rennin of
the gastric juice, is not understood.

*Digestion of Fat.*--Several theories have been proposed at different times
regarding the digestion and absorption of fat. Among these, what is known
as the "solution theory" seems to have the greatest amount of evidence in
its favor. According to this theory, the fat, under the influence of the
steapsin, absorbs water and splits into two substances, recognized as
glycerine and fatty acid. This finishes the process so far as the
glycerine is concerned, as this is soluble in water; but the fatty acid,
which (from certain fats) is insoluble in water,(62) requires further
treatment. The fatty acid is now supposed to be acted on in one, or both,
of the following ways: 1. To be dissolved as fatty acid by the action of
the bile (since bile is capable of dissolving it under certain
conditions). 2. To be converted by the sodium carbonate into a form of
soap which is soluble in water.

The emulsification of fat is known to occur in the small intestine. By
this process the fat is separated into minute particles which are
suspended in water, but not changed chemically, the mixture being known as
an _emulsion_. While this is believed by some to be an actual process of
digestion, the advocates of the solution theory claim that it is a process
accompanying and aiding the conversion of fat into fatty acid and
glycerine.(63)

*The Intestinal Juice* is a clear liquid with an alkaline reaction,
containing water, mineral salts, and certain proteid substances that may
act as enzymes. It assists in bringing about an alkaline condition in the
small intestine and aids in the reduction of cane sugar and maltose to the
simple sugars, dextrose and levulose. Since it is difficult to obtain this
liquid in sufficient quantities for experimenting, its uses have not been
fully determined. Recent investigators, however, assign to it an important
place in the work of digestion.

*Work of the Small Intestine.*--The small intestine is the most important
division of the alimentary canal. It serves as a receptacle for holding
the food while it is being acted upon; it secretes the intestinal juice
and mixes the food with the digestive fluids; it propels the food toward
the large intestine; and, in addition to all this, serves as an organ of
absorption.

Digestion is practically finished in the small intestine, and a large
portion of the reduced food is here absorbed. There is always present,
however, a variable amount of material that is not digested. This,
together with a considerable volume of liquid, is passed into

*The Large Intestine.*--The large intestine is a tube from five to six feet
in length and averaging about one and one half inches in diameter. It
begins at the lower right side of the abdominal cavity, forms a coil which
almost completely surrounds the coil of small intestine, and finally
terminates at the surface of the body (Figs. 2, 71 and 73). It has three
divisions, known as the caecum, the colon, and the rectum.

                                [Fig. 73]

Fig. 73--*Passage from small into large intestine.* At the ileo-caecal valve
             is the narrowest constriction of the food canal.

_The caecum_ is the pouch-like dilatation of the large intestine which
receives the lower end of the small intestine. It measures about two and
one half inches in diameter and has extending from one side a short,
slender, and blind tube, called the _vermiform appendix_. This structure
serves no purpose in digestion, but appears to be the rudiment of an organ
which may have served a purpose at some remote period in the history of
the human race. The caecum gradually blends into the second division of the
large intestine, called the colon.

_The colon_ consists of four parts, described as the ascending colon, the
transverse colon, the descending colon, and the sigmoid flexure, or
sigmoid colon. The first three divisions are named from the direction of
the movement of materials through them and the last from its shape, which
is similar to that of the Greek letter sigma ({~GREEK CAPITAL LETTER SIGMA~}).

_The rectum_ is the last division of the large intestine It is a nearly
straight tube, from six to eight inches in length, and connects with the
external surface of the body.

The general structure of the large intestine is similar to that of the
small intestine, and, like the small intestine, it is held in place by the
peritoneum. It differs from the small intestine, however, in its lining of
mucous membrane and in the arrangement of the muscular coat. The mucous
membrane presents a smooth appearance and has no villi, while the
longitudinal layer of the muscular coat is limited to three narrow bands
that extend along the greater length of the tube (Fig. 74). These bands
are shorter than the coats, and draw the large intestine into a number of
shallow pouches, by which it is readily distinguished from the small
intestine (Fig. 71).

                                [Fig. 74]

 Fig. 74--*Section of large intestine*, showing the coats. 1. Serous coat.
  2. Circular layer of muscle. 3. Submucous coat. 4. Mucous membrane. 5.
         Muscular bands extending lengthwise over the intestine.

*Work of the Large Intestine.*--The large intestine serves as a receptacle
for the materials from the small intestine. The digestive fluids from the
small intestine continue their action here, and the dissolved materials
also continue to be absorbed. In these respects the work of the large
intestine is similar to that of the small intestine. It does, however, a
work peculiar to itself in that it collects and retains undigested food
particles, together with other wastes, and ejects them periodically from
the canal.

*Work of the Alimentary Muscles.*--The mechanical part of digestion is
performed by the muscles that encircle the food canal. Their uses, which
have already been mentioned in connection with the different organs of
digestion, may be here summarized: They supply the necessary force for
masticating the food. They propel the food through the canal. They mix the
food with the different juices. At certain places they partly or
completely close the passage until a digestive process is completed. They
may even cause a reverse movement of the food, as in vomiting. All of the
alimentary muscles, except those around the mouth, are involuntary. Their
work is of the greatest importance.

*Other Purposes of the Digestive Organs.*--The digestive organs serve other
important purposes besides that of dissolving the foods. They provide
favorable conditions for passing the dissolved material into the blood.
They dispose of such portions of the foods as fail, in the digestive
processes, to be reduced to a liquid state. A considerable amount of waste
material is also separated from the blood by the glands of digestion
(especially the liver), and this is passed from the body with the
undigested portions of food. Then the food canal (stomach in particular)
is a means of holding, or storing, food which is awaiting the processes of
digestion. Considering the number of these purposes, the digestive organs
are remarkably simple, both in structure and in method of operation.

HYGIENE OF DIGESTION

Many of the ills to which flesh is heir are due to improper methods of
taking food and are cured by observing the simple rules of eating. Habit
plays a large part in the process and children should, for this reason, be
taught early to eat properly. Since the majority of the digestive
processes are involuntary and the food, after being swallowed, is
practically beyond control, careful attention must be given to the proper
mastication of the food and to such other phases of digestion as are under
control.

*Necessity for Thorough Mastication.*--Mastication prepares the food for
the digestive processes which follow. Unless the food has been properly
masticated, the digestive fluids in the stomach and intestines cannot act
upon it to the best advantage. When the food is carefully chewed, a larger
per cent of it is actually digested--a point of importance where economy in
the use of food needs to be practiced.

A fact not to be overlooked is that one cannot eat hurriedly and practice
thorough mastication. The food must not be swallowed in lumps, but reduced
to a finely divided and pulpy mass. This requires time. The one who
hurries through the meal is necessarily compelled to bolt his food. Thirty
minutes is not too long to give to a meal, and a longer period is even
better.

Perhaps the most important result of giving plenty of time to the taking
of food is that of _stimulating the digestive glands to a proper degree of
activity_. That both the salivary and gastric glands are excited by the
sight, smell, and thought of food and, through taste, by the presence of
food in the mouth, has been fully demonstrated. Food that is thoroughly
masticated and relished will receive more saliva and gastric juice, and
probably more of other juices, than if hastily chewed and swallowed. This
has a most important bearing upon the efficiency of the digestive
processes.

*Order of Taking Food.*--There has been evolved through experience a rather
definite order of taking food, which our knowledge of the process of
digestion seems to justify. The heavy foods (proteids for the most part)
are eaten first; after which are taken starchy foods and fats; and the
meal is finished off with sweetmeats and pastry.(64) The scientific
arguments for this order are the following:

1. By receiving the first of the gastric flow the proteids can begin
digesting without delay. Since these are the main substances acted on in
the stomach, the time required for their digestion is shortened by eating
them first.

2. Sugar, being of the nature of predigested starch, quickly gets into the
blood and _satisfies the relish_ for food. The result of taking sugar
first may be to cause one to eat less than he needs and to diminish the
activity of the glands.

3. Fat or grease, if taken first, tends to form a coating over the walls
of the stomach and around the material to be digested. This prevents the
juices from getting to and mixing with the foods upon which they are to
act.

4. Starch following the proteids, for the most part, does not so quickly
come in contact with the gastric juice. This enables the ptyalin of the
saliva to continue its action for a longer time than if the starch were
eaten first.

*Liquids during the Meal.*--Liquids as ordinarily taken during the meal are
objectionable. They tend to diminish the secretion of the saliva and to
cause rapid eating. Instead of eating slowly and swallowing the food only
so fast as the glands can supply the necessary saliva, the liquid is used
to wash the food down. Water or other drinks should be taken after the
completion of the meal or when the mouth is completely free from food.
Even then it should be taken in small sips. While the taking of a small
amount of water in this way does no harm, a large volume has the effect of
weakening the gastric juice. Most of the water needed by the body should
be taken between meals.

*The State of Mind* has much to do with the proper digestion of the food.
Worry, anger, fear, and other disturbed mental states are known to check
the secretion of fluids and to interfere with the digestive processes.
While the cultivation of cheerfulness is important for its general
hygienic effects, it is of especial value in relation to digestion.
Intense emotions, either during or following the meal, should if possible
be avoided. The table is no place for settling difficulties or
administering rebuke. The conversation, on the other hand, should be
elevating and joy giving, thereby inducing a desirable reactionary
influence upon the digestive processes.

*Care of the Teeth.*--The natural teeth are indispensable for the proper
mastication of the food. Of especial value are the molars--the teeth that
grind the food. The development of the profession of dentistry has made
possible the preservation of the teeth, even when naturally poor, as long
as one has need of them. To preserve the teeth they must be kept clean.
They should be washed at least once a day with a soft-bristled brush, and
small particles of food, lodged between them, should be removed with a
wooden pick. The biting of hard substances, such as nuts, should be
avoided, on account of the danger of breaking the enamel, although the
chewing of tough substances is considered beneficial.

Decayed places in the teeth should be promptly filled by the dentist. It
is well, even when decayed places are not known to exist, to have the
teeth examined occasionally in order to detect such places before they
become large. On account of the expense, pain, and inconvenience there is
a tendency to put off dental work which one knows ought to be done.
Perhaps in no other instance is procrastination so surely punished. The
decayed places become larger and new points of decay are started; and the
pain, inconvenience, and expense are increased proportionately.

*The Natural Appetite* should be followed with reference to both the kind
and the amount of food eaten. No system of knowledge will ever be devised
which can replace the appetite as an aid in the taking of food. _It is_
_nature's means of indicating the needs of the body_. The natural appetite
may be spoiled, however, by overeating and by the use of highly seasoned
foods, or by indulging in stimulants during the meal. It is spoiled in
children by too free indulgence in sweetmeats. By cultivating the natural
appetite and heeding its suggestions, one has at his command an almost
infallible guide in the taking of food.

*Preparation of Meals.*--The cooking of food serves three important
purposes. It renders the food more digestible, relieving the organs of
unnecessary work; it destroys bacteria that may be present in the food,
diminishing the likelihood of introducing disease germs into the body; and
it makes the food more palatable, thereby supplying a necessary stimulus
to the digestive glands. While the methods employed in the preparation of
the different foods have much to do with the ease with which they are
digested and with their nourishing qualities, the scope of our subject
does not permit of a consideration of these methods.

*Quantity of Food.*--Overeating and undereating are both objectionable from
a hygienic standpoint. Overeating, by introducing an unnecessary amount of
food into the body, overworks the organs of digestion and also the organs
of excretion. It may also lead to the accumulation of burdensome fat and
of harmful wastes. On the other hand, the taking of too little food
impoverishes the blood and weakens the entire body. As a rule, however,
more people eat too much than too little, and to quit eating before the
appetite is fully satisfied is with many persons a necessary precaution.
The power of self-control, valuable in all phases of life, is
indispensable in the avoidance of overeating.

*Frequency of Taking Food.*--Eating between meals is manifestly an
unhealthful practice. The question has also been raised as to whether the
common habit of eating three times a day is best suited to all classes of
people. Many people of weak digestive organs have been benefited by the
plan of two meals a day, while others adopt the plan of eating one heavy
meal and two light ones. Either plan gives the organs of digestion more
time to rest and diminishes the liability of overeating. On the other
hand, those doing heavy muscular work can hardly derive the energy which
they need from less than three good meals a day. Though no definite rule
can be laid down, there is involved a hygienic principle which all should
follow: _Meals should not overlap_. The stomach should be free from food
taken at a previous meal before more is introduced into it. When this
principle is not observed, material ferments in the stomach, causing
indigestion and other disorders. It should be noted, however, that the
overlapping may be due to overeating as well as to eating too frequently.

*Dangers from Impure Food.*--Food is frequently the carrier of disease
germs and for this reason requires close inspection (page 128). Typhoid
fever, a most dangerous disease, is usually contracted through either
impure food or impure water (Chapter XXIII). One safeguard against disease
germs, as stated above, is thorough cooking. Too much care cannot be
exercised with reference to the water for drinking purposes. Water which
is not perfectly clear, which smells of decaying material, or which forms
a sediment on standing is usually not fit to drink. It can, however, be
rendered comparatively harmless by boiling. The objections which many
people have to drinking boiled water are removed when it is boiled the day
before it is used, so as to give it time to cool, settle, and replace the
air driven off by the boiling.

*Care of the Bowels.*--In considering the hygiene of the alimentary canal,
the fact that it is used as a means of separating the impurities from the
body must not be overlooked. Frequently, through lack of exercise,
negligence in evacuating the bowels, or other causes, a weakened condition
of the canal is induced which results in the retention of impurities
beyond the time when they should be discharged. This is a great annoyance
and at the same time a menace to the health.

In most cases this condition can be relieved, and prevented from
recurring, by observing the following habits: 1. Have a regular time each
day for evacuating the bowels. This is a most important factor in securing
the necessary movements. 2. Drink a cup of cold water on rising in the
morning and on retiring at night. 3. Eat generously of fruits and other
coarse foods, such as corn bread, oatmeal, hominy, cabbage, etc. 4.
Practice persistently such exercises as bring the abdominal muscles into
play. These exercises strengthen indirectly the muscles of the canal. 5.
Avoid overwork, especially of the nervous system.

*Alcohol and Digestion.*--Though exciting temporarily a greater flow of the
digestive fluids, alcoholic drinks taken in any but very small quantities
are considered detrimental to the work of digestion. Large doses <DW44>
the action of enzymes, inflame the mucous lining of the stomach,(65) and
bring about a diseased condition of the liver. It may be noted, however,
that the bad effects of alcoholic beverages upon the stomach, the liver,
and the body in general are less pronounced when these are taken as a part
of the regular meals.

*Effects of Tea and Coffee.*--In addition to the stimulating agent
caffeine, tea and coffee contain a bitter, astringent substance, known as
tannin. On account of the tannin these beverages tend to <DW44> digestion
and to irritate the lining of the stomach--effects that may be largely
obviated by methods of preparing tea and coffee which dissolve little of
the tannin. (They should be made without continued boiling or steeping.)
The caffeine may do harm through its stimulating effect upon the nervous
system (page 56) and through the introduction of a special waste into the
body. In chemical composition caffeine closely resembles a waste, called
uric acid, and in the body is converted into this substance. If one is in
a weakened condition, the uric acid may fail to be oxidized to urea, as
occurs normally, or to be thrown off as uric acid. In this case it
accumulates in the body, causing rheumatism and related diseases. It thus
happens that while some people may use tea and coffee without detriment,
others are injured by them.

*Summary.*--The main structure in the digestive system is the alimentary
canal. This provides cavities where important dissolving processes take
place, and tubes for joining these cavities, while glands connecting with
the canal supply the necessary liquids for changing and dissolving the
foods. The general plan of digestion is that of passing the food through
the canal, beginning with the mouth, and of acting on it at various
places, with the final result of reducing most of it to the liquid state.
The digestive fluids supply water which acts as a solvent and carries the
active chemical agents, or enzymes, that convert the insoluble foods into
substances that are soluble. The muscles in the walls of the canal perform
the mechanical work of digestion, while the nervous system controls and
regulates the activity of the various organs concerned in this work.

Exercises.--1. State the general purpose of digestion. How does digested
food differ from that not digested?

2. Name all the divisions of the alimentary canal in the order in which
the food passes through them.

3. What other work besides digestion is carried on by the alimentary
canal?

4. What is gained by the mastication of the food? Why should mastication
precede the other processes of digestion?

5. What is the work of the tongue in digestion?

6. State the purposes served by the gastric juice.

7. Give reasons for regarding the small intestine as the most important
division of the food canal.

8. At what places, and by the action of what liquids, are fats, proteids,
and starch digested?

9. What enzymes are found in the pancreatic juice? What is the digestive
action of each?

10. Describe the work performed by the muscles of the stomach, the mouth,
the esophagus, and the small intestine.

11. What advantages are derived from the use of cooked food?

12. State the advantages of drinking pure water.

13. If all the food that one needs to take at a single meal can be
thoroughly masticated in fifteen minutes, why is it better to spend a
longer time at the table?

14. What is meant by the overlapping of meals? What bad results follow?
How avoided?

PRACTICAL WORK

Examine a dissectible model of the human abdomen (Fig. 75), noting the
form, location, and connection of the different organs. Find the
connection of the esophagus with the stomach, of the stomach with the
small intestine, and of the small intestine with the large intestine.
Sketch a general outline of the cavity, and locate in this outline its
chief organs.

Where it is desirable to learn something of the actual structure of the
digestive organs, the dissection of the abdomen of some small animal is
necessary. On account of unpleasant features likely to be associated with
such a dissection, however, this work is not recommended for immature
pupils.

                                [Fig. 75]

      Fig. 75--Model for demonstrating the abdomen and its contents.

*Dissection of the Abdomen.* (Optional)--For individual study, or for a
small class, a half-grown cat is perhaps the best available material. It
should be killed with chloroform, and then stretched, back downward, on a
board, the feet being secured to hold it in place.

The teacher should make a preliminary examination of the abdomen to see
that it is in a fit condition for class study. If the bladder is
unnaturally distended, its contents may be forced out by slight pressure.
The following materials will be needed during the dissection, and should
be kept near at hand: a sharp knife with a good point, a pair of heavy
scissors, a vessel of water, some cotton or a damp sponge, and some fine
cord. During the dissection the specimen should be kept as clean as
possible, and any escaping blood should be mopped up with the cotton or
the sponge. The dissection is best carried out by observing the following
order:

1. Cut through the abdominal wall in the center of the triangular space
where the ribs converge. From here cut a slit downward to the lower
portion of the abdomen, and sideward as far as convenient. Tack the
loosened abdominal walls to the board, and proceed to study the exposed
parts. Observe the muscles in the abdominal walls, and the fold of the
_peritoneum_ which forms an apron-like covering over the intestines.

2. Observe the position of the stomach, liver, spleen, and intestines, and
then, by pushing the intestines to one side, find the kidneys and the
bladder.

3. Study the liver with reference to its location, size, shape, and color.
On the under side, find the gall bladder, from which a small tube leads to
the small intestine. Observe the portal vein as it passes into the liver.
As the liver is filled with blood, neither it nor its connecting blood
vessels should be cut at this time.

4. Trace out the continuity of the canal. Find the esophagus where it
penetrates the diaphragm and joins the stomach. Find next the union of the
stomach with the small intestine. Then, by carefully following the coils
of the small intestine, discover its union with the large intestine.

5. Within the first coil of the small intestine, as it leaves the stomach,
find the _pancreas_. Note its color, size, and branches. Find its
connection with the small intestine.

6. Beginning at the cut portion of the abdominal wall, lift the thin
lining of the peritoneum and carefully follow it toward the back and
central portion of the abdomen. Observe whether it extends back of or in
front of the kidneys, the aorta, and the inferior vena cava. Find where it
leaves the wall as a _double_ membrane, the _mesentery_, which surrounds
and holds in place the large and small intestines. Sketch a coil of the
intestine, showing the mesentery.

7. Find in the center of the coils of small intestine a long, slender body
having the appearance of a gland. This is the beginning of the _thoracic
duct_ and is called the _receptacle of the chyle_. From this the thoracic
duct rapidly narrows until it forms a tiny tube difficult to trace in a
small animal.

8. Cut away about two inches of the small intestine from the remainder,
having first tied the tube on the two sides of the section removed. Split
it open for a part of its length, and wash out its contents. Observe its
coats. Place it in a shallow vessel containing water, and examine the
mucous membrane with a lens to find the _villi_. Make a drawing of this
section, showing the coats.

9. Study the connection of the small intestine with the large. Split them
open at the place of union, wash out the contents, and examine the
ileo-caecal valve.

10. Observe the size, shape, and position of the kidneys. Do they lie in
front of or back of the peritoneum? Do they lie exactly opposite each
other? Note the connection of each kidney with the aorta and the inferior
vena cava by the renal artery and the renal vein. Find a slender tube, the
_ureter_, running from each kidney to the bladder. Do the ureters connect
with the top or with the base of the bladder? Show by a sketch the
connection of the kidneys with the large blood vessels and the bladder.

*To demonstrate the Teeth.*--Procure from the dentist a collection of
different kinds of teeth, both sound and decayed.

(_a_) Examine external surfaces of different kinds of teeth, noting
general shape, cutting or grinding surfaces, etc. Make a drawing of an
incisor and also of a molar.

(_b_) After soaking some of the teeth for a couple of days in warm water
saw one of them in two lengthwise, and another in two crosswise, and
smooth the cut surfaces with fine emery or sand paper. Examine both kinds
of sections, noting arrangement and extent of dentine, enamel, and pulp.
Make drawings.

(_c_) Examine a decayed tooth. Which substance of the tooth appears to
decay most readily? Why is it necessary to cut away a part of the tooth
before filling?

(_d_) Test the effect of acids upon the teeth by leaving a tooth over
night in a mixture of one part hydrochloric acid to four parts water, and
by leaving a second tooth for a couple of days in strong vinegar. Examine
the teeth exposed to the action of acids, noting results.

*To show the Importance of Mastication.*--Fill two tumblers each half full
of water. Into one put a lump of rock salt. Into the other place an equal
amount of salt that has been finely pulverized. Which dissolves first and
why?

*To illustrate Acid and Alkaline Reactions.*--To a tumbler half full of
water add a teaspoonful of hydrochloric or other acid, as vinegar. To a
second tumbler half full of water add an equal amount of cooking soda.
Taste each liquid, noting the sour taste of the acid, and the alkaline
taste of the soda. Hold a piece of red litmus paper in the soda solution,
noting that it is turned blue. Then hold a piece of blue litmus paper in
the acid solution, noting that it is turned red. Add acid to the soda
solution, and soda to the acid solution, until the conditions are
reversed, testing with the red and blue litmus papers.

Hold, for a minute or longer, a narrow strip of red litmus paper in the
mouth, noting any change in the color of the paper. Repeat, using blue
litmus paper. What effect, if any, has the saliva upon the color of the
papers? Has the mouth an acid or an alkaline reaction?

*To show the Action of Saliva on Starch.*--1 (Optional). Prepare starch
paste by mixing half a teaspoonful of starch in half a pint of water and
heating the mixture to boiling. Place some of this in a test tube and thin
it by adding more water. Then add a small drop of iodine solution (page
136) to the solution of starch. It should turn a deep blue color. This is
the test for starch.

Now collect from the mouth, in a clean test tube, two or three
teaspoonfuls of saliva. Add portions of this to small amounts of fresh
starch solution in two test tubes. Let the tubes stand for five or ten
minutes surrounded by water having about the temperature of the body. Test
for changes that have occurred as follows:

(_a_) To one tube add a little of the iodine solution. If it does not turn
blue, it shows that the starch has been converted into some other
substance by the saliva, (_b_) To the other tube add a few drops of a very
dilute solution of copper sulphate. Then add sodium (or potassium)
hydroxide, a few drops at a time, until the precipitate which first forms
dissolves and turns a deep blue. Then gradually heat the upper portion of
the liquid to boiling. If it turns an orange or yellowish red color, the
presence of a form of sugar (maltose or dextrose) is proved. See page 136.

2. Hold some powdered starch in the mouth until it completely dissolves
and observe that it gradually acquires a sweetish taste. This shows the
change of starch into sugar.

*To illustrate the Action of the Gastric Juice.*--Add to a tumbler two
thirds full of water as much scale pepsin (obtained from a drug store) as
will stay on the end of the large blade of a penknife. Then add enough
hydrochloric acid to give a slightly sour taste. Place in the artificial
gastric juice thus prepared some boiled white of egg which has been finely
divided by pressing it through a piece of wire gauze. Also drop in a
single large lump. Keep in a warm place (about the temperature of the
body) for several hours or a day, examining from time to time. What is the
general effect of the artificial gastric juice upon the egg?

*To illustrate Effect of Alcohol upon Gastric Digestion.*--Prepare a
tumbler half full of artificial gastric juice as in the above experiment,
and add 10 cubic centimeters of this to each of six clean test tubes
bearing labels. To five of the tubes add alcohol from a burette as
follows: (1) .5 c.c., (2) 1 c.c., (3) 1.5 c.c., (4) 2 c.c., and (5) 3
c.c., leaving one tube without alcohol. Now add to each tube about 1/4
gram of finely divided white of egg from the experiment above, and place
all of the tubes in a beaker half full of water. Keep the water a little
above the temperature of the body for several hours, examining the tubes
at intervals to note the progress of digestion. Inferences.

CHAPTER XI - ABSORPTION, STORAGE, AND ASSIMILATION

The dissolved nutrients, to reach the cells, must be transferred from the
alimentary canal to the blood stream. This process is known as
_absorption_. In general, absorption means the penetration of a liquid
into the pores of a solid, and takes place according to the simple laws of
molecular movements. The absorption of food is, however, not a simple
process, and the passage takes place through an _active_ (living)
membrane. Another difference is that certain foods undergo chemical change
while being absorbed.

*Small Intestine as an Organ of Absorption.*--While absorption may occur to
a greater or less extent along the entire length of the alimentary canal,
most of it takes place at the small intestine. Its great length, its small
diameter, and its numerous blood vessels all adapt the small intestine to
the work of absorption. The transverse folds in the mucous membrane, by
retarding the food in its passage and by increasing the absorbing surface,
also aid in the process. But of greatest importance are the minute
elevations that cover the surface of the mucous membrane, known as

*The Villi.*--Each single elevation, or villus, has a length of about one
fiftieth of an inch and a diameter about half as great (_A_, Fig. 76), and
contains the following essential parts:

1. An outer layer of epithelial cells, resting upon a connective tissue
support.

2. A small lymph tube, called a _lacteal_, which occupies the center of
the villus and connects at the base with other lymph tubes, also called
lacteals (_B_, Fig. _76_).

3. A network of capillaries.

The villi are structures especially adapted to the work of absorption, and
they are found only in the small intestine. The mucous membrane in all
parts of the canal, however, is capable of taking up some of the digested
materials.

                                [Fig. 76]

Fig. 76--*The villi.* _A._ Diagram of a small section of mucous membrane of
      small intestine. 1. Villi. 2. Small glands, called _crypts_.

 _B._ Diagram showing structure of villi. 1. Small artery. 2. Lacteal. 3.
Villus showing termination of the lacteal. 4. Villus showing capillaries.
5. Villus showing both the lacteal and the capillaries. 6. Small vein. 7.
                        Layer of epithelial cells.

*Work of Capillaries and Lacteals.*--The capillaries and lacteals act as
receivers of material as it passes through the layer of epithelial cells
covering the mucous membrane. The lacteals take up the digested fats,(66)
and the capillaries receive all the other kinds of nutrients. These
vessels do not, of course, retain the absorbed materials, but pass them
on. Their final destination is the general circulation, which they reach
by two well-defined channels, or routes.

*Routes to the Circulation.*--The two routes from the place of absorption
to the general circulation are as follows:

1. _Route taken by the Fat._--The fat is conveyed by the lacteals from the
villi to the receptacle of the chyle. At this place it mingles with the
lymph from the lower parts of the body, and with it passes through the
thoracic duct to the left subclavian vein. Here it enters the general
circulation. Thus, to reach the general circulation, the fat has to pass
through the villi, the lacteals, the receptacle of the chyle, and the
thoracic duct (Fig. 77). Its passage through these places, like the
movements in all lymph vessels, is slow, and it is only gradually admitted
to the blood stream.

                                [Fig. 77]

 Fig. 77--*Diagram of routes* from food canal to general circulation. See
                                  text.

2. _Route of All the Nutrients except Fat._--Water and salts and the
digested proteids and carbohydrates, in passing into the capillaries, mix
there with the blood. But this blood, instead of flowing directly to the
heart, is passed through the portal vein to the liver, where it enters a
_second set of capillaries_ and is brought very near the liver cells. From
the liver it is passed through the hepatic veins into the inferior vena
cava, and by these it is emptied into the right auricle. This route then
includes the capillaries in the mucous membrane of the stomach and
intestines, the branches of the portal vein, the portal vein proper, the
liver, and the hepatic veins (Fig. 77). In passing through the liver, a
large portion of the food material is temporarily retained for a purpose
and in a manner to be described later (page 177).

*Absorption Changes.*--During digestion the insoluble foods are converted
into certain soluble materials, such as peptones, maltose, and
glycerine,--the conversion being necessary to their solution. A natural
supposition is that these materials enter and become a part of the blood,
but examination shows them to be absent from this liquid. (See Composition
of the Blood, page 30.) There are present in the blood, however,
substances closely related to the peptones, maltose, glycerine, etc.;
substances which have in fact been formed from them. During their transfer
from the food canal, the dissolved nutrients undergo changes, giving rise
to the materials in the blood. Thus are the serum albumin and serum
globulin of the blood derived from the peptones and proteoses; the
dextrose, from the maltose and other forms of sugar; and the fat droplets,
from the glycerine, fatty acid, and soluble soap.

While considerable doubt exists as to the cause of these changes and as to
the places also where some of them occur, their purpose is quite apparent.
The materials forming the dissolved foods, although adapted to absorption,
are not suited to the needs of the body, and if introduced in this form
are likely to interfere with its work.(67) They are changed, therefore,
into the forms which the body can use.

*A Second Purpose of Digestion.*--Comparing the digestive changes with
those of absorption, it is found that they are of a directly opposite
nature; that while digestion is a process of tearing down, or
separating,--one which reduces the food to a more finely divided
condition--there is in absorption a process of building up. From the
comparatively simple compounds formed by digestion, there are formed
during absorption the more complex compounds of the blood. The one
exception is dextrose, which is a simple sugar; but even this is combined
in the liver and the muscles to form the more complex compound known as
glycogen. (See Methods of Storage, below.) These facts have suggested a
second purpose of digestion--that of reducing foods to forms sufficiently
simple to enable the body to construct out of them the more complex
materials that it needs. Evidence that digestion serves such a purpose is
found in the fact that both proteids and carbohydrates are reduced to a
simpler form than is necessary for dissolving them.(68)

*The Storage of Nutriment.*--For some time after the taking of a meal, food
materials are being absorbed more rapidly than they can be used by the
cells. Following this is an interval when the body is taking no food, but
during which the cells must be supplied with nourishment. It also happens
that the total amount of food absorbed during a long interval may be in
excess of the needs of the cells during that time; and it is always
possible, as in disease, that the quantity absorbed is not equal to that
consumed. To provide against emergencies, and to keep up a uniform supply
of food to the cells, it is necessary that the body store up nutrients in
excess of its needs.

*Methods of Storage.*--The general plan of storage varies with the
different nutrients as follows:

1. _The carbohydrates_ are stored in the form of _glycogen_. This, as
already stated (page 120), is a substance closely resembling starch. It is
stored in the cells of both the liver and the muscles, but mainly in the
liver (Fig. 78). It is a chief function of the liver to collect the excess
of dextrose from the blood passing through it, and to convert it into
glycogen, which it then stores within its cells. It does not, however,
separate all of the dextrose from the blood, a small amount being left for
supplying the immediate needs of the tissues. As this is used, the
glycogen in the liver is changed back to dextrose and, dissolving, again
finds its way into the blood. In this way, the amount of dextrose in the
blood is kept practically constant. The carbohydrates are stored also by
converting them into fat.

                                [Fig. 78]

  Fig. 78--*Liver cells* where is stored the glycogen. _C._ Capillaries.

                                [Fig. 79]

  Fig. 79--*Stored-up fat.* The figure shows four connective tissue cells
 containing small particles of fat. 1. Nucleus. 2. Protoplasm. 3. Fat. 4.
                        Connective tissue fibers.

2. _The fat_ is stored for the most part in the connective tissue. Certain
of the connective tissue cells have the property of taking fat from the
blood and of depositing it within their inclosing membranes (Fig. 79).
When this is done to excess, and the cells become filled with fat, they
form the so-called _adipose tissue_. Most of this tissue is found under
the skin, between the muscles, and among the organs occupying the
abdominal cavity. If one readily takes on fat, it may also collect in the
connective tissue around the heart. The stored-up fat is redissolved as
needed, and enters the blood, where it again becomes available to the
active cells.

3. _The proteids_ form a part of all the tissues, and for this reason are
stored in larger quantities than any of the other food substances. The
large amount of proteid found in the blood may also be looked upon as
storage material. The proteids in the various tissues are spoken of as
_tissue proteids_, and those in the blood as _circulating proteids_. The
proteids of the tissues serve the double purpose of forming a working part
of the cell protoplasm, and of supplying reserve food material. That they
are available for supplying energy, and are properly regarded as _storage
material_, is shown by the rapid loss of proteid in starving animals. When
the proteids are eaten in excess of the body's need for rebuilding the
tissues, they are supposed to be broken up in such a manner as to form
glycogen and fat, which may then be stored in ways already described.

*General Facts Relating to Storage.*--The form into which the food is
converted for storage in the body is that of _solids_--the form that takes
up the least amount of space. These solids are of such a nature that they
can be changed back into their former condition and, by dissolving,
reenter the blood.

Only energy-yielding foods are stored. Water and salts, though they may be
absorbed in excess of the needs of the body, are not converted into other
substances and stored away. Oxygen, as already stated (page 108), is not
stored. The interval of storage may be long or short, depending upon the
needs of the body. In the consumption of stored material the glycogen is
used first, then as a rule the fat, and last of all the proteids.

*Storage in the Food Canal.*--Not until three or four hours have elapsed
are all the nutrients, eaten at a single meal, digested and passed into
the body proper. The undigested food is held in reserve, awaiting
digestion, and is only gradually absorbed as this process takes place. It
may properly, on this account, be regarded as _stored material_. That such
storage is of advantage is shown by the observed fact that substances
which digest quickly (sugar, dextrin, "predigested foods," etc.) do not
supply the needs of the body so well as do substances which, like starch
and proteids, digest slowly. Even substances digesting quite slowly
(greasy foods and pastry), since they can be stored longer in the food
canal, may be of real advantage where, from hard work or exposure, the
body requires a large supply of energy for some time. These "stay by" the
laborer, giving him strength after the more easily digested foods have
been used up. Storage by the food canal is limited chiefly to the stomach.

*Regulation of the Food Supply to the Cells.*--The storage of food
materials is made to serve a second purpose in the plan of the body which
is even more important than that of supplying nourishment to the cells
during the intervals when no food is being taken. It is largely the means
whereby the rate of supply of materials to the cells is regulated. The
cells obtain their materials from the lymph, and the lymph is supplied
from the blood. Should food substances, such as sugar, increase in the
blood beyond a low per cent, they are converted into a form, like
glycogen, in which they are held in reserve, or, for the time being,
placed beyond the reach of the cells. When, however, the supply is
reduced, the stored-up materials reenter the blood and again become
available to the cells. By this means their rate of supply to the cells is
practically constant.

We are now in a position to understand why carbohydrates, fats, and
proteids are so well adapted to the needs of the body, while other
substances, like alcohol, which may also liberate energy, prove injurious.
It is because foods are of such a chemical nature that they are adapted in
all respects to the body plan of taking up and using materials, while the
other substances are lacking in some particular.

                                [Fig. 80]

    Fig. 80--*Diagrams illustrating the relation of nutrients* and the
 non-relation of these to alcohol. _A._ Inter-relation and convertibility
           of proteids, fats, and carbohydrates (after Hall).

_B._ Diagram showing disposition of alcohol if this substance is taken in
  quantity corresponding to that of the nutrients (F.M.W.). The alcohol
         thrown off as waste is unoxidized and yields no energy.

*Why Alcohol is not a Food.*--If the passage of alcohol through the body is
followed, it is seen, in the first place, that it is a simple liquid and
undergoes no digestive change; and in the second place, that it is rapidly
absorbed from the stomach in both weak and concentrated solutions. This
introduces it quickly into the blood, and once there, it diffuses rapidly
into the lymph and then into the cells. Since the body cannot store
alcohol or convert it into some nutrient that can be stored (Fig. 80),
_there is no way of_ _regulating the amount that shall be present in the
blood, or of supplying it to the cells as their needs require_. They must
take it in excess of their needs, regardless of the effect, at least until
the organs of excretion can throw off the surplus as waste. Compared with
proteid, carbohydrates, or fats, alcohol is an _unmanageable_ substance in
the body. Attempting to use it as a food is as foolish as trying to burn
gasolene or kerosene in an ordinary wood stove. It may be done to a
limited extent, but is an exceedingly hazardous experiment. Not being
adapted to the body method of using materials, alcohol cannot be classed
as a food.

*Assimilation.*--Digestion, absorption, circulation, and storage of foods
are the processes that finally make them available to the cells in the
different parts of the body. There still remains another process for these
materials to undergo before they serve their final purposes. This last
process, known as _assimilation_, is the appropriation of the food
material by the cell protoplasm. In a sense the storage of fat by
connective tissue cells and of glycogen by the liver cells is
assimilation. The term is limited, however, to the disposition of material
with reference to its final use. Whether all the materials used by the
cells actually become a part of the protoplasm is not known. It is known,
however, that the cells are the places where most of the oxidations of the
body occur and that materials taking part in these oxidations must, at
least, come in close contact with the protoplasm. Assimilation, then, is
the last event in a series of processes by which oxygen, food materials,
and cell protoplasm are brought into close and _active_ relations. The
steps leading up to assimilation are shown in Table II.

                  TABLE II. THE PASSAGE OF MATERIALS TO THE CELLS
MATERIALS     DIGESTION     ABSORPTION     ROUTE TO       STORAGE       CONDITION
                                           THE GENERAL                  IN THE
                                           CIRCULATION                  BLOOD
Proteids      Changed       In passing     Through the    Become a      As proteids
              into          into the       portal vein    part of the   in
              proteoses     capillaries,   to the         protoplasm    colloidal
              and           the            liver and      of all the    solution.
              peptones by   proteoses      from there     cells.
              the action    and            through the
              of the        peptones       hepatic
              gastric and   change into    veins into
              pancreatic    the            the
              juices.       proteids of    inferior
                            the blood.     vena cava.
Fat           Changed       In passing     Through the    As fat in     Chiefly as
              into fatty    into the       lacteals to    the cells     minute oil
              acid,         lacteals,      the            of            droplets.
              glycerine,    the            thoracic       collective
              and           glycerine      duct, by       tissue.
              soluable      unites with    which it is
              soap by the   the soluable   emptied
              bile and      soap and       into the
              pancreatic    fatty acid     left
              juice.        to form the    subclavian
                            oil droplets   vein.
                            of the
                            blood.
Starch        Reduced to    Enters the     Through the    As glycogen   As dextrose
              some of the   capillaries    portal         chiefly by    in
              different     as dextrose.   vein,          the liver,    solution.
              forms of                     liver,         but to some
              sugar, as                    hepatic        extent by
              maltose,                     veins, into    muscle
              dextrose,                    inferior       cells.
              etc.                         vena cava.
Water         Undergoes     Taken up by    Both           Is not        As the
              no change.    both the       routes, but    stored in     water which
                            lacteals and   mostly by      the sense     serves as a
                            capillaries,   way of the     that energy   carrier of
                            but to the     liver.         foods are.    all the
                            greater                                     other
                            extent by                                   constituents
                            the                                         of the
                            capilaries.                                 blood.
Common salt   Undergoes     Taken up by    By way of      Not stored.   In solution.
              no change.    the            portal
                            capillaries    vein,
                            without        liver, and
                            undergoing     hepatic
                            apparent       veins into
                            change.        inferior
                                           vena cava.
Oxygen                      Taken up by    Already in     Is not        United with
                            the            the general    stored.       the
                            capillaries    circulation.                 hemoglobin
                            at the                                      and to a
                            lungs.                                      small extent
                                                                        in solution
                                                                        in the
                                                                        plasma.

*Tissue Enzymes.*--The important part played by enzymes in the digestion of
the food has suggested other uses for them in the body. It has been
recently shown that many of the chemical changes in the tissues are in all
probability due to the presence of enzymes. An illustration of what a
tissue enzyme may do is seen in the changes which fat undergoes. In order
for the body to use up its reserve fat, it must be transferred from the
connective tissue cells, where it is stored, to the cells of the active
tissues where it is to be used. This requires that it be reduced to the
form of a solution and that it reenter the blood. In other words, it must
be _redigested_. For bringing about these changes a substance identical in
function with the steapsin of the pancreatic juice has been shown to exist
in several of the tissues.

Although this subject is still under investigation, it may be stated with
certainty that there are present in the tissues, enzymes that change
dextrose to glycogen and _vice versa_, that break down and build up the
proteids, and that aid in the oxidations at the cells. The necessity for
such enzymes is quite apparent.

*Summary.*--The digested nutrients are taken up by the capillaries and the
lymph vessels and transferred by two routes to the circulation. In passing
from the alimentary canal into the circulation the more important of the
foods undergo changes which adapt them to the needs of the body. Since
materials are absorbed more rapidly than they are used, means are provided
for storing them and for supplying them to the cells as their needs
require. _Capability of storage is an essential quality of energy-yielding
foods_; and substances, such as alcohol, which lack this quality are not
adapted to the needs of the body. For causing the chemical changes that
occur in the storage of foods, as well as the oxidations at the cells, the
presence of active agents, or enzymes, is necessary.

*Exercises.*--1. In what respects does the absorption of food materials
from the alimentary canal differ from the absorption of a simple liquid by
a solid?

2. In what different ways is the small intestine especially adapted to the
work of absorption?

3. What are the parts of a villus? What are the lacteals? Account for the
name.

4. What part is played by the capillaries and the lacteals in the work of
absorption? How does their work differ?

5. What changes, if any, take place in water, common salt, fat, proteids,
and carbohydrates during absorption?

6. What double purpose is served by the processes of digestion?

7. Trace the passage of proteids, fats, and carbohydrates from the small
intestine into the general circulation.

8. What is the necessity for storing nutrients in the body? Why is it not
also necessary to store up oxygen?

9. In what form and at what places is each of the principal nutrients
stored?

10. How is the rate of supply of food to the cells regulated? Why is the
body unable to regulate the supply of alcohol to the cells when this
substance is taken?

11. Explain Fig. 80, page 181. What becomes of the alcohol if this is
taken in any but very small quantities?

12. State the general purpose of enzymes in the body. Name the enzymes
found in each of the digestive fluids. What ones are found in the tissues?

PRACTICAL WORK

Illustrate the ordinary meaning of the term "absorption" by bringing the
end of a piece of crayon in contact with water, or a piece of blotting
paper in contact with ink, noting the passage of the liquid into the
crayon or the paper. Show how absorption from the food canal differs from
this kind of absorption.

Show by a diagram similar to Fig. 77 the two routes by which the foods
pass from the alimentary canal into the blood stream.

CHAPTER XII - ENERGY SUPPLY OF THE BODY

If one stops taking food, it becomes difficult after a time for him to
move about and to keep warm. These results show that food has some
relation to the energy of the body, for motion and heat are forms of
energy. The relation of oxygen to the supply of energy has already been
discussed (Chapter VIII). We are now to inquire more fully into the energy
supply of the body, and to consider those conditions which make necessary
the introduction of both food and oxygen for this purpose.

*Kinds of Bodily Energy.*--The healthy body has at any time a considerable
amount of _potential_, or reserve, energy,--energy which it is not using at
the time, but which it is able to use as its needs require. When put to
use, this energy is converted into such forms of _kinetic_ energy(69) as
are indicated by the different kinds of bodily power. These are as
follows:

1. _Power of Motion._--The body can move itself from place to place and it
can give motion to things about it.

2. _Heat Power._--The body keeps itself warm and is able to communicate
warmth to its surroundings.

3. _Nervous Power._--Through the nervous system the body exercises the
power of control over its different parts.

As motion, heat, and nervous power the body uses most of its energy.

*The Source of Bodily Energy.*--As already indicated, the energy of the
body is supplied through the food and the oxygen. These contain energy in
the potential form, which becomes kinetic (active) through their uniting
with each other in the body. Somewhat as the power of the steam engine is
derived from the combustion of fuel in the furnaces, the energy of the
body is supplied through the oxidations at the cells. How the food and
oxygen come to possess energy is seen by a study of the general methods by
which energy is stored up and used.

                                [Fig. 81]

       Fig. 81--*Simple device* for storing energy through gravity.

*Simple Methods of Storing Energy.*--Energy is stored by converting the
kinetic into the potential form. Two of the simplest ways of doing this
are the following:

1. _Storing of Energy through Gravity._--On account of the attraction
between the earth and all bodies upon the earth, the mere lifting of a
weight puts it in a position where gravity can cause it to move (Fig. 81).
As a consequence _the raising of bodies above the earth's surface is a
means of storing energy_--the energy remaining stored until the bodies
fall. As they fall, the stored-up (potential) energy becomes kinetic and
can be made to do work.

2. _Storing of Energy through Elasticity._--Energy is stored also by doing
work in opposition to elasticity, as in bending a bow or in winding a
clock spring. The bending, twisting, stretching, or compressing of elastic
substances puts them in a condition of _strain_ which causes them to exert
a pressure (called elastic force) that tends to restore them to their
former condition. Energy stored by this means becomes active as the
distorted or compressed substance returns to its former shape or volume.

These simple methods of storing energy will serve to illustrate the
general principles upon which such storage depends:

1. To store energy, energy must be expended, or work done.

2. The work must be against some force, such as gravity or elasticity,
which can undo the work, i.e., bring about an effect opposite to that of
the work.

3. The stored energy becomes active (kinetic) as the force through which
the energy was stored undoes the work, or puts the substance upon which
the work was done into its former condition (gravity causing bodies to
fall, etc.).

These principles are further illustrated by the

*Storing of Energy through Chemical Means.*--A good example of storing
energy by chemical means is that of decomposing water with electricity. If
a current of electricity is passed through acidulated water in a suitable
apparatus (Fig. 82), the water separates into its component gases, oxygen
and hydrogen. These gases now have power (energy) which they did not
possess before they were separated. The hydrogen will burn in the oxygen,
giving heat; and if the two gases are mixed in the right proportions and
then ignited, they explode with violence. This energy was derived from the
electricity. It was stored by _decomposing_ the water.

                                [Fig. 82]

  Fig. 82--*Storing energy by chemical means.* Apparatus for decomposing
                         water with electricity.

Energy is stored by chemical means by causing it to do work in opposition
to the force of chemism, or chemical affinity. Instead of changing the
form of bodies or moving them against gravity, it overcomes the force that
causes atoms to unite and to hold together after they have united. Since
in most cases the atoms on separating from any given combination unite at
once to form other combinations, we may say that _energy is stored when
strong chemical combinations are broken up and weak ones formed_. Energy
stored by this means becomes active when the atoms of weak combinations
unite to form combinations that are strong.(70)

*How Plants store the Sun's Energy.*--The earth's supply of energy comes
from the sun. While much of this, after warming and lighting the earth's
surface, is lost by radiation, a portion of it is stored up and retained.
The sun's energy is stored both through the force of gravity(71) and by
chemical means, the latter being the more important of the two methods.
Plants supply the means for storing it chemically (Fig. 83). Attention has
already been called to the fact (page 112) that growing plants are
continually taking carbon dioxide into their leaves from the air. This
they decompose, adding the carbon to compounds in their tissues and
returning the oxygen to the air. It is found, however, that this process
does not occur unless the plants are exposed to sunlight. The sunlight
supplies the energy for overcoming the attraction between the atoms of
oxygen and the atoms of carbon, while the plant itself serves as the
instrument through which the sunlight acts. The energy for decomposing the
carbon dioxide then comes from the sun, and through the decomposition of
the carbon dioxide the sun's energy is stored--becomes potential. It
remains stored until the carbon of the plant again unites with the oxygen
of the air, as in combustion.

                                [Fig. 83]

   Fig. 83--*Nature's device* for storing energy from the sun. See text.

*The Sun's Energy in Food and Oxygen.*--Food is derived directly or
indirectly from plants and sustains the same relation to the oxygen of the
air as do the plants themselves. (The elements in the food have an
attraction for the oxygen, but are separated chemically from it.) On
account of this relation they have potential energy--the energy derived
through the plant from the sun. When a person eats the food and breathes
the oxygen, this energy becomes the possession of the body. It is then
converted into kinetic energy as the needs of the body require.

                                [Fig. 84]

  Fig. 84--*Simple apparatus* for illustrating transformation of energy.
      Potential energy is converted into heat and heat into motion.

*From the Sun to the Cells.*--It thus appears that the body comes into
possession of energy, and is able to use it, through a series of
transferences and transformations that can be traced back to the sun.(72)
Coming to the earth as kinetic energy, it is transformed into potential
energy and stored in the compounds of plants and in the oxygen of the air.
Through the food and the oxygen the potential energy is transferred to the
cells of the body. Then by the uniting of the food and the oxygen at the
cells (oxidation), the potential becomes kinetic energy and is used by the
body in doing its work. The phrase "Child of the Sun" has sometimes been
applied to man to express his dependence upon the sun for his supply of
energy.

*Why Oxygen and Food are Both Necessary.*--The necessity for introducing
both oxygen and food into the body for the purpose of supplying energy is
now apparent. The energy which is used in the body is not the energy of
food alone. Nor is it the energy of oxygen alone. It belongs to both. It
is due to their attraction for each other and their condition of
separation. It cannot, therefore, become kinetic except through their
union. To introduce one of these substances into the body without the
other, would neither introduce the energy nor set it free. They must both
be introduced into the body and there caused to unite.

*Bodily Control of Energy.*--A fact of importance in the supply of energy
to the body is that the rate of transformation (changing of potential to
kinetic) is just sufficient for its needs. It is easily seen that too
rapid or too slow a rate would prove injurious. The oxidations at the
cells are, therefore, under such control that the quantity of kinetic
energy supplied to the body as a whole, and to the different organs, is
proportional to the work that is done. This is attained, in part at least,
through the ability of the body to store up the food materials and hold
them in reserve until they are to be oxidized (page 180).

*Animal Heat and Motion.*--Most of the body's energy is expended as heat in
keeping warm. It is estimated that as much as five sixths of the whole
amount is used in this way. The proportion, however, varies with different
persons and is not constant in the same individual during different
seasons of the year. This heat is used in keeping the body at that
temperature which is best suited to carrying on the vital processes. All
parts of the body, through oxidation, furnish heat. Active organs,
however, such as the muscles, the brain, and the glands (especially the
liver), furnish the larger share. The blood in its circulation serves as a
_heat distributer_ for the body and keeps the temperature about the same
in all its parts (page 33).

Next to the production of heat, in the consumption of the body's energy,
is the production of motion. This topic will be considered in the study of
the muscular system (Chapter XV).

*Some Questions of Hygiene.*--The heat-producing capacity of the body
sustains a very important relation to the general health. A sudden chill
may result in a number of derangements and is supposed to be a
predisposing cause of _colds_. One's capacity for producing heat may be so
low that he is unable to respond to a sudden demand for heat, as in going
from a warm room into a cold one. As a consequence, the body is unable to
protect itself against unavoidable exposures.

_Impairment of the heat-producing capacity_ is brought about in many ways.
Several diseases do this directly, or indirectly, to quite an extent. In
health too great care in protecting the body from cold is the most potent
cause of its impairment. Staying in rooms heated above a temperature of
70 deg. F., wearing clothing unnecessarily heavy, and sleeping under an excess
of bed clothes, all diminish the power of the body to produce heat. They
accustom it to producing only a small amount, so that it does not receive
sufficient of what might be called _heat-producing exercise_. Lack of
physical exercise in the open air, as well as too much time spent in
poorly lighted and ventilated rooms, tends also to reduce one's ability to
produce heat. Moreover, since most of the heat of the body comes from the
union of oxygen and food materials at the cells, a lack of either of these
will interfere with the production of heat.

*Results of Exhaustion.*--Through overwork, or excesses in pleasurable
pursuits, one may make greater demands upon the energy of his body than it
can properly supply. The resulting condition, known as _exhaustion_, is
not only a matter of temporary inconvenience, but may through repetition
lead to a serious impairment of the health. It should be noted, in this
connection, that the energy of the body is spent in two general ways:
first, in carrying on the vital processes; and second, in the performance
of voluntary activities. Since, in all cases, there is a limit to one's
energy, it is easily possible to expend so much in the voluntary
activities that the amount left is not sufficient for the vital processes.
This leads to various disturbances and, among other things, renders the
body less able to supply itself with energy.

*The Problem of Increasing One's Energy.*--Since the energy supply is kept
up through the food and the oxygen, it might be inferred that the
introduction of these substances into the body in larger amounts would
increase the energy at one's disposal. This does not necessarily follow.
Oxidation at the cells is preceded by digestion, absorption, circulation,
and assimilation. It is followed and influenced by the removal of wastes
from the body. A careful study of the problem leads to the conclusion that
while the energy supply to the body does depend upon the introduction of
the proper amounts of food and oxygen, it also depends upon the efficiency
of the vital processes. The maximum amount of energy may, therefore, be
expected when the body is in a condition of perfect health. Hence, one
desiring to increase the amount of his energy must give attention to all
those conditions that improve the health.

*Effect of Stimulants on the Energy Supply.*--In the effort to get out of
the body as much as possible of work or of pleasure, various stimulants,
such as alcohol, tobacco, and strong tea and coffee, have been used.
Though these have the effect of giving a temporary feeling of strength and
of enabling the individual in some instances to accomplish results which
he could not otherwise have brought about, the general effect of their use
is to lessen, rather than to increase, the sum total of bodily power. The
student, for example, who drinks strong coffee in order to study late at
night is able to command less energy on the day following. While enabling
him to draw upon his reserve of nervous power for the time being, the
coffee deprives him of sleep and needed rest.

The danger of stimulants, so far as energy is concerned, is this: they
tend to exhaust the bodily reserve so that there is not sufficient left
for properly running the vital processes. Evidences of their weakening
effect are found in the feeling of discomfort and lassitude which result
when stimulants to which the body has become accustomed are withdrawn. Not
until one gets back his bodily reserve is he able to work normally and
effectively. Increase in bodily energy comes through health and not
through the use of stimulants.

*Summary.*--The body requires a continuous supply of energy. To obtain this
supply, materials possessing potential, or stored-up, energy are
introduced into it. The free oxygen of the air and the substances known as
foods, on account of the chemical relations which they sustain to each
other, contain potential energy and are utilized for supplying the body.
So long as the foods are not oxidized, the energy remains in the potential
form, but in the process of oxidation the potential energy is changed to
kinetic energy and made to do the work of the body.

*Exercises.*--1. In what different ways does the body use energy?

2. Show that a stone lying against the earth has no energy, while the same
stone above the earth has energy.

3. How does potential energy differ from kinetic energy?

4. What kind of energy is possessed by a bent bow? By a revolving wheel?
By a coiled spring? By the wind? By gunpowder?

5. How does decomposing water with electricity store energy?

6. Account for the energy possessed by the oxygen of the air and food
substances.

7. Trace the energy supply of the body back to the sun.

8. Why must both oxygen and food be introduced into the body in order to
supply it with energy?

9. How may overwork and overexercise diminish the energy supply of the
body?

10. How may one increase the amount of his energy?

PRACTICAL WORK

*Suggested Experiments.*--1. The change of kinetic into potential energy
may be shown by stretching a piece of rubber, by lifting a weight, and by
separating the armature from a magnet.

2. The change of potential into kinetic energy may be shown by letting
weights fall to the ground, by releasing the end of a piece of stretched
rubber, and by burning substances.

3. The change of one form of kinetic energy to another may be illustrated
by rubbing together two pieces of wood until they are heated, by ringing a
bell, and by causing motion in air or in water by heating them. If
suitable apparatus is at hand, the transformation of electrical energy
into heat, light, sound, and mechanical motion can easily be shown.

4. A weight connected by a cord with some small machine and made to run
it, will help the pupil to grasp the general principles in the storage of
energy through gravity. A vessel of water on a high support from which the
water is siphoned on to a small water wheel will serve the same purpose.

5. The storing of energy by chemical means may be illustrated by
decomposing potassium chlorate with heat or by decomposing water by means
of a current of electricity.

6. Study the transfer of energy from the body to surrounding objects, as
in moving substances and lifting weights.

Fill a half gallon jar two thirds full of water and carefully take the
temperature with a chemical thermometer. Hold the hand in the water for
four or five minutes and take the temperature again. Inference.

CHAPTER XIII - GLANDS AND THE WORK OF EXCRETION

In our study so far we have been concerned mainly with the introduction of
materials into the body. We are now to consider the removal of materials
from the body. The structures most directly concerned in this work are
known as

*Glands.*--As generally understood, glands are organs that prepare special
liquids in the body and pour them out upon free surfaces. These liquids,
known as _secretions_, are used for protecting exposed parts, lubricating
surfaces that rub against each other, digesting food, and for other
purposes. They differ widely in properties as well as in function, but are
all alike in being composed chiefly of water. The water, in addition to
being necessary to the work of particular fluids, serves in all cases as a
carrier of solid substances which are dissolved in it.

*General Structure of Glands.*--While the various glands differ greatly in
size, form, and purpose, they present striking similarities in structure.
All glands contain the following parts:

1. Gland, or secreting, cells. These are _specialized_ cells for the work
of secretion and are the active agents in the work of the gland. They are
usually cubical in shape.

2. A basement membrane. This is a thin, connective tissue support upon
which the secreting cells rest.

3. A network of capillary and lymph vessels. These penetrate the tissues
immediately beneath the secreting cells.

4. A system of nerve fibers which terminate in the secreting cells and in
the walls of the blood vessels passing to the glands.

These structures--secreting cells, basement membrane, capillary and lymph
vessels, and nerve fibers--form the essential parts of all glands. The
capillaries and the lymph vessels supply the secreting cells with fluid,
and the nerves control their activities.

*Kinds of Glands.*--Glands differ from one another chiefly in the
arrangement of their essential parts.(73) The most common plan is that of
arranging the parts around a central cavity formed by the folding or
pitting of an exposed surface. Many such glands are found in the mucous
membrane, especially that lining the alimentary canal, and are most
numerous in the stomach, where they supply the gastric juice. If these
glands have the general form of tubes, they are called _tubular_ glands;
if sac-like in shape, they are called _saccular_ glands. Both the tubular
and the saccular glands may, by branching, form a great number of similar
divisions which are connected with one another, and which communicate by a
common opening with the place where the secretion is used. This forms a
_compound_ gland which, depending on the structure of the minute parts,
may be either a _compound tubular_ or a _compound saccular_ gland. The
larger of the compound saccular glands are also called _racemose_ glands,
on account of their having the general form of a cluster, or raceme,
similar to that of a bunch of grapes. The general structure of the
different kinds of glands is shown in Fig. 85.

                                [Fig. 85]

Fig. 85--*Diagram illustrating evolution of glands.* _A._ Simple secreting
surface. 1. Gland cells. 2. Basement membrane. 3. Blood vessel. 4. Nerve.
   _B._ Simple tubular gland. _C._ Simple saccular gland. _D._ Compound
  tubular gland. _E._ Compound saccular gland. _F._ A compound racemose
gland with duct passing to a free surface. _G._ Relation of food canal to
   different forms of glands. The serous coat has a secreting surface.

*Nature of the Secretory Process.*--At one time the gland was regarded
merely as a kind of filter which separated from the blood the ingredients
found in its secretions. Recent study, however, of several facts relating
to secretion has led to important modifications of this view. The
secretions of many glands are known to contain substances that are not
found in the blood, or, if present, are there in exceedingly small
amounts. Then again the cells of certain glands have been found to undergo
marked changes during the process of secretion. If, for example, the cells
of the pancreas be examined after a period of rest, they are found to
contain small granular bodies. On the other hand, if they are examined
after a period of activity, the granules have disappeared and the cells
themselves have become smaller (Fig. 86). The granules have no doubt been
used up in forming the secretion. These and other facts have led to the
conclusion that secretion is, in part, the separation of materials without
change from the blood, and, in part, a process by which special substances
are prepared and added to the secretion. According to this view the gland
plays the double role of a _filtering apparatus_ and of a _manufacturing
organ_.

                                [Fig. 86]

Fig. 86--*Secreting cells from the pancreas* (after Langley). _A._ After a
 period of rest. _B._ After a short period of activity. C. After a period
  of prolonged activity. In _A_ and _B_ the nuclei are concealed by the
           granules that accumulate during the resting period.

*Kinds of Secretion.*--In a general way all the liquids produced by glands
may be considered as belonging to one or the other of two classes, known
as the _useful_ and the _useless_ secretions. To the first class belong
all the secretions that serve some purpose in the body, while the second
includes all those liquids that are separated as waste from the blood. The
first are usually called _true secretions_, or secretions proper, while
the second are called _excretions_. The most important glands producing
liquids of the first class are those of digestion (Chapter X).

*Excretory Work of Glands.*--The process of removing wastes from the body
is called _excretion_. While in theory excretion may be regarded as a
distinct physiological act, it is, in fact, leaving out the work of the
lungs, but a phase of the work of glands. From the cells where they are
formed, the waste materials pass into the lymph and from the lymph they
find their way into the blood. They are removed from the blood by glands
and then passed to the exterior of the body.

*The Necessity for Excretion* is found in the results attending oxidation
and other chemical changes at the cells (page 107). Through these changes
large quantities of materials are produced that can no longer take any
part in the vital processes. They correspond to the ashes and gases of
ordinary combustion and form wastes that must be removed. The most
important of these substances, as already noted (page 110), are carbon
dioxide, water, and urea.(74) A number of mineral salts are also to be
included with the waste materials. Some of these are formed in the body,
while others, like common salt, enter as a part of the food. They are
solids, but, like the urea, leave the body dissolved in water.

Waste products, if left in the body, interfere with its work (some of,
them being poisons), and if allowed to accumulate, cause death. Their
removal, therefore, is as important as the introduction of food and oxygen
into the body. The most important of the excretory glands are

*The Kidneys.*--The kidneys are two bean-shaped glands, situated in the
back and upper portion of the abdominal cavity, one on each side of the
spinal column. They weigh from four to six ounces each, and lie between
the abdominal wall and the peritoneum. Two large arteries from the aorta,
called the _renal arteries_, supply them with blood, and they are
connected with the inferior vena cava by the _renal veins_. They remove
from the blood an exceedingly complex liquid, called the _urine_, the
principal constituents of which are water, salts of different kinds,
coloring matter, and urea. The kidneys pass their secretion by two slender
tubes, the _ureters_, to a reservoir called the _bladder_ (Fig. 87).

                                [Fig. 87]

   Fig. 87--*Relations of the kidneys.* (Back view.) 1. The kidneys. 2.
 Ureters. 3. Bladder. 4. Aorta. 5. Inferior vena cava. 6. Renal arteries.
                             7. Renal veins.

*Structure of the Kidneys.*--Each kidney is a compound tubular gland and is
composed chiefly of the parts concerned in secretion. The ureter serves as
a duct for removing the secretion, while the blood supplies the materials
from which the secretion is formed. On making a longitudinal section of
the kidney, the upper end of the ureter is found to expand into a
basin-like enlargement which is embedded in the concave side of the
kidney. The cavity within this enlargement is called the _pelvis of the
kidney_, and into it project a number of cone-shaped elevations from the
kidney substance, called the _pyramids_ (Fig. 88).

From the summits of the pyramids extend great numbers of very small tubes
which, by branching, penetrate to all parts of the kidneys. These are the
_uriniferous tubules_, and they have their beginnings at the outer margin
of the kidney in many small, rounded bodies called the _Malpighian
capsules_ (_A_, Fig. 88). Each capsule incloses a cluster of looped
capillaries and connects with a single tubule (Fig. 89). From the capsule
the tubule extends toward the concave side of the kidney and, after
uniting with similar tubules from other parts, finally terminates at the
pyramid. Between its origin and termination, however, are several
convolutions and one or more loops or turns. After passing a distance many
times greater than from the surface to the center of the kidney, the
tubule empties its contents into the expanded portion of the ureter.

                                [Fig. 88]

    Fig. 88--*Sectional view of kidney.* 1. Outer portion or cortex. 2.
 Medullary portion. 3. Pyramids. 4. Pelvis. 5. Ureter. _A._ Small section
   enlarged to show the tubules and their connection with the capsules.

                                [Fig. 89]

Fig. 89--*Malpighian capsule* highly magnified (Landois). _a._ Small artery
  entering capsule and forming cluster of capillaries within. _e._ Small
vein leaving capsule and branching into _c_, a second set of capillaries,
                  _h._ Beginning of uriniferous tubule.

The uriniferous tubules are lined with secreting cells. These differ
greatly at different places, but they all rest upon a basement membrane
and are well supplied with capillaries. These cells provide one means of
separating wastes from the blood (Fig. 90).

                                [Fig. 90]

  Fig. 90--*Diagram illustrating renal circulation.* 1. Branch from renal
artery. 2. Branch from renal vein. 3. Small artery branches, one of which
  enters a Malpighian capsule (5). 6. Small vein leaving the capsule and
branching into the capillaries (7) which surround the uriniferous tubules.
4. Small veins which receive blood from the second set of capillaries. 8.
                Tubule showing lining of secreting cells.

*Blood Supply to the Kidneys.*--The method by which the kidneys do their
work is suggested by the way in which the blood circulates through them.
The renal artery entering each kidney divides into four branches and these
send smaller divisions to all parts of the kidney. At the outer margin of
the kidney, called the _cortex_, the blood is passed through _two sets of
capillaries_. The first forms the clusters in the Malpighian capsules and
receives the blood directly from the smallest arteries. The second forms a
network around the uriniferous tubules and receives the blood which has
passed from the capillary clusters into a system of small veins (Fig. 90).
From the last set of capillaries the blood is passed into veins which
leave the kidneys where the artery branches enter, uniting there to form
the main renal veins.

*Work of the Kidneys.*--Why should the blood pass through two systems of
capillaries in the kidneys? This is because the separation of waste is
done in part by the Malpighian capsules and in part by the uriniferous
tubules. Water and salts are removed chiefly at the capsules, while the
remaining solid constituents of the urine pass through the secreting cells
that line the tubules. It was formerly believed that the kidneys obtained
their secretion by a process of filtration from the blood, but this belief
has been gradually modified. The prevailing view now is that the processes
of filtration and secretion are both carried on by the kidneys,--that the
capillary clusters in the Malpighian bodies serve as delicate filters for
the separation of water and salts, while the secreting cells of the
tubules separate substances by the process of secretion.

On account of the large volume of blood passing through the kidneys this
liquid is still a bright red color as it flows into the renal veins (Fig.
90). The kidney cells require oxygen, but the amount which they remove
from the blood is not sufficient to affect its color noticeably. The blood
in the renal veins, having given up most of its impurities and still
retaining its oxygen, is considered the purest blood in the body.

*Urea* is the most abundant solid constituent of the urine and is the
chief waste product arising from the oxidation of nitrogenous substances
in the body. Although secreted by the cells lining the uriniferous
tubules, it is not formed in the kidneys. The secreting cells simply
separate it from the blood where it already exists. The muscles also have
been suggested as a likely source of urea, for here the proteids are
broken down in largest quantities; but the muscles produce little if any
urea. Its production has been found to be the _work of the liver_. In the
muscular tissue, and in the other tissues as well, the proteids are
reduced to a lower order of compounds, such as the compounds of ammonia,
which pass into the blood and are then taken up by the liver. By the
action of the liver cells these are converted into urea and this is turned
back into the blood. From the blood the urea is separated by the secreting
cells of the kidneys.

*Work of the Liver.*--The liver, already described as an organ of digestion
(page 152), assists in the work of excretion both by changing waste
nitrogenous compounds into urea and by removing from the blood the wastes
found in the bile. While the chief work of the liver is perhaps not that
of excretion, its functions may here be summarized. The liver is, first of
all, a _manufacturing organ_, producing, as we have seen, three distinct
products--bile, glycogen, and urea. On account of the nature of the urea
and the bile, the liver is properly classed as an _excretory organ_; but
in the formation of the glycogen it plays the part of a _storage organ_.
Then, on account of the use made of the bile after it is passed into the
food canal, the liver is also classed as a _digestive organ_. These
different functions make of the liver an organ of the first importance.

*Excretory Work of the Food Canal.*--The glands connected with the food
canal, other than the liver, while secreting liquids that aid in
digestion, also separate waste materials from the blood. These are passed
into the canal, whence they leave the body with the undigested portions of
the food and the waste from the liver. Though the nature and quantity of
the materials removed by these glands have not been fully determined,
recent investigations have tended to enhance the importance attached to
this mode of excretion.

*The Perspiratory Glands.*--The perspiratory, or sweat, glands are located
in the skin. They belong to the type of simple tubular glands and are very
numerous over the entire surface of the body. A typical sweat gland
consists of a tube which, starting at the surface of the cuticle,
penetrates to the under portion of the true skin and there forms a
ball-shaped coil. The coiled extremity, which forms the secreting portion,
is lined with secreting cells and surrounded by a network of capillaries.
The portion of the tube passing from the coil to the surface serves as a
duct (Figs. 91 and 121).

                                [Fig. 91]

 Fig. 91--*Diagram of section through a sweat gland.* _a._ Outer layer of
 skin or cuticle. _b._ Dermis or true skin. _d, e._ Sections of the tube
forming the coiled portion of the gland. _c._ Duct passing to the surface.
               The other structures of the skin not shown.

The sweat glands secrete a thin, colorless fluid, called _perspiration_,
or sweat. This consists chiefly of water, but contains a small per cent of
salts and of urea. The excretory work of these glands seems not to be so
great as was formerly supposed, but they supplement in a practical way the
work of the kidneys and, during diseases of these organs, show an increase
in excretory function to a marked degree. The perspiration also aids in
the regulation of the temperature of the body (Chapter XVI).

*Excretory Work of the Lungs.*--While the lungs cannot be regarded as
glands, they do a work in the removal of waste from the body which must be
considered in the general process of excretion. They are especially
adapted to the removal of gaseous substances from the blood, and it is
through them that most of the carbon dioxide leaves the body. The lungs
remove also a considerable quantity of water. This is of course in the
gaseous form, being known as water vapor.

*Ductless Glands and Internal Secretion.*--Midway in function between the
glands that secrete useful liquids and those that remove waste materials
from the blood is a class of bodies, found at various places, known as the
_ductless glands._ They are so named from their having the general form of
glands and from the fact that they have no external openings or ducts.
They prepare special materials which are passed into the blood and which
are supposed to exert some beneficial effect either upon the blood or upon
the tissues through which the blood circulates. The most important of the
ductless glands are the thyroid gland, located in the neck; the suprarenal
bodies, situated one just over each kidney; and the thymus gland, a
temporary gland in the upper part of the chest. The spleen and the
lymphatic glands (page 68) are also classed with the ductless glands. The
liver, the pancreas, and (according to some authorities) the kidneys, in
addition to their external secretions, produce materials that pass into
the blood. They perform in this way a function like that of the ductless
glands. The work of glands in preparing substances that enter the blood is
known as _internal secretion._

*Quantity of Excretory Products.*--If the weight of the normal body be
taken at intervals, after growth has been attained, there will be found to
be practically no gain or loss from time to time. This shows that
materials are leaving the body as fast as they enter and that the tissues
are being torn down as fast as they are built up. It also shows that
substances do not remain in the body _permanently_, but only so long
perhaps as is necessary for them to give up their energy, or serve some
additional purpose in the ever changing protoplasm. The excretory organs
then remove from the body a quantity of material that is equal in weight
to the materials absorbed by the organs of digestion and respiration. This
is estimated for the average individual to be about five pounds daily. The
passage of waste from the body is summarized in Table III.

      TABLE III. THE PASSAGE OF WASTE MATERIALS FROM THE BODY
Materials   State       How Formed       Condition in   How Removed
                        in the Body      the Blood      from the
                                                        Blood
Carbon      Gas         By the           Dissolved in   Separated
dioxide                 oxidation of     the plasma     from the
                        the carbon       and in loose   blood at the
                        of proteids,     combination    alveoli of
                        carbohydrates,   with salts     the lungs
                        and fats.        in the         and then
                                         blood.         forced
                                                        through the
                                                        air passages
                                                        into the
                                                        atmosphere.
Urea        Solid       By the           Dissolved in   Removed by
                        oxidation in     the plasma.    the
                        the liver of                    uriniferous
                        nitrogenous                     tubules of
                        compounds.                      the kidneys
                                                        and to a
                                                        small extent
                                                        by the
                                                        perspiratory
                                                        glands.
Water       Liquid      By the           As water.      Removed by
                        oxidation of                    all the
                        the hydrogen                    organs of
                        of proteids,                    excretion,
                        carbohydrates,                  but in the
                        and fats.                       largest
                        Amount formed                   quantities
                        in the body is                  by the
                        small.                          kidneys and
                                                        the skin.
Salts       Solid                        Dissolved in   By the
                                         the plasma.    kidneys,
                                                        liver, and
                                                        skin.

HYGIENE

The separation of wastes from the body has such a close relation to the
health that all conditions affecting it should receive the most careful
attention. Their retention beyond the time when they should be discharged
undoubtedly does harm and is the cause of many bodily disorders.

*Value of Water.*--As a rule the work of excretion is aided by drinking
_freely_ of pure water. As water is the natural dissolver and transporter
of materials in the body, it is generally conceded by hygienists and
physicians that the taking of plenty of water is a healthful practice.
People do not as a rule drink a sufficient amount of water, about three
pints per day being required by the average adult, in addition to that
contained in the food. Most of the water should, of course, be taken
between meals, although the sipping of a small amount during meals does
not interfere with digestion. As stated elsewhere, the taking of a cup of
water on retiring at night and again on rising in the morning is very
generally recommended.

*Protection of Kidneys and Liver.*--The kidneys and liver are closely
related in their work and in many instances are injured or benefited by
the same causes. Both, as already stated (page 124), are liable to injury
from an _excess of proteid food_, especially meats, and also by a
condition of inactivity of the bowels (page 166). The free use of alcohol
also has an injurious effect on both of these organs.(75) On the other
hand, increasing the activity of the skin has a beneficial effect upon
them, especially the kidneys. Exercise and bathing, which tend to make the
skin more active, are valuable aids both in ridding the body of impurities
and in lessening the work of the other excretory organs. One having a
disease of the kidneys, however, needs to exercise great care in bathing
on account of the bad results which follow getting chilled.

*Special Care after Certain Diseases.*--Certain diseases, as measles,
diphtheria, scarlet fever, and typhoid fever, sometimes have the effect of
weakening the kidneys (and other vital organs) and of starting disease in
them. When this occurs it is usually the result of exposure or of
over-exertion while the body is in a weakened condition. Severe chilling
at such a time, by driving blood from the surface to the parts within,
often causes inflammation of the kidneys. On recovering from any wasting
disease one should exercise great caution both in resuming his regular
work and in exposing his body to wet or cold.

*Misunderstood Symptoms.*--Pains in the small of the back, an increase in
the secretions of the kidneys, and a sediment in the urine very naturally
suggest some disorder of the kidneys. It is a fact, however, that these
symptoms have little or no relation to the state of the kidneys and may
occur when the kidneys are in a perfectly healthy condition. The kidneys
are not located in the small of the back, but above this place, so that
pains in this region are evidently not from the kidneys, while the
increase in the flow of the urine may arise from a number of causes, one
of which is an increase of certain waste products passed into the blood.
The symptoms referred to are frequently the results of nervous exhaustion,
resulting from overstudy, worry, eye strain, or some other condition that
overtaxes the nervous system. When this is the case, relief is obtained
through resting the nerves. Actual disease of the kidneys can only be
determined through a chemical and microscopic examination of the urine. To
resort to some patent medicine for kidney trouble without knowing that
such trouble exists, as is sometimes done, is both foolish and unhygienic.

*Alcoholic Beverages and the Elimination of Waste.*--Causing as it does
such serious diseases as cirrhosis of the liver and Bright's disease of
the kidneys (footnote, page 210), alcohol will greatly interfere in this
way with the elimination of waste. There is also evidence to the effect
that it interferes with waste elimination before the stage is reached of
causing disease of these organs. Researches have shown that alcohol
increases the amount of uric acid in the body and decreases the amount of
urea found in the urine. The conclusion to be drawn is that alcohol
interferes in some way with the change of the harmful uric acid into the
comparatively harmless urea--an interference which in some instances
results in great harm. It has also been shown that malted liquors, such as
beer and ale, contain substances which, like the caffein of tea and coffee
(page 167), are readily converted into uric acid.(76) Wines contain acids
which may also act injuriously. The harm which such substances do is, of
course, additional to that caused by the alcohol.

*Summary.*--As a result of the oxidations and other changes at the cells,
substances are produced that can no longer serve a purpose in the body.
They are of the nature of waste, and their continuous removal from the
body is as necessary to the maintenance of life as the introduction of
food and oxygen. The organs whose work it is to remove the waste,
excepting the lungs, are glands; and the material which they remove are of
the nature of secretions. From the cells, the waste passes through the
lymph in the blood. From the blood it is separated by the excretory organs
and passed to the exterior of the body.

*Exercises.*--1. What general purposes are served by the glands in the
body?

2. What are the parts common to all glands? What purpose is served by each
of these parts?

3. How do tubular glands differ in structure from saccular glands? What is
a racemose gland? Why so called?

4. Describe the nature of the secretory process.

5. What conditions render necessary the formation of waste materials in
the body? Why must these be removed?

6. How do the waste materials get from the cells to the organs of
excretion?

7. Show by a drawing the connections of the kidneys with the large blood
vessels and the bladder. Name parts of drawing.

8. In what do the uriniferous tubes have their beginning? In what do they
terminate? With what are they lined?

9. Why should the blood pass through two sets of capillaries in the
kidneys?

10. Bright's disease of the kidneys affects the uriniferous tubes and
interferes with their work. What impurity is then left in the blood?

11. Trace water and salts from the Malpighian capsules to the bladder,
naming parts through which they pass.

12. Trace carbon dioxide from the cells to the outside atmosphere.

13. How does the quantity of material introduced into the body compare
with that which is removed by the organs of excretion?

14. Name two ways of lessening the work of the kidneys.

15. Why is the drinking of plenty of pure water a healthful practice?

PRACTICAL WORK

*To suggest the Double Work of Glands.*--Prepare a simple filter by fitting
a piece of porous paper into a glass funnel. Through this pass pure water
and also water having salt dissolved in it and containing some sediment,
as sand. The water and the dissolved salt pass through, while the sediment
remains on the filter. Now substitute a fresh piece of paper in the funnel
and drop on its surface a little solid coloring matter, such as cochineal.
Again pass the liquid through the funnel. This time it comes through
, the color being added by the filter. Compare the filter and
materials filtered to the gland and the materials concerned in secretion
(blood, the liquid secreted, substances added by the gland, etc.).

                                [Fig. 92]

   Fig. 92--*The physiological scheme.* Diagram suggesting the essential
 relation of the bodily activities. See Summary of Part I, page 215, and
                      Summary of Part II, page 413.

SUMMARY OF PART I

The body is an organization of different kinds of cells; it grows through
the growth and reproduction of these cells; and its life as a whole is
maintained by providing such conditions as will enable the cells to keep
alive. Of chief importance in the work of the body is a nutrient fluid
which supplies the cells with food and oxygen and relieves them of waste.
A moving portion of this fluid, called the blood, serves as a transporting
agent, while another portion, called the lymph, passes the materials
between the blood and the cells. Through their effects upon the blood and
the lymph, the organs of circulation, respiration, digestion, and
excretion minister in different ways to the cells, and aid in the
maintenance of life. By their combined action two distinct movements are
kept up in the body, as follows:

1. An _inward_ movement which carries materials from the outside of the
body toward the cells.

2. An _outward_ movement which carries materials from the cells to the
outside of the body.

Passing _inward_ are the oxygen and food materials _in a condition to
unite with each other_ and thereby change their potential into kinetic
energy. Passing _outward_ are the oxygen and the elements that formed the
food materials _after having united_ at the cells and liberated their
energy.

As a final and all-important result, there is kept up a _continuous series
of chemical changes_ in the cells. These liberate the energy, provide
special substances needed by the cells, and preserve the life of the body
(Fig. 92).

In the chapters which follow, we are to consider the problem of adjusting
the body to and of bringing it into proper relations with its
surroundings.

PART II: MOTION, COORDINATION, AND SENSATION

CHAPTER XIV - THE SKELETON

One necessary means of establishing proper relations between the body and
its surroundings is _motion_.(77) Not only can the body move itself from
place to place, but it is able to move surrounding objects as well. In the
production of motion three important systems are employed--the muscular
system, the nervous system, and a system of mechanical devices which are
found mainly in the skeleton. The muscular system supplies the energy for
operating the mechanical devices, while the nervous system controls the
movements.(78) Although the skeleton serves other purposes, such as giving
shape to the body and protecting certain organs, its main use is that of
an aid in the production of motion.

*Skeleton Tissues.*--The tissues employed in the construction of the
skeleton are the osseous, the cartilaginous, and the connective tissues.
These are known as the supporting tissues of the body. They form the
bones, supply the elastic pads at the ends of the bones, and furnish
strong bands, called ligaments, for fastening the bones together. The
skeleton forms about 16 per cent of the weight of the body. Its tissues,
being of a more durable nature than the rest of the body, do not so
readily decay. Especially is this true of the osseous tissue, which may be
preserved indefinitely, after removal from the body, by simply keeping it
dry.

*The Bones.*--The separate units, or parts, of which the skeleton is
constructed are called bones. They are the hard structures that can be
felt in all parts of the body, and they comprise nearly the entire amount
of material found in the prepared skeleton. As usually estimated, the
bones are 208 in number. They vary greatly in size and shape in different
parts of the body.

*Composition and Properties of Bones.*--The most noticeable and important
properties of the bones are those of hardness, stiffness, and toughness.
Upon these properties the uses of the bones depend. These properties may,
in turn, be shown to depend upon the presence in osseous tissue of two
essentially different kinds of substance, known as the _animal matter_ and
the _mineral matter_. If a bone is soaked in an acid, the mineral matter
is dissolved out, and as a result it loses its properties of hardness and
stiffness. (See Practical Work.) This is because the mineral matter
supplies these properties, being composed of substances which are hard and
closely resemble certain kinds of rock. The chief materials forming the
mineral matter are calcium phosphate and calcium carbonate.

On the other hand, burning a bone destroys the animal matter. When this is
done the bone loses its toughness, and becomes quite brittle. The property
of toughness is, therefore, supplied by the animal matter. This consists
mainly of a substance called _ossein_, which may be dissolved out of the
bones by boiling them. Separated from the bones it is known as _gelatine_.
The blood vessels and nerves in the bones, and the protoplasm of the bone
cells, are also counted in with the animal matter.

                                [Fig. 93]

 Fig. 93--*Section of a long bone* (_tibia_), showing the gross structure.

If a dry bone from a full-grown, but not old, animal be weighed before and
after being burned, it is found to lose about one third of its weight.
From this we may conclude that about one third of the bone by weight is
animal matter and two thirds is mineral matter. This proportion, however,
varies with age, the mineral matter increasing with advance of years.

*Gross Structure of Bones.*--The gross structure of the bones is best
learned by studying both dry and fresh specimens. (See Practical Work.)
The ends of the bones are capped by a layer of smooth, elastic cartilage,
while all the remaining surface is covered by a rather dense sheath of
connective tissue, called the _periosteum_. Usually the central part of
the long bones is hollow, being filled with a fatty substance known as the
_yellow marrow_. Around the marrow cavity the bone is very dense and
compact, but most of the material forming the ends is porous and spongy.
These materials are usually referred to as the _compact substance_ and the
_cancellous_, or _spongy, substance_ of the bones (Fig. 93).

The arrangement of the compact and spongy substance varies with the
different bones. In the short bones (wrist and ankle bones, vertebrae,
etc.) and also in the flat bones (skull bones, ribs, shoulder blades,
etc.) there is no cavity for the yellow marrow, all of the interior space
being filled with the spongy substance. The _red marrow_, relations of
which to the red corpuscles of the blood have already been noted (page
27), occupies the minute spaces in the spongy substance.

                                [Fig. 94]

 Fig. 94--*Cross section of bone showing minute structure.* Magnified. 1.
 Surface layer of bone. 2. Deeper portion. 3. Haversian canals from which
pass the canaliculi. 4. A lacuna. Observe arrangement of lacunae at surface
                          and in deeper portion.

*Minute Structure of Bone.*--A microscopic examination of a thin slice of
bone taken from the compact substance shows this to be porous as well as
the spongy substance. Two kinds of small channels are found running
through it in different directions, known as the Haversian canals and the
canaliculi (Fig. 94). These serve the general purpose of distributing
nourishment through the bone. The _Haversian canals_ are larger than the
canaliculi and contain small nerves and blood vessels, chiefly capillaries
(Fig. 95). They extend lengthwise through the bone. The _canaliculi_ are
channels for conveying lymph. They pass out from the Haversian canals at
right angles, going to all portions of the compact substance except a thin
layer at the surface. In the surface layer of the bone the canaliculi are
in communication with the periosteum.

                                [Fig. 95]

 Fig. 95--*Section showing Haversian canal and contents*, highly magnified
  (after Schaefer). 1. Arterial capillary. 2. Venous capillary. 3. Nerve
                         fibers. 4. Lymph vessel.

*The Bone Cells.*--Surrounding the Haversian canals are thin layers of bone
substance called the _laminae_, and within these are great numbers of
irregular bodies, known as the _lacunae_. The walls of the lacunae are hard
and dense, but within each is an open space. In this lies a flattened
body, having a nucleus, which is recognized as the _bone cell_, or the
bone corpuscle (Fig. 96). It appears to be the work of the bone cells to
deposit mineral matter in the walls surrounding them and in this way to
supply the properties of hardness and stiffness to the bones. The
canaliculi connect with the lacunae in all parts of the bone, causing them
to appear under the microscope like so many burs fastened together by
their projecting spines (Fig. 94).

                                [Fig. 96]

  Fig. 96--*Bone cell* removed from the lacuna and very highly magnified.
                        (From Quain's _Anatomy_.)

*How the Bone Cells are Nourished.*--The bone cells, like all the other
cells of the body, are nourished by the lymph that escapes from the blood.
This passes through the canaliculi to the cells in the different parts of
the bone, as follows:

1. The cells in the surface layer of the bone receive lymph from the
capillaries in the periosteum.(79) It gets to them through the short
canaliculi that run out to the surface.

2. The cells within the interior of the bone receive their nourishment
from the small blood vessels in the Haversian canals. Lymph from these
vessels is conveyed to the cells through the canaliculi that connect with
the Haversian canals.

*Plan and Purpose of the Skeleton.*--The framework of the body is such as
to adapt it to a _movable_ structure. Obviously the different parts of the
body cannot be secured to a foundation, as are those of a stationary
building, but must be arranged after a plan that is conducive to motion. A
moving structure, as a wagon or a bicycle, has within it some strong
central part to which the remainder is joined. The same is true of the
skeleton. That part to which the others are attached is a long, bony axis,
known as the _spinal column_. Certain parts, as the ribs and the skull,
are attached directly to the spinal column, while others are attached
indirectly to it. The arrangement of all the parts is such that the spinal
column is made the central, cohering portion of the skeleton and also of
the whole body.

Besides the general arrangement of the parts of the skeleton, there is
such a grouping of the bones in each of its main divisions as will enable
them to serve definite purposes. In most places they form mechanical
devices for supplying special movements, and in certain places they
provide for the support or protection of important organs. In most cases
there is a definite combination of different bones, forming what is called
the bone group.

                                [Fig. 97]

                       Fig. 97--The human skeleton.

*Bone Groups.*--On account of the close relation between the bones of the
same group, they cannot profitably be studied as individual bones, but
each must be considered as a part of the group to which it belongs. By
first making out the relation of a given bone to its group, its value to
the whole body can be determined. The most important of the groups of
bones are as follows:

1. _The Spinal Column._--This group consists of twenty-four similarly
shaped bones, placed one above the other, called the _vertebrae_, and two
bones found below the vertebrae, known as the sacrum and the coccyx (Fig.
98). These twenty-six bones supply the central axis of the body, support
the head and upper extremities, and inclose and protect the spinal cord.

                                [Fig. 98]

                        Fig. 98--The spinal column.

The upper seven vertebrae form the neck and are called the _cervical_
vertebrae. They are smaller and have greater freedom of motion than the
others. The first and second cervical vertebrae, known as the _atlas_ and
the _axis_, are specially modified to form a support for the head and
provide for its movements. The head rests upon the atlas, forming with it
a hinge joint (used in nodding to indicate "yes"); and the atlas turns
upon an upward projection of the axis forming a pivot joint (used in
shaking the head to indicate "no").

The next twelve vertebrae, in order below the cervical, are known as the
_thoracic_ vertebrae. They form the back part of the framework of the
thorax and have little freedom of motion. The five vertebrae below the
thoracic are known as the _lumbar_ vertebrae. These bones are large and
strong and admit of considerable motion. Below the last lumbar vertebra is
a wedge-shaped bone which has the appearance of five vertebrae fused
together. This bone, known as the _sacrum_, connects with the large bones
which form the pelvic girdle. Attached to the lower end of the sacrum is a
group of from two to four small vertebrae, more or less fused, called the
_coccyx_.

                                [Fig. 99]

 Fig. 99--*Two views of a lumbar vertebra.* _A._ From above. _B._ From the
       side. 1. Body. 2, 3, 4, 5. Projections from the neural arch.

*The Joining of the Vertebrae.*--A typical vertebra consists of a heavy,
disk-shaped portion in front, called the _body_, which is connected with a
ring-like portion behind, called the _neural arch_. The body and the
neural arch together encircle a round opening which is a part of the canal
that contains the spinal cord (Fig. 99). From the neural arch are seven
bony projections, or processes, three of which serve for the attachment of
muscles and ligaments, while the other four, two above and two below, are
for the interlocking of the vertebrae with each other. The separate
vertebrae are joined together in the spinal column, as follows:

_a._ Between the bodies of adjacent vertebrae are disks of elastic
cartilage. Each disk is about one fourth of an inch thick and is grown
tight onto the face of the vertebra above and also onto the face of the
vertebra below. By means of these disks a very close connection is secured
between the vertebrae on the front side of the column.

_b._ On the back of the column, the downward projections from the neural
arch of each vertebra above fit into depressions found in the neural arch
of the vertebra below. This _interlocking_ of the vertebrae, which is most
marked in the lumbar region, strengthens greatly the back portion of the
column.

_c._ To further secure one bone upon the other, numerous ligaments pass
from vertebra to vertebra on all sides of the column.

2. _The Skull._--The skull is formed by the close union of twenty-two
irregular bones. These fall naturally into two subgroups--the cranium and
the face (Fig. 100). The _cranium_ consists of eight thin, curved bones
which inclose the space, called the _cranial cavity_, that holds the
brain. The _face group_, consisting of fourteen bones, provides cavities
and supports for the different organs of the face, and supplies a movable
part (the inferior maxillary) which, with the bones above (superior
maxillary), forms the machine for masticating the food.

                                [Fig. 100]

Fig. 100--*The skull (Huxley).* The illustration shows most of the bones of
                                the skull.

3. _The Thorax._--This group contains twenty-four bones of similar form,
called _ribs_, and a straight flat bone, called the _sternum_, or
breastbone (Fig. 101). The ribs connect with the spinal column behind, and
all but the two lowest ones connect with the sternum in front, and, by so
doing, inclose the thoracic cavity. As already stated (page 85), the bones
of the thorax form a mechanical device, or machine, for breathing. The
ribs are so arranged that the volume of the thorax is increased by
elevating them and diminished by depressing them, enabling the air to be
forced into and out of the lungs.

                                [Fig. 101]

                     Fig. 101--*Bone groups of trunk.*

4. _The Shoulder and Pelvic Girdles._--These groups form two bony
supports--one at the upper and the other at the lower portion of the
trunk--which serve for the attachment of the arms and legs (Fig. 101). The
_shoulder girdle_ is formed by four bones--two clavicles, or collar bones,
and two scapulae, or shoulder blades. The clavicle on either side connects
with the upper end of the sternum and serves as a _brace_ for the
shoulder, while the scapula forms a socket for the humerus (the large bone
of the arm) and supplies many places for the attachment of muscles.

The _pelvic girdle_ consists of two large bones of irregular shape, called
the _innominate_ bones. They connect behind with the sacrum and in front
they connect, through a small pad of cartilage, with each other. On the
inside of the girdle is a smooth, basin-shaped support for the contents of
the abdomen, but on the outside the bones are rough and irregular and
provide many places for the attachment of muscles and ligaments. Each
innominate bone has a deep, round socket into which the end of the femur
(the long bone of the leg) accurately fits.

5. _The Arm and Hand Groups._--A long bone, the _humerus_, connects the arm
with the shoulder and gives form to the upper arm. In the forearm are two
bones, the _radius_ and the _ulna_, which connect at one end with the
humerus and at the other with the bones of the wrist (Fig. 102).

                                [Fig. 102]

                  Fig. 102--*Bone groups of arm and leg.*

A group of eight small, round bones is found in the wrist, known as the
_carpal_ bones. These are arranged in two rows and are movable upon one
another. Five straight bones, the _metacarpals_, connect with the wrist
bones and form the framework for the palm of the hand. Attached to the
metacarpals are the bones of the fingers and thumb. These form an
interesting group of fourteen bones, called the _phalanges of the fingers_
(Fig. 102).

The bones of the hand provide a mechanical device, or machine, for
grasping, and the arm serves as a device for moving this grasping machine
from place to place. The work of the arm, in this respect, is not unlike
that of a revolving crane upon the end of which is a grab-hook. The hand
without the arm to move it about would be of little use.

6. _The Leg and Foot Groups._--These correspond in form and arrangement to
the bones of the arm and hand. Since, however, the leg and foot are used
for purposes different from those of the arm and hand, certain differences
in structure are to be found. The _patella_, or kneepan, has no
corresponding bone in the arm; and the _carpus_, or ankle, which
corresponds to the wrist, contains seven instead of eight bones. The bones
of the foot and toes are the same in number as those of the hand and
fingers, but they differ greatly in size and form and have less freedom of
motion. The _femur_, which gives form to the thigh, is the longest bone of
the body. The _tibia_, or shin bone, and the _fibula_, the slender bone by
its side, give form to the lower part of the leg (Fig. 102).

The legs are mechanical devices (walking machines) for moving the body
from place to place. The feet serve both as supports for the body and as
levers for pushing the body forward. By their attachment to the legs they
may be placed in all necessary positions for supporting and moving the
body.

The different bone groups are shown in Fig. 97 and named in Table IV.

*Adaptation to Special Needs.*--When any single bone is studied in its
relation to the other members of the group to which it belongs or with
particular reference to its purpose in the body, its adaptation to some
special place or use is at once apparent. Each bone serves some special
purpose, and to this purpose it is adapted by its form and structure. Long
bones, like the humerus and femur, are suited to giving strength, form,
and stiffness to certain parts, while irregular bones, like the vertebrae
and the pelvic bones, are fitted for supporting and protecting organs.
Others, like the wrist and ear bones, make possible a peculiar kind of
motion, and still others, like the ribs, are adapted to more than one
purpose. The vast differences in shape, size, structure, and surface among
the various bones are but the conditions that adapt them to particular
forms of service in the body.

TABLE IV - THE PRINCIPAL BONES AND THEIR GROUPING IN THE BODY

      I. AXIAL SKELETON

            A. _Skull_, 28.

                  1. Cranium, 8.

                  _      a._ Frontal, forehead 1
                  _      b._ Parietal 2
                  _      c._ Temporal, temple 2
                  _      d._ Occipital 1
                  _      e._ Sphenoid 1
                  _      f._ Ethmoid 1

                  2. Face, 14.

                  _      a._ Inferior maxillary 1
                  _      b._ Superior maxillary 2
                  _      c._ Palatine, palate 2
                  _      d._ Nasal bones 2
                  _      e._ Vomer 1
                  _      f._ Inferior turbinated 2
                  _      g._ Lachrymal 2
                  _      h._ Malar, cheek bones 2

                  3. Bones of the Ears, 6.

                  _      a._ Malleus 2
                  _      b._ Incus 2
                  _      c._ Stapes 2

            B. _Spinal Column_, 26.

                  1. Cervical, or neck, vertebrae 7
                  2. Dorsal, or thoracic, vertebrae 12
                  3. Lumbar vertebrae 5
                  4. Sacrum 1
                  5. Coccyx 1

            C. _Thorax_, 25.

                  1. Ribs 24
                  2. Sternum 1

            D. _Hyoid_, 1 (at base of tongue).

      II. APPENDICULAR SKELETON

            A. _Shoulder girdle_ 4.

                  1. Clavicle, collarbone. 2
                  2. Scapula, shoulder blade 2

            B. _Upper extremities_, 60.

                  1. Humerus 2
                  2. Radius 2
                  3. Ulna 2
                  4. Carpal, wrist bones 16
                  5. Metacarpal 10
                  6. Phalanges of fingers 28

            C. _Pelvic girdle_, 2.

                  1. Osinnominatum 2

            D. _Lower extremities_, 60.

                  1. Femur, thigh bone 2
                  2. Tibia, shin bone 2
                  3. Fibula 2
                  4. Patella, kneepan 2
                  5. Tarsal, ankle bones 14
                  6. Metatarsal, instep bones 10
                  7. Phalanges of toes 28

ARTICULATIONS

Any place in the body where two or more bones meet is called an
articulation, or joint. At the place of meeting the bones are firmly
attached to each other, thereby securing the necessary coherence of the
skeleton. The large number of bones, and consequently of articulations,
are necessary for the different movements of the body and also on account
of the manner in which the skeleton develops, or grows. Articulations are
classed with reference to their freedom of motion, as _movable_, _slightly
movable_, and _immovable_ articulations.

Most of the _immovable_ articulations are found in the skull. Here
irregular, tooth-like projections from the different bones enable them to
interlock with one another, while they are held firmly together by a thin
layer of connective tissue. The wavy lines formed by articulations of this
kind are called _sutures_ (Fig. 100).

The best examples of joints that are _slightly_, but not freely, _movable_
are found in the front of the spinal column. The cartilaginous pads
between the vertebrae permit, by their elasticity, of a slight bending of
the column in different directions. These movements are caused, not by one
bone gliding over another, but by compressions and extensions of the
cartilage. Between the vertebrae in the back of the spinal column, however,
there is a slight movement of the bone surfaces upon one another.

*Structure of the Movable Joints.*--By far the most numerous and important
of the joints are those that are freely movable. Such joints are strongly
constructed and endure great strain without dislocation, and yet their
parts move over each other easily and without friction. The ends of the
bones are usually enlarged and have specially formed projections or
depressions which fit into corresponding depressions or elevations on the
bones with which they articulate. In addition to this the articular
surfaces are quite smooth and dense, having no Haversian canals, and they
are covered with a layer of cartilage. Strong ligaments pass from one bone
to the other to hold each in its place (_A, _Fig. 103). Some of these
consist simply of bands, connecting the joint on its different sides,
while others form continuous sheaths around the joint.

                                [Fig. 103]

     Fig. 103--*Outside and inside view of knee joint.* 1. Tendons. 2.
Ligaments. 3. Cartilage. 4. Space containing synovial fluid. This space is
   lined, except upon the articular surfaces, by the synovial membrane.

The interior of the joint, except where the bone surfaces rub upon each
other, is covered with a serous lining, called the _synovial membrane_
(_B_, Fig. 103). This secretes a thick, viscid liquid, the _synovial
fluid_, which prevents friction. The synovial membrane does not cover the
ends of the bones, but passes around the joint and connects with the bones
at their edges so as to form a closed sac in which the fluid is retained.

*Kinds of Movable Joints.--*The different kinds of movable joints are the
ball and socket joint, the hinge joint, the pivot joint, the condyloid
joint, and the gliding joint. These are constructed and admit of motion,
as follows:

1. In the _ball and socket_ joint the ball-shaped end of one bone fits
into a cup-shaped cavity in another bone, called the socket. The best
examples of such joints are found at the hips and shoulders. The ball and
socket joint admits of motion in all directions.

2. In the _hinge_ joint the bones are grooved and fit together after the
manner of a hinge. Hinge joints are found at the elbows and knees and also
in the fingers. The hinge joint gives motion in but two directions--forward
and backward.

3. A _pivot_ joint is formed by the fitting of a pivot-like projection of
one bone into a ring-like receptacle of a second bone, so that one, or the
other, is free to turn. A good example of the pivot joint is found at the
elbow, where the radius turns upon the humerus. Another example is the
articulation of the atlas with the axis vertebra as already noted. The
pivot joint admits of motion around an axis.

4. The _condyloid_ joint is formed by the fitting of the ovoid
(egg-shaped) end of one bone into an elliptical cavity of a second bone.
Examples of condyloid joints are found at the knuckles and where the wrist
bones articulate with the radius and ulna. They move easily in two
directions, like hinge joints, and slightly in other directions.

5. _Gliding_ joints are formed by the articulation of plain (almost flat)
surfaces. Examples of gliding joints are found in the articulations
between the bones of the wrist and those of the ankle. They are the
simplest of the movable joints and are formed by one bone gliding, or
slipping, upon the surface of another.

*The Machinery of the Body.*--A machine is a contrivance for directing
energy in doing work. A sewing machine, for example, so directs the energy
of the foot that it is made to sew. Through its construction the machine
is able to produce just that form of motion needed for its work, and no
other forms, so that energy is not wasted in the production of useless
motion. The places in machines where parts rub or turn upon each other are
called _bearings_, and extra precautions are taken in the construction and
care of the bearings to prevent friction.

The body cannot properly be compared to any single machine, but must be
looked upon as a complex organization which employs a number of different
kinds of machines in carrying on its work. The majority of these machines
are found in the skeleton. The bones are the parts that are moved, and the
joints serve as bearings. Connected with the bones are the muscles that
supply energy, and attached to the muscles are the nerves that control the
motion. Other parts also are required for rendering the machines of the
body effective in doing work. These are supplied by the tissues connected
with the bones and the muscles.

HYGIENE OF THE SKELETON

Of chief concern in the hygiene of the skeleton is the proper _adjustment_
of its parts. The efficiency of any of the body machines is impaired by
lack of proper adjustment. Not only this, but because of the fact that the
skeleton forms the groundwork of the whole body--muscles, blood vessels,
nerves, everything in fact, being arranged with reference to it--any lack
of proper adjustment of the bones interferes generally with the
arrangement and work of tissues and organs. The displaced bones may even
compress blood vessels and nerves and interfere, in this way, with the
nourishment and control of organs remote from the places where the
displacements occur. For these reasons the proper adjustment of the
different parts of the skeleton supplies one of the essential conditions
for preserving the health.

*Hygienic Importance of the Spinal Column.*--What has been said about the
adjustment of the skeleton in general applies with particular force to the
spinal column. The spinal column serves both as the central axis of the
body and as the container of the spinal cord. Thirty-one pairs of nerves
pass between the vertebrae to connect the spinal cord with different parts
of the body, and two important arteries (the vertebral) pass through a
series of small openings in the bones of the neck to reach the brain.
Unnatural curves of the spine throw different parts of the body out of
their natural positions, diminish the thoracic and abdominal cavities,
and, according to the belief of certain physicians, compress the nerves
that pass from the cord to other parts of the body. Slightly misplaced
vertebrae in the neck, by compressing the vertebral arteries, may also
interfere with the supply of blood

                                [Fig. 104]

        Fig. 104--A tendency toward spinal curvature (after Mosher)

                                [Fig. 105]

 Fig. 105--Effect on spinal column of improper position in writing. (From
                       Pyle's _Personal Hygiene._)

*How the Skeleton becomes Deformed*--We are accustomed to look upon the
skeleton as a rigid framework which can get out of its natural form only
through severe strain or by violence. This view is far from being correct.
On account of their necessary freedom of motion, the bones, especially
those of the spinal column, are easily slipped from their normal
positions; and where improper attitudes are frequently assumed, or
continued through long periods of time, the skeleton gradually becomes
deformed (Fig. 104). For example, the habit of always sleeping on the same
side with a high pillow may develop a bad crook in the neck; and the ugly
curves, assumed so frequently in writing (80) (Fig. 105), and also in
standing, when the weight is shifted too much on one foot, may become
permanent. Then the habit of reclining in a chair with the hips resting on
the front of the seat often deforms the back and causes a drooping of the
shoulders. In fact, slight displacements of the vertebrae come about so
easily _through incorrect positions_, that they may almost be said to
"occur of themselves" where active measures are not taken to preserve the
natural form of the body. The very few people who have perfectly formed
bodies show to what an extent has been overlooked an essential law of
hygiene.

*Prevention of Skeletal Deformities.*--Those deformities of the skeleton
that are acquired through improper positions are prevented by giving
sufficient attention to the positions assumed in sitting, standing, and
sleeping, and also to the posture in various kinds of work. In sitting the
trunk should be erect and the hips should touch the back of the chair. One
should not lounge in the ordinary chair. In standing the body should be
erect, the shoulders back and down, the chest pushed slightly up and
forward, and the chin slightly depressed, while the weight should, as a
rule, rest about equally on the two feet. The habit of leaning against
some object when standing (the pupil in reciting often leans on his desk)
should be avoided. In sleeping the pillow should be of the right thickness
to support the head on a level with the spinal column and should not be
too soft. If one sleeps on his back, no pillow is required. It is best not
to acquire the habit of sleeping always on the same side.

Where one is compelled by his work to assume harmful positions, these
should be corrected by proper exercises, and by cultivating opposing
positions during the leisure hours. Much is to be accomplished through
those forms of physical exercise which develop the muscles whose work it
is to keep the body in an upright position.

*School Furniture.*--It has long been observed that school children are
more subject to curvature of the spine and other deformities of the
skeleton than the children who do not attend school. While this is due
largely to faulty positions assumed by the pupils at their work, it has
been suggested that the school furniture may be in part to blame for these
positions. Investigations of this problem have shown that most of the
school desks and seats in use in our public schools are unhygienically
constructed, in that they _force_ pupils into unnatural positions. School
seats should support the pupil in a natural position, both in the use of
his books and in writing, and there are many arguments in favor of the
so-called "adjustable" school furniture. Fig. 106 shows the seat and desk
designed by the Boston, Mass., Schoolhouse Commission after much study and
experimenting and used in the Boston schools. This furniture, which
provides a seat adjustable for height, having a back rest also adjustable
for height, and a desk which is likewise provided with a vertical
adjustment, supplies all essential hygienic requirements. It is to be
hoped that school furniture of this character may in the near future come
into general use.

                                [Fig. 106]

    Fig. 106--Adjustable seat and desk used in schools of Boston, Mass.

*Correction of Skeletal Deformities.*--It is, of course, easier to prevent
deformities of the skeleton by giving attention to proper positions, than
to correct them after they have occurred. It should also be noted that
severe deformities cannot be corrected by the individual for himself, but
these must come under the treatment of specialists in this line of medical
work. In mild cases of spinal curvature, drooping of the head, and round
shoulders, the individual _can_ benefit his condition. By working to
"substitute a correct attitude for the faulty one,"(81) he can by
persistence bring about marked improvements. It is better, however, to
have the advice and aid of a physical director, where this is possible. It
should also be borne in mind that the correction of skeletal deformities
requires effort through a long period of time, especially where the
deformities are pronounced; and one lacking the will power to persist will
not secure all the results which he seeks.

*"Setting Up" Exercises.*--The splendid carriage of students from military
schools shows what may be accomplished in securing erectness of form where
proper attention is given to this matter. The military student gets his
fine form partly through his exercises in handling arms, but mainly
through his so-called "setting up" drill. As a suggestion to one desiring
to improve the form of his body, a modification of the usual "setting up"
drill is here given:

1. Standing erect, with the heels together, the feet at an angle of 45 deg.,
and hands at the sides, bring the arms to a horizontal position in front,
little fingers touching and nails down. From this position raise the hands
straight over the head, bringing the palms gradually together. Then with a
backward sweeping movement, return the hands again to the sides. Repeat
several times.

2. With the feet as in the above exercise, bring the hands and the arms to
a level with the shoulders, palms down, elbows bent, middle fingers of the
two hands touching, and the extended thumbs touching the chest. Keeping
the palms down and the arms on a level with the shoulders, extend the
hands as far sideward and backward as possible, returning each time to the
first position. As the hands move out, inhale deeply (through the nose),
and as they are brought back, exhale quickly (through the mouth). Repeat
several times.

3. With the arms at the sides and the feet side by side and touching,
bring the hands in a circular movement to a vertical position over the
head, and lock the thumbs. Keeping the knees straight and the thumbs
locked, bend forward, letting the hands touch the ground if possible, and
then bring the body and hands again to the vertical position. Then by a
backward sweeping movement, return the hands again to the sides. Repeat.

While these exercises may be practiced whenever convenient, it is best to
set apart some special time each day for them, as on retiring at night or
on rising in the morning.

*Hygienic Footwear.*--A necessary aid to erectness of position in standing
and walking is a properly fitting shoe. Heels that are too high tilt the
body unnaturally forward, and shoes that cause any kind of discomfort in
walking lead to unnatural positions in order to protect the feet. Shoes
should fit snugly, being neither too large nor too small. Many shoes,
however, are unhygienically constructed, and no attempt should be made to
wear them. Certainly is this true of styles that approach the "French
heel" or the "toothpick toe" (Fig. 107). However, many styles of shoes are
manufactured that are both hygienic and neat fitting. Rubber heels, on
account of their elasticity, are to be preferred to those made of leather.

                                [Fig. 107]

     Fig. 107--Heels and toes of unhygienic and of hygienic footwear.

*The Skeleton in Childhood and Old Age.*--Certain peculiarities are found
to exist in the bones of children and of old people which call for special
care of the skeleton during the first and last periods of life. The bones
of children are soft, lacking mineral matter, and are liable to become
bent For this reason, children who are encouraged to walk at too early an
age may bend the thigh bones, causing the too familiar "bow-legs." These
bones may also be bent by having children sit on benches and chairs which
are too high for the feet to reach the floor, and which do not provide
supports for the feet. Wholesome food, fresh air, sunlight, and exercise
are also necessary to the proper development of the bones of children.
Where these natural conditions are lacking, as in the crowded districts of
cities, children often suffer from a disease known as "rickets," on
account of which their bones are unnaturally soft and easily bent.

On account of the accumulation of mineral matter, the bones of elderly
people become brittle and are easily broken, and from lack of vigor of the
bone cells they heal slowly after such injuries occur. This makes the
breaking of a bone by an aged person a serious matter. Old people should,
as far as possible, avoid liabilities to falls, such as going rapidly up
and down stairs, or walking on icy sidewalks, and should use the utmost
care in getting about. In old people also the cartilage between the bones
softens, increasing the liability of getting misshaped. Special attention,
therefore, should be given to erectness of form, and to such exercises as
tend to preserve the natural shape of the body.

*Treatment of Fractures.*--A fractured bone always requires the aid of a
surgeon, and no time should be lost in securing his services. In the
meantime the patient should be put in a comfortable position, and the
broken limb supported above the rest of the body. Though the breaking of a
bone is not, as a rule, a serious mishap, it is necessary that the very
best skill be employed in setting it. Any failure to bring the ends of the
broken bone into their normal relations permanently deforms the limb and
interferes with its use.

*Dislocations and Sprains.*--Dislocations, if they be of the larger joints,
also require the aid of the surgeon in their reduction and sometimes in
their subsequent treatment. Simple dislocations of the finger joints,
however, may be reduced by pulling the parts until the bones can be
slipped into position.

_A sprain_, which is an overstrained condition of the ligaments
surrounding a joint, frequently requires very careful treatment. When the
sprain is at all serious, a physician should be called. Because of the
limited supply of blood to the ligaments, they are slow to heal, and the
temptation to use the joint before it is fully recovered is always great.
Massage(82) judiciously applied to a sprained joint, by bringing about a
more rapid change in the blood and the lymph, is beneficial both in
relieving the pain, and in hastening recovery.

*Summary.*--The skeleton, or framework of the body, is a structure which is
movable as a whole and in most of its parts. It preserves the form of the
body, protects important organs, and supplies the mechanical devices, or
machines, upon which the muscles act in the production of motion. The
skeleton is adapted to its purposes through the number and properties of
the bones, and through the cartilage and connective tissue associated with
the bones. The places where the different bones connect one with another
are known as joints, and most of these admit of motion. The preservation
of the natural form of the skeleton is necessary, both for its proper
action and for the health of the body.

*Exercises.*--1. State the main purpose of the skeleton. What is the
necessity for so many bones in its construction?

2. How may the per cent of animal and of mineral matter in a bone be
determined?

3. What properties are given the bones by the animal matter? What by the
mineral matter?

4. Locate the bone cells. What is their special function?

5. State the plan by which nourishment is supplied to the bone cells in
different parts of the bone.

6. Give the uses of the periosteum.

7. State the purpose of the Haversian canals. Of the canaliculi.

8. Give functions of the spinal column.

9. Name the different materials used in the construction of a joint and
the purpose served by each.

10. Name four mechanical devices, or machines, found in the skeleton and
state the purpose served by each.

11. Name one or more of the body machines not located in the skeleton.

12. Of what advantage is the peculiar shape of the lower jaw? Of the ribs?
Of the bones of the pelvic girdle?

13. State the importance of preserving the natural form of the skeleton.
How are unnatural curves produced in the spinal column?

14. How may slight deformities of the skeleton be corrected?

15. What different systems are employed in the body in the production of
motion? What is the special function of each?

PRACTICAL WORK

To obtain clear ideas of the form and functions of the bones, a careful
examination of a prepared and mounted skeleton is necessary. Many of the
bones, however, may be located and their general form made out from the
living body. Bones of the lower animals may also be studied to advantage.

*Experiments to show the Composition of Bone.*--1. Examine a slender bone,
like that in a chicken's leg. Note that it resists bending and is
difficult to break. Note also that it is elastic--that, when slightly bent,
it will spring back.

2. Soak such a bone over night in a mixture of one part hydrochloric acid
and four parts water. Then ascertain by bending, stretching, and twisting
what properties the bone has lost. The acid has dissolved out the mineral
matter.

3. Burn a small piece of bone in a clear gas flame, or on a bed of coals,
until it ceases to blaze and turns a white color. Can the bone now be bent
or twisted? What properties has it lost and what retained? What substance
has been removed from the bone by burning?

*Observation on the Gross Structure of Bone.*--1. Procure a long, dry bone.
(One that has lain out in the field until it has bleached will answer the
purpose excellently.) Test its hardness, strength, and stiffness. Saw it
in two a third of the distance from one end, and saw the shorter piece in
two lengthwise. Compare the structure at different places. Find rough
elevations on the outside for the attachment of muscles, and small
openings into the bone for the entrance of blood vessels and nerves. Make
drawings to represent the sections.

2. Procure a fresh bone from the butcher shop. Note the difference between
it and the dry bone. Examine the materials surrounding the sides and
covering the ends of the bone. Saw through the enlarged portion at the end
and examine the red marrow. Saw through the middle of the bone and observe
the yellow marrow.

*To show the Minute Structure of the Bone.*--Prepare a section of bone for
microscopic study as follows: With a jeweler's saw cut as thin a slice as
possible. Place this upon a good-sized whetstone, not having too much
grit, and keeping it wet rub it under the finger, or a piece of leather,
until it is thin enough to let the light shine through. The section may
then be washed and examined with the microscope. If the specimen is to be
preserved for future study, it may be mounted in the usual way, but with
_hard_ balsam. Prepare and study both transverse and longitudinal
sections, making drawings. The sections should be prepared from bones that
are thoroughly dry but which have not begun to decay.

*To show the Structure of a Joint.*--Procure from a butcher the joint of
some small animal (hog or sheep). Cut it open and locate the cartilage,
synovial membrane, and ligaments. Observe the shape and surface of the
rubbing parts and the strength of the ligaments.

CHAPTER XV - THE MUSCULAR SYSTEM

As already stated, the skeleton, the nervous system, and the muscular
system are concerned in the production of motion. The skeleton and the
nervous system, however, serve other purposes in the body, while the
muscular system is devoted exclusively to the production of motion. For
this reason it is looked upon as the special _motor_ system. The muscular
tissue is the most abundant of all the tissues, forming about 41 per cent
of the weight of the body.

*Properties of Muscles.*--The ability of muscular tissue to produce motion
depends primarily upon two properties--the property of irritability and the
property of contractility. _Irritability_ is that property of a substance
which enables it to respond to a stimulus, or to act when acted upon.
_Contractility_ is the property which enables the muscle when stimulated
to draw up, thereby becoming shorter and thicker (a condition called
contraction), and when the stimulation ceases, to return to its former
condition (of relaxation). The property of contractility enables the
muscles to produce motion. Irritability is a condition necessary to their
control in the body.

*Kinds of Muscular Tissue.*--Three kinds of muscular tissue are found in
the body. These are known as the _striated_, or striped, muscular tissue;
the _non-striated_, or plain, muscular tissue; and the _muscular tissue of
the heart_. These are made up of different kinds of muscle cells and act
in different ways to cause motion. The striated muscular tissue far
exceeds the others in amount and forms all those muscles that can be felt
from the surface of the body. The non-striated muscle is found in the
walls of the food canal, blood vessels, air passages, and other tubes of
the body; while the muscular tissue of the heart is confined entirely to
that organ.

*Striated Muscle Cells.*--The cells of the striated muscles are slender,
thread-like structures, having an average length of 1-1/2 inches (35
millimeters) and a diameter of about 1/400 of an inch (60 {~GREEK SMALL LETTER MU~}). Because of
their great length they are called fibers, or fiber cells. They are marked
by a number of dark, transverse bands, or stripes, called striations,(83)
which seem to divide them into a number of sections, or disks (Fig. 108).
A thin sac-like covering, called the _sarcolemma_, surrounds the entire
cell and just beneath this are a number of nuclei.(84)

                                [Fig. 108]

Fig. 108--*A striated muscle cell* highly magnified, showing striations and
    nuclei. Attached to the cell is the termination of a nerve fiber.

Within the sarcolemma are minute fibrils and a semiliquid substance,
called the _sarcoplasm_. At each end the cell tapers to a point from which
the sarcolemma appears to continue as a fine thread, and this, by
attaching itself to the inclosing sheath, holds the cell in place. Most of
the muscle cells receive, at some portion of their length, the termination
of a nerve fiber. This penetrates the sarcolemma and spreads out upon a
kind of disk, having several nuclei, known as the _end plate_.

*The "Muscle-organ."*--We must distinguish between the term "muscle" as
applied to the muscular tissue and the term as applied to a working group
of muscular tissue, which is an organ. In the muscle, or muscle-organ, is
found a definite grouping of muscle fibers such as will enable a large
number of them to act together in the production of the same movement. An
examination of one of the striated muscles shows the individual fibers to
lie parallel in small bundles, each bundle being surrounded by a thin
layer of connective tissue. (See Practical Work.) These small bundles are
bound into larger ones by thicker sheaths and these in turn may be bound
into bundles of still larger size (Fig. 109). The sheaths surrounding the
fiber bundles are connected with one another and also with the outer
covering of the muscle, known as

                                [Fig. 109]

 Fig. 109--*Diagram* of a section of a muscle, showing the perimysium and
                       the bundles of fiber cells.

                                [Fig. 110]

Fig. 110--*A muscle-organ in position.* The tendons connect at one end with
 the bones and at the other end with the fiber cells and perimysium. (See
                                  text.)

*The Perimysium.*--The plan of the muscle-organ is revealed through a study
of the perimysium. This is not limited to the surface of the muscle, as
the name suggests, but properly includes the sheaths that surround the
bundles of fibers. Furthermore, the surface perimysium and that within the
muscle are both continuous with the strong, white cords, called _tendons_,
that connect the muscles with the bones. By uniting with the bone at one
end and blending with the perimysium and fiber bundles at the other, the
tendon forms a very secure attachment for the muscle. The perimysium and
the tendon are thus the means through which the fiber cells in any
muscle-organ are made to _pull together_ upon the same part of the body
(Fig. 110).

*Purpose of Striated Muscles.*--The striated muscles, by their attachments
to the bones, supply motion to all the mechanical devices, or machines,
located in the skeleton. Through them the body is moved from place to
place and all the external organs are supplied with such motion as they
require. Because of the attachment of the striated muscles to the
skeleton, and their action upon it, they are called _skeletal_ muscles. As
most of them are under the control of the will, they are also called
_voluntary_ muscles. They are of special value in adapting the body to its
surroundings.

*Structure of the Non-striated Muscles.*--The cells of the non-striated
muscles differ from those of the striated muscles in being decidedly
spindle-shaped and in having but a single well-defined nucleus (Fig. 111).
Furthermore, they have no striations, and their connection with the nerve
fibers is less marked. They are also much smaller than the striated cells,
being less than one one-hundredth of an inch in length and one
three-thousandth of an inch in diameter.

In the formation of the non-striated muscles, the cells are attached to
one another by a kind of muscle cement to form thin sheets or slender
bundles. These differ from the striated muscles in several particulars.
They are of a pale, whitish color, and they have no tendons. Instead of
being attached to the bones, they usually form a distinct layer in the
walls of small cavities or of tubes (Fig. 111). Since they are controlled
by the part of the nervous system which acts independently of the will,
they are said to be _involuntary_. They contract and relax slowly.

                                [Fig. 111]

 Fig. 111--*Non-striated muscle cells.* _A._ Cross section of small artery
    magnified, showing (1) the layer of non-striated cells. _B._ Three
                   non-striated cells highly magnified.

*Work of the Non-striated Muscles.*--The work of the non-striated muscles,
both in purpose and in method, is radically different from that of the
striated. They do not change the _position_ of parts of the body, as do
the striated muscles, but they alter the _size_ and _shape_ of the parts
which they surround. Their purpose, as a rule, is to move, or control the
movement of, materials within cavities and tubes, and they do this by
means of the _pressure_ which they exert. Examples of their action have
already been studied in the propulsion of the food through the alimentary
canal and in the regulation of the flow of blood through the arteries
(pages 159 and 49). While they do not contract so quickly, nor with such
great force as the striated muscles, their work is more closely related to
the vital processes.

*Structure of the Heart Muscle.*--The cells of the heart combine the
structure and properties of the striated and the non-striated muscle
cells, and form an intermediate type between the two. They are
cross-striped like the striated cells, and are nearly as wide, but are
rather short (Fig. 112). Each cell has a well-defined nucleus, but the
sarcolemma is absent. They are placed end to end to form fibers, and many
of the cells have branches by which they are united to the cells in
neighboring fibers. In this way they interlace more or less with each
other, but are also cemented together. They contract quickly and with
great force, but are not under control of the will. Muscular tissue of
this variety seems excellently adapted to the work of the heart.

                                [Fig. 112]

Fig. 112--*Muscle cells from the heart*, highly magnified (after Schaefer).

*The Muscular Stimulus.*--The inactive, or resting, condition of a muscle
is that of relaxation. It does work through contracting. It becomes
active, or contracts, only when it is being acted upon by some force
outside of itself, and it relaxes again when this force is withdrawn. Any
kind of force which, by acting on muscles, causes them to contract, is
called a _muscular stimulus_. Electricity, chemicals of different kinds,
and mechanical force may be so applied to the muscles as to cause them to
contract. These are _artificial_ stimuli. So far as known, muscles are
stimulated _naturally_ in but one way. This is through the nervous system.
The nervous system supplies a stimulus called the _nervous impulse_, which
reaches the muscles by the nerves, causing them to contract. By means of
nervous impulses, all of the muscles (both voluntary and involuntary) are
made to contract as the needs of the body for motion require.

*Energy Transformation in the Muscle.*--The muscle serves as a kind of
engine, doing work by the transformation of potential into kinetic energy.
Evidences of this are found in the changes that accompany contraction.
Careful study shows that during any period of contraction oxygen and food
materials are consumed, waste products, such as carbon dioxide, are
produced, and heat is liberated. Furthermore, the _blood supply to the
muscle_ is such that the materials for providing energy may be carried
rapidly to it and the products of oxidation as rapidly removed. Blood
vessels penetrate the muscles in all directions and the capillaries lie
very near the individual cells (Fig. 113). Provision is made also, through
the nervous system, for _increasing_ the blood supply when the muscle is
at work. From these facts, as well as from the great force with which the
muscle contracts, one must conclude that the muscle is a _transformer of
energy_--that within its protoplasm, chemical changes take place whereby
the potential energy of oxygen and food is converted into the kinetic
energy of motion.

                                [Fig. 113]

                    Fig. 113--*Capillaries* of muscles.

*Plan of Using Muscular Force.*--Two difficulties have to be overcome in
the using of muscular force in the body. The first of these is due to the
fact that the muscles exert their force _only when they contract_. They
can pull but not push. Hence, in order to bring about the opposing
movements(85) of the body, each muscle must work against some force that
produces a result directly opposite to that which the muscle produces.
Some of the muscles (those of breathing) work against the elasticity of
certain parts of the body; others (those that hold the body in an upright
position), to some extent against gravity; and others (the non-striated
muscle in arteries), against pressure. But in most cases, _muscles work
against muscles_.

                                [Fig. 114]

 Fig. 114--*The muscle pair* that operates the forearm. For names of these
                          muscles, see Fig. 119.

The striated, or skeletal, muscles are nearly all arranged after the
last-named plan. As a rule a pair of muscles is so placed, with reference
to a joint, that one moves the part in one direction, and the other moves
it in the opposite direction. From the kinds of motion which the various
muscle pairs produce, they are classified as follows:

1. _Flexors and Extensors._--The flexor muscles bend and the extensors
straighten joints (Fig. 114).

2. _Adductors and Abductors._--The adductors draw the limbs into positions
parallel with the axis of the body and the abductors draw them away.

3. _Rotators_ (two kinds).--The rotators are attached about pivot joints
and bring about twisting movements.

4. _Radiating and Sphincter Muscles. _--The radiating muscles open and the
sphincter muscles close the natural openings of the body, such as the
mouth.

The pupil should locate examples of the different kinds of muscle pairs in
his own body.

*Exchange of Muscular Force for Motion.*--The second difficulty to be
overcome in the use of muscular force in the body is due to the fact that
the muscles contract through _short_ distances, while it is necessary for
most of them to move portions of the body through _long_ distances. It may
be easily shown that the longest muscles of the body do not shorten more
than three or four inches during contraction. To bring about the required
movements of the body, which in some instances amount to four or five
feet, requires that a large proportion of the muscular force be exchanged
for motion. The machines of the skeleton, while providing for motion in
definite directions, also provide the means whereby _strong forces_,
acting through _short distances_, are made to produce movements of _less
force_, through _long distances_. The mechanical device employed for this
purpose is known as

*The Lever.*--The lever may be described as a stiff bar which turns about a
fixed point of support, called the _fulcrum_. The force applied to the bar
to make it turn is called the _power_, and that which is lifted or moved
is termed the _weight_. The weight, the power, and the fulcrum may occupy
different positions along the bar and this gives rise to the three kinds
of levers, known as levers of the first class, the second class, and the
third class (Fig. 115). In levers of the _first class_ the fulcrum
occupies a position somewhere between the power and the weight. In the
_second class_ the weight is between the fulcrum and the power. In the
_third class_ the power is between the fulcrum and the weight.

                                [Fig. 115]

    Fig. 115--*Classes of levers. I.* Two levers of first class showing
fulcrums in different positions. II. Lever of second class. III. Lever of
 third class. _F._ Fulcrum. _P._ Power. _W._ Weight. _a._ Power-arm. _b._
                               Weight-arm.

*Application to the Body.*--In the body the bones serve as levers; the
turning points, or fulcrums, are found at the joints; the muscles supply
the power; and parts of the body, or things to be lifted, serve as
weights. For these levers to _increase_ the motion of the muscles, it is
necessary that the muscles be attached to the bones _near the joints_, and
that the parts to be moved be located at some distance from the joints. In
other words the (muscle) power-arm must be _shorter_ than the (body)
weight-arm.(86)

Examining Fig. 116, it is seen that the distances moved by the power and
weight vary as their respective distances from the fulcrum. That is to
say, if the weight is twice as far from the fulcrum as the power, it will
move through twice the distance, and if three times as far, through three
times the distance. Thus the muscles, by acting through short distances
(on the short arms of levers), are able to move portions of the body
(located on the long arms) through long distances. Can all three classes
of levers be used in this way in the body?

                                [Fig. 116]

Fig. 116--*Motion producing levers.* Diagrams show relative distances moved
by the power and weight in levers having the power nearer the fulcrum than
      is the weight. _F._ Fulcrum. _P, P'._ Power. _W, W'._ Weight.

*Classes of Levers found in the Body.*--Practically all of the levers of
the body belong either to the first class or the third class. In both of
these the muscle power can be applied to the short arm of the lever,
thereby moving the body weight through a longer distance than the muscle
contracts (Fig. 116). In the levers of the second class, however, the
weight occupies this position, being situated _between_ the power and
fulcrum (Fig. 117). The weight, therefore, _cannot_ move farther than the
power in this lever. It must always move a shorter distance. While such a
lever is of great advantage in lifting heavy weights outside of the body,
it cannot be used for increasing the motion of the muscles. For this
reason no well-defined levers of the second class are present in the
body.(87)

                                [Fig. 117]

 Fig. 117--*Weight lifting levers.* Diagrams show relative distances moved
  by the power and weight in levers having the weight nearer the fulcrum
    than is the power. _F._ Fulcrum. _P, P'._ Power. _W, W'._ Weight.

                                [Fig. 118]

 Fig. 118--*Diagram of the foot lever.* _F._ Fulcrum at ankle joint. _W._
  Body weight expressed as pressure against the earth. While the muscle
power acts through the distance _ab_, the fulcrum support (body) is forced
                        through the distance _FE_.

*Loss of Muscular Force.*--Using a small spring balance for measuring the
power, a light stick for a lever, and a small piece of metal for a weight,
and arranging these to represent some lever of the body (as the forearm),
it is easily shown that the gain in motion causes a corresponding loss in
muscular power. (See Practical Work.) If, for example, the balance is
attached two inches from the fulcrum and the weight twelve inches, the
pull on the balance is found to be six times greater than the weight that
is being lifted. If other positions are tried, it is found that the power
exerted in each case is as many times greater than the weight as the
weight-arm is times longer than the power-arm.

Applying this principle to the levers of the body, it is seen that the
gain in motion is at the expense of muscular force, or, as we say,
_muscular force is exchanged for motion_. This exchange is greatly to the
advantage of the body; for while the ability to lift heavy weights is
important, the ability to move portions of the body rapidly and through
long distances is much more to be desired.

*Important Muscles.*--There are about five hundred separate muscles in the
body. These vary in size, shape, and plan of attachment, to suit their
special work. Some of those that are prominent enough to be felt at the
surface are as follows:

_Of the head_: The _temporal_, in the temple, and the _masseter_, in the
cheek. These muscles are attached to the lower jaw and are the chief
muscles of mastication.

_Of the neck_: The _sterno-mastoids_, which pass between the mastoid
processes, back of the ears, and the upper end of the sternum. They assist
in turning the head and may be felt at the sides of the neck (Fig. 119).

_Of the upper arm_: The _biceps_ on the front side, the _triceps_ behind,
and the _deltoid_ at the upper part of the arm beyond the projection of
the shoulder.

                                [Fig. 119]

           Fig. 119--Back and front views of important muscles.

_Of the forearm_: The _flexors_ of the fingers, on the front side, and the
_extensors_ of the fingers, on the back of the forearm (Fig. 119).

_Of the hand_: The _adductor pollicis_ between the thumb and the palm.

_Of the trunk_: The _pectoralis major_, between the upper front part of
the thorax and the shoulder; the _trapezius_, between the back of the
shoulders and the spine; the _rectus abdominis_, passing over the abdomen
from above downward; and the _erector spinae_, found in the small of the
back.

_Of the hips_: The _glutens maximus_, fastened between the lower back part
of the hips and the upper part of the femur.

_Of the upper part of the leg_: The _rectus femoris_, the large muscle on
the front of the leg which connects at the lower end with the kneepan.

_Of the lower leg_: The _tibialis anticus_ on the front side, exterior to
the tibia, and the _gastrocnemius_, the large muscle in the calf of the
leg. This is the largest muscle of the body, and is connected with the
heel bone by the _tendon of Achilles_ (Fig. 119).

The use of these muscles is, in most instances, easily determined by
observing the results of their contraction.

HYGIENE OF THE MUSCLES

The hygiene of the muscles is almost expressed by the one word _exercise_.
It is a matter of everyday knowledge that the muscles are developed and
strengthened by use, and that they become weak, soft, and flabby by
disuse. The effects of exercise are, however, not limited to the large
muscles attached to the skeleton, but are apparent also upon the
involuntary muscles, whose work is so closely related to the vital
processes. While it is true that exercise cannot be applied directly to
the involuntary muscles, it is also true that exercise of the voluntary
muscles causes a greater activity on the part of those that are
involuntary and is indirectly a means of exercising them.

*Exercise and Health.*--In addition to its effects upon the muscles
themselves, exercise is recognized as one of the most fundamental factors
in the preservation of the health. Practically every process of the body
is stimulated and the body as a whole invigorated by exercise properly
taken. On the other hand, a lack of exercise has an effect upon the entire
body somewhat similar to that observed upon a single muscle. It becomes
weak, lacks energy, and in many instances actually loses weight when
exercise is omitted. This shows exercise to supply an actual need and to
be in harmony with the nature and plan of the body.

*How Exercise benefits the Body.*--In accounting for the healthful effects
of exercise, it must be borne in mind that the body is essentially a
motion-producing structure. Furthermore, its plan is such that the
movements of its different parts aid indirectly the vital processes. The
student will recall instances of such aid, as, for example, the assistance
rendered by muscular contractions in the circulation of the blood and
lymph, due to the valves in veins and lymph vessels, and the assistance
rendered by abdominal movements in the propulsion of materials through the
food canal. A fact not as yet brought out, however, is that _exercise
stimulates nutritive changes in the cells_, thereby imparting to them new
vigor and vitality. While this effect of exercise cannot be fully
accounted for, two conditions that undoubtedly influence it are the
following:

1. Exercise causes the blood to circulate more rapidly.

2. Exercise increases the movement of the lymph through the lymph vessels.

The increase in the flow of the blood and the lymph causes changes to take
place more rapidly in the liquids around the cells, thereby increasing the
supply of food and oxygen, and hastening the removal of waste.

*One should plan for Exercise.*--Since exercise is demanded by the nature
and plan of the body, to neglect it is a serious matter. People do not
purposely omit exercise, but from lack of time or from its interference
with the daily routine of duties, the needed amount is frequently not
taken. Especially is this true of students and others who follow sedentary
occupations. People of this class should plan for exercise as they plan
for the other great needs of the body--food, sleep, clothing, etc. It is
only by making a sufficient amount of muscular work or play a regular part
of the daily program that the needs of the body for exercise are
adequately supplied.

*Amount and Kind of Exercise.*--The amount of exercise required varies
greatly with different individuals, and definite recommendations cannot be
made. For each individual also the amount should vary with the physical
condition and the other demands made upon the energy. One in health should
exercise sufficiently to keep the muscles firm to the touch and the body
in a vigorous condition.

Of the many forms of exercise from which one may choose, the question is
again one of individual adaptability and convenience. While the different
forms of exercise vary in their effects and may be made to serve different
purposes, the consideration of these is beyond the scope of an elementary
text. As a rule one will not go far wrong by following his inclinations,
observing of course the conditions under which exercise is taken to the
best advantage.

*General Rules for Healthful Exercise.*--That exercise may secure the best
results from the standpoint of health, a number of conditions should be
observed: 1. It should not be excessive or carried to the point of
exhaustion. Severe physical exercise is destructive to both muscular and
nervous tissues. 2. It should, if possible, be of an interesting nature
and taken in the open air. 3. It should be counter-active, that is,
calling into play those parts of the body that have not been used during
the regular work.(88) 4. It should be directed toward the weak rather than
toward the strong parts of the body. 5. When one is already tired from
study, or other work, it should be taken with moderation or omitted for
the time being. (For exercise of the heart muscle and the muscular coat of
the blood vessels see pages 55 and 57.)

*Massage.*--In lieu of exercise taken in the usual way, similar effects are
sometimes obtained by a systematic rubbing, pressing, stroking, or
kneading of the skin and the muscles by one trained in the art. This
process, known as massage, may be gentle or vigorous and is subject to a
variety of modifications. Massage is applied when one is unable to take
exercise, on account of disease or accident, and also in the treatment of
certain bodily disorders. A weak ankle, wrist, or other part of the body,
or even a bruise, may be greatly benefited by massage. The flow of blood
and lymph is stimulated, causing new materials to be passed to the
affected parts and waste materials to be removed. Massage, however, should
never be applied to a boil, or other infected sore. The effect in this
case would be to spread the infection and increase the trouble.

*Summary.*--Motion is provided for in the body mainly through the muscle
cells. These are grouped into working parts, called muscles, which in turn
are attached to the movable parts of the body. The striated muscles, as a
rule, are attached to the mechanical devices found in the skeleton, and
bring about the voluntary, movements. The non-striated muscles surround
the parts on which they act, and produce involuntary movements. Both,
however, are under the control of the nervous system. To bring about the
opposing movements of the body, the striated muscles are arranged in
pairs; and to increase their motion, the bones are used as levers.
Physical exercise is necessary both for the development of the muscles and
for the health and vigor of the entire body.

*Exercises.*--1. Compare the striated and non-striated muscles with
reference to structure, location, and method of work.

2. In what respects is the muscular tissue of the heart like the striated,
and in what respects like the non-striated, muscular tissue?

3. If muscles could push as well as pull, would so many be needed in the
body? Why?

4. Locate muscles that work to some extent against elasticity and gravity.

5. Locate five muscles that act as flexors; five that act as extensors;
two that act as adductors; and two as abductors. Locate sphincter and
radiating muscles.

6. By what means does the nervous system control the muscles?

7. Give proofs of the change of potential into kinetic energy during
muscular contraction.

8. Define the essential properties of muscular tissue and state the
purpose served by each.

9. Describe a lever. For what general purpose are levers used in the body?
What other purpose do they serve outside of the body?

10. Why are levers of the second class not adapted to the work of the
body?

11. Name the class of lever used in bending the elbow; in straightening
the elbow; in raising the knee; in elevating the toes; and in biting. Why
is one able to bite harder with the back teeth than with the front ones
when the same muscles are used in both cases?

12. Measure the distance from the middle of the palm of the hand to the
center of the elbow joint. Find the attachment of the tendon of the biceps
muscle to the radius and measure its distance to the center of the elbow
joint. From these distances calculate the force with which the biceps
contracts in order to support a weight of ten pounds on the palm of the
hand.

13. How does exercise benefit the health? How does a short walk "clear the
brain" and enable one to study to better advantage?

14. When exercisers taken for its effects upon the health, what conditions
should be observed?

PRACTICAL WORK

The reddish muscle found in a piece of beef is a good example of striated
muscle. The clear ring surrounding the intestine of a cat (shown by cross
section) and the outer portion of the preparation from the cow's stomach,
sold at the butcher shop under the name of _tripe_, are good examples of
non-striated muscular tissue. The heart of any animal, of course, shows
the heart muscle.

*To show the Structure of Striated Muscle.*--Boil a tough piece of beef, as
a cut from the neck, until the connective tissue has thoroughly softened.
Then with some pointed instrument, separate the main piece into its fiber
bundles and these in turn into their smallest divisions. The smallest
divisions obtainable are the muscle cells or fibers.

*To show Striated Fibers.*--Place a small muscle from the leg of a frog in
a fifty-per-cent solution of alcohol and leave it there for half a day or
longer. Then cover with water on a glass slide, and with a couple of fine
needles tease out the small muscle threads. Protect with a cover glass and
examine with a microscope, first with a low and then with a high power.
The striations, sarcolemma, and sometimes the nuclei and nerve plates, may
be distinguished in such a preparation.

*To show Non-striated Cells.*--Place a clean section of the small intestine
of a cat in a mixture of one part of nitric acid and four parts of water
and leave for four or five hours. Thoroughly wash out the acid with water
and separate the muscular layer from the mucous membrane. Cover a small
portion of the muscle with water on a glass slide and tease out, with
needles, until it is as finely divided as possible. Examine with a
microscope, first with a low and then with a high power. The cells appear
as very fine, spindle-shaped bodies.

*To illustrate Muscular Stimulus and Contraction.*--Separate the muscles at
the back of the thigh of a frog which has just been killed and draw the
large sciatic nerve to the surface. Cut this as high up as possible and,
with a sharp knife and a small pair of scissors, dissect it out to the
knee. Now cut out entirely the large muscle of the calf of the leg (the
gastrocnemius), but leave attached to it the nerve, the lower tendon, and
the bones of the knee. Mount on an upright support, as shown in Fig. 120,
and fasten the tendon to a lever below by a thread or small wire hook:

                                [Fig. 120]

      Fig. 120--*Apparatus* for demonstrating properties of muscles.

1. Lay the nerve over the ends of the wires from a small battery which are
attached to the support at _A_, and arrange a second break in the circuit
at _B_. At this place the battery circuit is made and broken either by a
telegraph key or by simply touching and separating the wires. Note that
the muscle gives a single contraction, or twitch, both when the current is
made and when it is broken.

2. Remove the current and pinch the end of the nerve, noting the result.
With very fine wires, connect the battery directly to the ends of the
muscle. Stimulate by making and breaking the current as before. In this
experiment the muscle cells are stimulated by the direct action of the
current and not by the current acting on the nerve.

3. With the wires attached to either the muscle or the nerve, make and
break the current in rapid succession. This causes the muscle to enter
into a second contraction before it has relaxed from the first, and if the
shocks follow in rapid succession, to continue in the contracted state.
This condition, which represents the method of contraction of the muscles
in the body, is called _tetanus_.

NOTE.--In these experiments a twitching of the muscle is frequently
observed when no stimulus is being applied. This is due to the drying out
of the nerve and is prevented by keeping it wet with a physiological salt
solution. (See footnote, page 38.)

*To show the Action of Levers.*--With a light but stiff wooden bar, a
spring balance, and a wedge-shaped fulcrum, show:

1. The position of the weight, the fulcrum, and the power in the different
classes of levers, and also the weight-arm and the power-arm in each case.

2. The direction moved by the power and the weight respectively in the use
of the different classes of levers.

3. That when the power-arm and weight-arm are equal, the power equals the
weight and moves through the same distance.

4. That when the power-arm is longer than the weight-arm, the weight is
greater, but moves through a shorter distance than the power.

5. That when the weight-arm is longer than the power-arm, the power is
greater and moves through a shorter distance than the weight.

*To show the Loss of Power in the Use of the Body Levers.*--Construct a
frame similar to, but larger than, that shown in Fig. 120, (about 12
inches high), and hang a small spring balance (250 grams capacity) at the
place where the muscle is attached. Fasten the end of a lever to the
upright piece, at a point on a level with the end of the balance hook.
(The nail or screw used for this purpose must pass loosely through the
lever, and serve as a pivot upon which it can turn.) The lever should
consist of a light piece of wood, and should have a length at least three
times as great as the distance from the hook to the turning point. Connect
the balance hook with the lever by a thread or string, and then hang upon
it a small body of known weight. Note the amount of force exerted at the
balance in order to support the weight at different places on the lever.
At what point is the force just equal to the weight? Where is it twice as
great? Where three times? Show that the force required to support the
weight increases proportionally as the weight-arm and as the distance
through which the weight may be moved by the lever. Apply to the action of
the biceps muscle in lifting weights on the forearm.

*A Study of the Action of the Biceps Muscle.*--Place the fingers upon the
tendon of the biceps where it connects with the radius of the forearm.
With the forearm resting upon the table, note that the tendon is somewhat
loose and flaccid, but that with the slightest effort to raise the forearm
it quickly tightens. Now transfer the fingers to the body of the muscle,
and sweep the forearm through two or three complete movements, noting the
changes in the length and thickness of the muscle. Lay the forearm again
on the table, back of hand down, and place a heavy weight (a flatiron or a
hammer) upon the hand. Note the effort required to raise the weight, and
then shift it along the arm. Observe that the nearer it approaches the
elbow the lighter it seems. Account for the difference in the effort
required to raise the weight at different places. Does the effort vary as
the distance from the tendon?

CHAPTER XVI - THE SKIN

Protective coverings are found at all the exposed surfaces of the body.
These vary considerably at different places, each being adapted to the
conditions under which it serves. The most important ones are the _skin_,
which covers the entire external surface of the body; the _mucous
membrane_, which lines all the cavities that communicate by openings with
the external surface; and the _serous membrane_, which, including the
synovial membranes, lines all the closed cavities of the body. In addition
to the protection which it affords, the skin is one of the means by which
the body is brought into proper relations with its surroundings. It is
because of this function that we take up the study of the skin at this
time.

*The Skin* is one of the most complex structures of the body, and serves
several distinct purposes. It is estimated to have an area of from 14 to
16 square feet, and to have a thickness which varies from less than one
eighth to more than one fourth of an inch. It is thickest on the palms of
the hands and the soles of the feet, the places where it is most subject
to wear. It is made up of two distinct layers--an outer layer called the
_epidermis_, or cuticle, and an inner layer called the _dermis_, or cutis
vera (Fig. 121).

*The Dermis.*--This is the thicker and heavier of the two layers, and is
made up chiefly of connective tissue. The network of tough fibers which
this tissue supplies,  forms the essential body of the dermis and gives to
it its power of resistance. It is on account of the connective tissue that
the skins of animals can be converted into leather by tanning. A variety
of structures, including blood and lymph vessels, oil and perspiratory
glands, hair follicles, and nerves, are found embedded in the connective
tissue (Fig. 122). These aid in different ways in the work of the skin.

                                [Fig. 121]

  Fig. 121--*Section of skin* magnified, _a, b._ Epidermis, _b._ Pigment
  layer. _c._ Papillae, _d._ Dermis. _e._ Fatty tissue. _f, g, h._ Sweat
        gland and duct. _i, k._ Hair and follicle. _l._ Oil gland.

On the outer surface of the dermis are numerous elevations, called
_papillae_. These average about one one-hundredth of an inch in height, and
one two hundred and fiftieth of an inch in diameter. They are most
numerous on the palms of the hands, the soles of the feet, and the under
surfaces of the fingers and toes. At these places they are larger than in
other parts of the body, and are closely grouped, forming the parallel
curved ridges which cover the surfaces. Each papilla contains a loop of
capillaries and a small nerve, and many of them are crowned with touch
corpuscles (page 342).

                                [Fig. 122]

 Fig. 122--*Diagram* of section of skin showing its different structures.

*The Epidermis* is much thinner than the dermis. It is made up of several
layers of cells which are flat and scale-like at the surface, but are
rounded in form where the epidermis joins the dermis. The epidermis has
the appearance of being _moulded onto_ the dermis, filling up the
depressions between the papillae and having corresponding irregularities
(Fig. 121). No blood vessels are found in the epidermis, its nourishment
being derived from the lymph which reaches it from the dermis. Only the
part next to the dermis is made up of _living_ cells. These are active,
however, in the formation of new cells, which take the place of those that
are worn off at the surface. Some of the cells belonging to the inner
layer of epidermis contain _pigment granules_, which give the skin its
color (Fig. 121). The epidermis contains no nerves and is therefore
non-sensitive. The hair and the nails are important modifications of the
epidermis.

*A Hair* is a slender cylinder, formed by the union of epidermal cells,
which grows from a kind of pit in the dermis, called the _hair follicle_.
The oval and somewhat enlarged part of the hair within the follicle is
called the _root_, or _bulb_, and the uniform cylinder beyond the follicle
is called the _shaft_. Connected with the sides of the follicles are the
_oil_, or _sebaceous, glands_ (Figs. 121 and 122). These secrete an oily
liquid which keeps the hair and cuticle soft and pliable. Attached to the
inner ends of the follicles are small, involuntary muscles whose
contractions cause the roughened condition of the skin that occurs on
exposure to cold.

*A Nail* is a tough and rather horny plate of epidermal tissue which grows
from a depression in the dermis, called the _matrix_. The back part of the
nail is known as the _root_, the middle convex portion as the _body_, and
the front margin as _the free edge_ (Fig. 123). Material for the growth of
the nail is derived from the matrix, which is lined with active epidermal
cells and is richly supplied with blood vessels. Cells added to the root
cause the nail to grow in length (forward) and cells added to the under
surface cause it to grow in thickness. The cuticle adheres to the nail
around its entire circumference so that the covering over the dermis is
complete.

                                [Fig. 123]

      Fig. 123--*Section of end of finger* showing nail in position.

*Functions of the Skin.*--The chief function of the skin is that of
protection. It is able to protect the body on account of the tough
connective tissue in the dermis, the non-sensitive cells of the epidermis,
and also by the touch corpuscles and their connecting nerve fibers. This
protection is of at least three kinds, as follows:

1. _From mechanical injuries_ such as might result from contact with hard,
rough, or sharp objects. The main quality needed for resisting mechanical
injuries is _toughness_, and this is supplied both by the epidermis and by
the connective tissue of the dermis.

2. _From chemical injuries_ caused by contact with various chemical
agents, as acids, alkalies, and the oxygen of the air. The epidermis,
being of such a nature as to resist to a considerable extent the action of
chemical agents, affords protection from these substances. (89)

3. _From disease germs_ which are everywhere present. The epidermis is the
main protective agent against attacks of germs, but should the epidermis
be broken, they meet with further resistance from the fluids of the dermis
and the white corpuscles of the blood.

4. _From an excessive evaporation of liquid from the surface of the body_.
In the performance of this function, the skin is an important means of
keeping the tissues soft and the blood and lymph from becoming too
concentrated.

*Other Functions of the Skin.*--Through the perspiratory glands the skin is
an _organ of excretion_. While the secretion from a single gland is small,
the waste that leaves the body through all of the perspiratory glands is
considerable (90) (page 206). By means of the nerves terminating in the
touch corpuscles, the skin serves as the _organ of touch_, or feeling
(Chapter XX). To a slight extent also the skin may absorb liquid
substances, these being taken up by the blood and lymph vessels, and
perform a respiratory function, throwing off carbon dioxide. But the most
important function of the skin, in addition to protection, is that of
serving as

*An Organ of Adaptation.*--Forming, as it does, the boundary between the
body and its physical environment, the skin is perhaps the most important
agent through which the body is adapted to its immediate surroundings.
Evidence of this is found in the great variety of influences which are
able to affect the body through their action upon the nerves in the skin,
and in the changes which the epidermis undergoes on exposure. The latter
function is especially marked in the lower animals, the coverings of
epidermal tissue (hair, scales, feathers, etc.) adapting each species to
the physical conditions under which it lives. In man the most striking
example of adaptation through the skin is seen in the variations in the
quantity of blood circulating through it, corresponding to the changes in
the temperature outside of the body. These variations are of great
importance, having to do with the

*Maintenance of the Normal Temperature.*--It is necessary to the
continuance of life that the temperature of the body be kept at a nearly
uniform degree, called the _normal temperature_, which is about 98.6 deg. F.
The maintenance of the normal temperature depends mainly upon four
conditions: the chemical changes at the cells, the circulation of the
blood, the nervous system, and _the skin_. The chemical changes produce
the heat, the blood in its circulation distributes the heat over the body,
and the nervous system controls the heat-producing and distributing
processes (page 320). The skin is the chief means by which the body gets
rid of an excess of heat and, by so doing, avoids overheating. (91)

*How the Skin cools the Body.*--The skin is a means of ridding the body of
an excess of heat in at least two ways:

1. _By the conduction and radiation of heat from its surface_ as from a
stove. This goes on all the time, but varies with the amount of heat
brought to the surface by the blood.

2. _By the evaporation of the perspiration._ It is a well-established and
easily demonstrated principle that liquids in evaporating use up heat.(See
Practical Work.) It is also a matter of everyday experience that the
perspiration has a cooling effect upon the body and that its flow
increases with the amount of heat to be gotten rid of. The quantity of
perspiration secreted, and of heat disposed of through its evaporation,
also varies with the amount of blood circulating through the skin.

*Temperature Regulation by the Skin.*--Variations in the quantity of blood
circulating through the skin enable this organ to throw off just the right
amount of heat for keeping the body at the normal temperature. If it is
necessary for the body to rid itself of an excess of heat, the quantity of
blood circulating in the skin is increased. This brings the blood near the
surface, where more heat can be radiated and where it may cause an
increase in the perspiration. On the other hand, if the body is in danger
of losing too much heat, the circulation diminishes in the skin and
increases in the internal organs. This stops the rapid loss of heat from
the surface. The skin in this work is of course made to cooperate with
other parts of the body. That it is not the only organ concerned in
regulating the escape of heat is seen in the results that follow
sensations either of chilliness or of heat at the surface.

*Effects of Heat and Cold Sensations.*--Sensations, or feelings, of heat
and cold are made possible through the nerves which connect the brain with
the _temperature corpuscles_, found in the skin (page 343). As the warm
blood recedes from the skin, a sensation of cold is felt, but when the
blood returns, there is again the feeling of warmth. The sensation of cold
prompts one to seek a warmer place, or to put on more clothing; while the
sensation of heat, if it be oppressive, leads to activities of an opposite
kind. Prompted in this way by the sensations from the skin, one
voluntarily supplies the external conditions, such as clothing and heat,
that affect the body temperature.

*Alcohol and the Regulation of Temperature.*--Alcohol, through its effect
upon the nervous system, interferes seriously with the regulation of the
body temperature. By dilating the capillaries, it increases the
circulation in the skin and leads to an undue loss of heat. At the same
time the excess of blood in the skin causes a _feeling of warmth_ which
has led to the erroneous belief that alcohol is a heat producer. If taken
on a cold day, it deceives one about his true condition and leads to a
wasting of heat when it should be carefully economized. Not only is
alcohol of no value in maintaining the body temperature, but if taken
during severe exposure to cold, it becomes a menace to life itself.
Arctic, explorers and others exposed to severe cold have found that they
withstand cold far better when no alcohol at all is used.(92)

HYGIENE OF THE SKIN

Much of the hygiene of the skin is included in the problems of keeping it
warm and clean. It is kept warm by clothing; bathing is the method of
keeping it clean.

*Clothing* should be warm and loose-fitting. Woolen fabrics are to be
preferred in winter to cotton because, being poorer conductors of heat,
they afford better protection from the cold. But wool fails to absorb the
perspiration rapidly from the skin and to pass it to the outside where it
is evaporated. This, together with its tendency to irritate, makes woolen
clothing somewhat objectionable for wearing next to the skin. This
objection, however, is obviated by woolen underwear which is lined by a
thin weaving of cotton.

*Bathing.*--The solid material from the perspiration, which is left on the
skin, together with the oil from the oil glands and the dirt from the
outside, tends to close up the pores and develop offensive odors. Keeping
the skin clean is, for these reasons, necessary from both a health and a
social standpoint. While one should always keep clean, the frequency of
the bath will depend upon the season, the occupation of the individual,
and the nature and amount of the perspiration. As to the kind of bath to
be taken and the precautions to be observed, no specific rules can be laid
down. These must be determined by the facilities at hand and by the health
and natural vigor of the bather. Severe chilling of the body should be
avoided, especially by those in delicate health. If a hot bath is taken,
one should dash cold water over the body on finishing. One should then
quickly dry and rub the body with a coarse towel. The dash of cold water
closes the pores of the skin and lessens the liability of taking cold.

*The Tonic Bath.*--The cold bath has been found to have a beneficial effect
upon the general health beyond its effect upon the skin. When taken with
care as to the length of time and the degree of cold, decided tonic
effects are observed on the circulation and on the nervous system. The
rapid changes of temperature vigorously exercise the non-striated muscles
of the blood vessels (page 57) and the nerves controlling them. The
irritability of the nervous system in general is also lessened. For this
reason the cold bath is one of the best means of keeping both mind and
body in good condition during the warm months. Sponging off the body with
cold or tepid water before retiring is also an excellent aid in securing
sound sleep during the hot summer nights.

Danger from the cold bath arises through the shock to the nervous system
and the loss of heat from the body. It is avoided by using water whose
temperature is not too low and by limiting the time spent in the bath. A
brisk rubbing with a coarse towel should always follow the cold bath.
People past middle age are, as a rule, not benefited by the cold bath; and
those in delicate health, especially if inclined toward rheumatism, are
likely to be affected injuriously by it.

*Care of the Complexion.*--A good complexion is a natural accompaniment of
good health and depends primarily upon two conditions--a clear skin and an
active circulation of the blood through it. Clearness of the skin depends
largely upon the elimination of waste material from the body, and where
the solid wastes are not effectively removed through the natural channels
(the liver, kidneys, and bowels), blotches, sallowness of the skin, and
skin eruptions are likely to result. In seeking to clear the complexion,
attention must be given to all those agencies that favor the elimination
of waste, and especially should there be a free and thorough evacuation of
the bowels each day. The general health should also be looked after,
attention being given to exercise, fresh air, proper food,(93) sufficient
sleep, etc.

Bathing is the chief means employed for increasing the circulation in the
skin, although exercise which is sufficiently vigorous to cause one to
perspire freely is a valuable aid. A daily bath of warm or hot water,
finished off with a dash of cold, followed by a thorough rubbing of the
entire surface, and this by a kneading of the skin with the thumbs and
fingers, will in most cases bring about the desired results. A little
olive oil, thoroughly worked into the skin during the kneading process, is
beneficial where one lacks flesh or where the skin is dry and thin. The
olive oil is also beneficial where the baths are exhausting or render one
susceptible to cold. In rubbing and kneading, the skin should not be
bruised or irritated.

The much advertised "complexion beautifiers" which are applied directly to
the face frequently have the effect of clogging the pores and of causing
eruptions of the skin. On the other hand, certain authorities state that
the cold cream preparations may be of advantage in giving the skin a
desired softness, and that when judiciously used (the face being cleansed
after each application) they do no harm. Of the different kinds of face
powder those prepared from rice are considered the least injurious.

*Treatment of Skin Wounds.*--Skin wounds which may not be serious in
themselves frequently become so through getting infected with germs. Blood
poisoning often results from such infections, one of the worst forms being
_tetanus_, or lockjaw. A wound should be kept clean, and if it shows signs
of infection, it should be washed with some antiseptic solution. Or, it
may be cleansed with pure warm water and then covered with some antiseptic
ointment,(94) of which there are a number on the market. A weak solution
of carbolic acid (one part acid to twenty-five parts of water) makes an
excellent antiseptic wash. It may be used not only for cleansing wounds,
but also in counteracting the poisonous effects that follow the bites of
insects.

A wound resulting from the bite of an animal (cat or dog), even though
slight, should receive more serious attention, and as soon as possible
after the occurrence. Such wounds should be cauterized, and for this
purpose pure carbolic, acid (undiluted with water) may be used. A wooden
toothpick is dipped into the acid and this is worked about in the wound.
The acid is then washed out with warm water. A deep wound from a rusty
nail or a thorn should be treated in the same manner and should be kept
open, not being allowed to heal at the surface first. If one has reason to
believe he has been bitten by a mad dog, the wound should be cauterized as
above, and a physician should be summoned at once. Deep wounds from
explosives, or other causes, should also receive the attention of the
physician. Many cases of lockjaw result every year from wounds inflicted
by the toy pistols, firecrackers, etc., used in our Fourth of July
celebrations. These are due to the embedding in the skin or flesh of small
solid particles on which are lockjaw germs. Wounds of this nature should,
of course, receive the attention of the physician.

*Care of the Nails.*--Relief from a blood blister under the nail is secured
by boring a small hole through the nail with the sharp point of a
sterilized penknife (page 38). This simple bit of surgery not only
relieves the pain, but is frequently the only means of saving the nail.
Ingrown toe nails are relieved by scraping a broad strip in the middle of
the nail until very thin. This relieves the pressure, preventing the sides
of the nail from being forced into the toe. While the finger nails should
be trimmed in a curve, corresponding to the end of the finger, it is
recommended that the toe nails be cut straight across (Fig. 124), as this
method diminishes the pressure from the shoe and keeps the nails from
ingrowing. Shoes that pinch the toes should, of course, not be worn (page
238).

                                [Fig. 124]

            Fig. 124--Proper method of trimming nails of toes.

*Care of the Hair.*--Occasional washing of the hair is beneficial, but too
much wetting causes decay of the hair roots, which leads to its falling
out. The worst enemy of the hair is dandruff. A method of removing
dandruff which is highly recommended is that of rubbing olive oil into the
scalp and later of removing this with a cleansing shampoo. The olive oil
is placed on the scalp with a medicine dropper and thoroughly rubbed in
with the fingers. After three or four hours the hair is washed with soap
and water (any good toilet soap will do) and rinsed with pure water. The
hair is then dried, the surplus water being removed with a coarse towel.
Where the dandruff is very troublesome, this treatment may be given once
or twice a week; but in mild cases once a month is sufficient. Massage of
the scalp, by increasing the circulation at the hair roots, is beneficial,
but irritation by a fine-tooth comb, a stiff hair brush, or by other means
should be avoided. Frequent brushing and combing, however, are necessary
both for the good appearance of the hair and for spreading the oil
secreted by the glands at the hair roots.

*Summary.*--The skin forms the external covering of the body and also
serves additional purposes. It is a most important agency in adapting the
body to its physical surroundings, as shown by the part which it plays in
the regulation of the body temperature. The skin should be kept clean and
active, and skin wounds, even though small, should be guarded against
infection.

*Exercises.*--1. Name an example of each of the protective coverings of the
body.

2. Compare the dermis and the epidermis with reference to thickness,
composition, and function.

3. To what is the color of the skin due? How is the color of the skin
affected by the sunlight?

4. What modifications of the epidermis are found on our bodies? What are
found on the body of a chicken?

5. What different kinds of protection are provided by the skin?

6. How does the perspiration cool the body?

7. What change occurs in the circulation in the skin when the body is
becoming too cold? When becoming too warm? What is the purpose of these
changes?

8. How does alcohol cause one to _feel_ warm when he may be losing too
much of his heat?

9. What precaution should be observed by one in poor health, in taking a
bath?

10. How may the cold bath be a means of improving the general health?

PRACTICAL WORK

*Observations on the Skin and its Appendages.*--Examine the palm of the
hand with a lens. Note the small ridges which correspond to the rows of
papillae beneath the cuticle. In these find small pits, which are the
openings of the sweat glands.

2. Examine the epidermis on the back of the hand and palm. At which place
is it thickest and most resisting? Is it of uniform thickness over the
palm? Try picking it with a pin at the thickest place, noting if pain is
felt. Inference?

3. Examine a finger nail. Is the free edge or the root the thickest? Trim
closely the thumb nail and the nail of the middle finger of one hand and
try to pick up a pin, or other minute object, from a smooth, hard surface.
The result indicates what use of the nails? Suggest other uses.

4. Examine with a microscope under a low power hairs from a variety of
animals, as the horse, dog, cat, etc., noting peculiarities of form and
surface.

*To illustrate Cooling Effects of Evaporation.*--1. Wet the back of the
hand and move it through the air to hasten evaporation. Observe that, as
the hand dries, a sensation of cold is felt. Repeat the experiment, using
ether, alcohol, or gasolene instead of the water, noting the differences
in results. These liquids evaporate faster than water.

2. Wet the bulb of a thermometer with alcohol or water. Move it through
the air to hasten evaporation. Note and account for the fall of the
mercury.

CHAPTER XVII - STRUCTURE OF THE NERVOUS SYSTEM

*Cooerdination and Adjustment.*--If we consider for a moment the movements
of the body, we cannot fail to note the cooeperation of organs, one with
another. In the simple act of whittling a stick one hand holds the stick
and the other the knife, while the movements of each hand are such as to
aid in the whittling process. Examples of cooeperation are also found in
the taking of food, in walking, and in the performance of different kinds
of work. Not only is cooeperation found among the external organs, but our
study of the vital processes has shown that the principle of cooeperation
is carried out by the internal organs as well. The fact that all the
activities of the body are directed toward a common purpose makes the
cooeperation of its parts a necessity. The term "cooerdination" is employed
to express this cooeperation, or working together, of the different parts
of the body.

A further study of the movements of the body shows that many of them have
particular reference to things outside of it. In going about one naturally
avoids obstructions, and if anything is in the way he walks around or
steps over it. Somewhat as a delicate instrument (the microscope for
example) is altered or adjusted, in order to adapt it to its work, the
parts of the body, and the body as a whole, have to be _adjusted_ to their
surroundings. This is seen in the attitude assumed in sitting and in
standing, in the position of the hands for different kinds of work, in the
variations of the circulation of the blood in the skin, and in the
movements for protecting the body.(95)

*Work of the Nervous System.*--How are the different activities of the body
controlled and cooerdinated? How is the body adjusted to its surroundings?
The answer is found in the study of the nervous system. Briefly speaking,
the nervous system controls, cooerdinates, and adjusts the different parts
of the body by fulfilling two conditions:

1. It provides a complete system of connections throughout the body,
thereby bringing all parts into communication.

2. It supplies a means of controlling action (the so-called impulse) which
it passes along the nervous connections from one part of the body to
another.

The present chapter deals with the first of these conditions; the chapter
following, with the second.

*The Nerve Skeleton.*--If all the other tissues are removed, leaving only
the nervous tissue, a complete skeleton outline of the body still remains.
This nerve skeleton, as it has been called, has the general form of the
framework of bones, but differs from it greatly in the fineness of its
structures and the extent to which it represents every portion of the
body. An examination of a nerve skeleton, or a diagram of one (Fig. 125),
shows the main structures of the nervous system and their connection with
the different parts of the body.

Corresponding to the skull and the spinal column is a central nervous
axis, made up of two parts, the _brain_ and the _spinal cord_. From this
central axis white, cord-like bodies emerge and pass to different parts of
the body. These are called _nerve trunks_, and the smaller branches into
which they divide are called _nerves_. The nerves also undergo division
until they terminate as fine thread-like structures in all parts of the
body. The distribution of nerve terminations, however, is not uniform, as
might be supposed, but the skin and important organs like the heart,
stomach, and muscles are the more abundantly supplied. On many of the
nerves are small rounded masses, called _ganglia_, and from many of these
small nerves also emerge. At certain places the nerves and ganglia are so
numerous as to form a kind of network, known as a _plexus_.

                                [Fig. 125]

 Fig. 125--*Diagram of nerve skeleton.* The illustration shows the extent
  and general arrangement of the nervous tissue. _A._ Brain. _B._ Spinal
            cord. _N._ Nerve trunks and nerves. _G._ Ganglia.

It is through these structures--brain and spinal cord, nerve trunks and
nerves, ganglia and nerve terminations--that connections are established
between all parts of the body, but more especially between the surface of
the body and the organs within.

*The Neurons, or Nerve Cells.*--While a hasty examination of the nerve
skeleton is sufficient to show the connection of the nervous system with
all parts of the body, no amount of study of its gross structures reveals
the nature of its connections or suggests its method of operation. Insight
into the real nature of the nervous system is obtained only through a
study of its minute structural elements. These, instead of being called
cells, as in the case of the other tissues, are called _neurons_. The use
of this term, instead of the simpler one of nerve cell, is the result of
recent advances in our knowledge of the nervous system.(96)

                                [Fig. 126]

 Fig. 126--*Diagram of a mon-axonic neuron* (greatly enlarged except as to
      length). The central thread in the axon is the axis cylinder.

The neurons are in all respects cells. They differ widely, however, from
all the other cells of the body and are, in some respects, the most
remarkable of all cells. They are characterized by minute extensions, or
prolongations, which in some instances extend to great distances. Though
the neurons in certain parts of the body differ greatly in form and size
from those in other parts of the body, most of them may be included in one
or the other of two classes, known as _mon-axonic_ neurons and _di-axonic_
neurons.

*Mon-axonic Neurons.*--Neurons of this class consist of three distinct
parts, known as the cell-body, the dendrites, and the axon (Fig. 126).

The _cell-body_ has in itself the form of a complete cell and was at one
time so described. It consists of a rounded mass of protoplasm, containing
a well-defined nucleus. The protoplasm is similar to that of other cells,
but is characterized by the presence of many small granules and has a
slightly grayish color.

The _dendrites_ are short extensions from the cell-body. They branch
somewhat as the roots of a tree and form in many instances a complex
network of tiny rootlets. Their protoplasm, like that of the cell-body, is
more or less granular. The dendrites increase greatly the surface of the
cell-body, to which they are related in function.

The _axon_, or nerve fiber, is a long, slender extension from the
cell-body, which connects with some organ or tissue. It was at one time
described as a distinct nervous element, but later study has shown it to
be an outgrowth from the cell-body. The mon-axonic neurons are so called
from their having but a single axon.

*Di-axonic Neurons.*--Neurons belonging to this class have each a
well-defined cell-body and two axons, but no parts just like the dendrites
of mon-axonic neurons. The cell-body is smooth and rounded, and its axons
extend from it in opposite directions (Fig. 127).

                                [Fig. 127]

   Fig. 127--*Diagram of a di-axonic neuron.* The diagram shows only the
            conducting portion of the axon, or axis cylinder.

*Structure of the Axon.*--The axon, or nerve fiber, has practically the
same structure in both classes of neurons, being composed in most cases of
three distinct parts. In the center, and running the entire length of the
axon, is a thread-like body, called the _axis cylinder_ (Fig. 126). The
axis cylinder is present in all axons and is the part essential to their
work. It may be considered as an extension of the protoplasm from the
cell-body. Surrounding the axis cylinder is a thick, whitish-looking
layer, known as the _medullary sheath_, and around this is a thin
covering, called the _primitive sheath_, or neurilemma. The medullary
sheath and the primitive sheath are not, strictly speaking, parts of the
nerve cell, but appear to be growths that have formed around it. Certain
of the axons have no primitive sheath and others are without a medullary
sheath.(97)

*Form and Length of Axons.*--Where the axons terminate they usually
separate into a number of small divisions, thereby increasing the number
of their connections. Certain axons are also observed to give off branches
before the place of termination is reached (Fig. 131). These collateral
branches, by distributing themselves in a manner similar to the main
fiber, greatly extend the influence of a single neuron.

In the matter of length, great variation is found among the axons in
different parts of the body. In certain parts of the brain, for example,
are fibers not more than one one-hundredth of an inch in length, while the
axons that pass all the way from the spinal cord to the toes have a length
of more than three feet. Between these extremes practically all variations
in length are found.

*Arrangements of the Neurons.*--Nowhere in the body do the neurons exist
singly, but they are everywhere connected with each other to form the
different structures observed in the nerve skeleton. Two general plans of
connection are to be observed, known as the anatomical and the
physiological, or, more simply speaking, as the "side-by-side" and
"end-to-end" plans. The side-by-side plan is seen in that disposition of
the neurons which enables them to form the nerves and the ganglia, as well
as the brain and spinal cord. The end-to-end connections are necessary to
the work which the neurons do.

*Side-by-side Connections.*--On separating the ganglia and nerves into
their finest divisions, it is found that the nerves consist of axons,
while the ganglia are made up mainly of cell-bodies and dendrites. The
axons lie side by side in the nerve, being surrounded by the same
protective coverings, while the cell-bodies form a rounded mass or
cluster, which is the ganglion (Fig. 128). But the axons, in order to
connect with the cell-bodies, must terminate within the ganglion, so that
they too form a part of it. To some extent, also, axons pass through
ganglia with which they make no connection. The neurons in the brain and
spinal cord also lie side by side, but their arrangement is more complex
than that in the nerves and ganglia.

                                [Fig. 128]

 Fig. 128--*Diagrams illustrating arrangement of neurons.* _A, B._ Ganglia
 and short segments of nerves. 1. Ganglion. 2. Nerve. In the ganglion of
 _A_ are end-to-end connections of different neurons; in the ganglion of
  _B_ are the cell-bodies of di-axonic neurons. _C._ Section of a nerve
 trunk. 1. Epineurium consisting chiefly of connective tissue. 2. Bundles
  of nerve fibers. 3. Covering of fiber bundle, or perineurium. 4. Small
                             artery and vein.

The side-by-side arrangement of the neurons shows clearly the structure of
the ganglia and nerves. The nerve is seen to be a bundle of axons, or
nerve fibers, held together by connective tissue, while the ganglion is
little more than a cluster of cell-bodies. Their connection is necessarily
very close, for the same group of neurons will form, with their axons, the
nerve, and, with their cell-bodies, the ganglion (Fig. 128).

*End-to-end Connections.*--These consist of loose end-to-end unions of the
fiber branches of certain neurons with the dendrites of other neurons. The
purpose of such connections is to provide the means of communication
between different parts of the body. There appears to be no actual uniting
of the fiber branches with the dendrites, but they come into relations
sufficiently close to establish _conduction pathways_, and these extend
throughout the body (Fig. 129). They connect all parts of the body with
the brain and spinal cord, while connections within the brain and cord
bring the parts into communication with each other.

                                [Fig. 129]

Fig. 129--*Diagram of a nerve path* starting at the skin, extending through
the spinal cord, and passing out to muscles. A division of this path also
                            reaches the brain.

*Nature of the Nervous System.*--The nervous system represents the sum
total of the neurons in the body. In some respects it may be compared to
the modern telephone system. The neurons, like the electric wires, connect
different places with a central station (the brain and spinal cord), and
through the central station connections are established between the
different places in the system. As the separate wires are massed together
to form cables, the neurons are massed to form the gross structures of the
nervous system. The nervous system, however, is so radically different
from anything found outside of the animal body that no comparison can give
an adequate idea of it. We now pass to a study of the gross structures
observed in the nerve skeleton.

*Divisions of the Nervous System.*--While all of the nervous structures are
very closely blended, forming one complete system for the entire body,
this system presents different divisions which may, for convenience, be
studied separately. As physiologists have become better acquainted with
the human nervous system, different schemes of classification have been
proposed. The following outline, based upon the location of the different
parts, presents perhaps the simplest view of the entire group of nervous
structures:

                                 [Table]

*The Central Division.*--This division of the nervous system lies within
the cranial and spinal cavities, and consists of the brain and the spinal
cord. The brain occupying the cranial cavity and the spinal cord in the
spinal cavity connect with each other through the large opening at the
base of the skull to form one continuous structure. The brain and cord are
the most complicated portions of the nervous system, and the ones most
difficult to understand.

                                [Fig. 130]

                Fig. 130--*Diagram of divisions of brain.*

*The Brain.*--The brain, which is the largest mass of nervous tissue in the
body, weighs in the average sized man about 50 ounces, and in the average
sized woman about 44 ounces.(98) It may be roughly divided into three
parts, which are named from their positions (in lower animals) the
forebrain, the midbrain, and the hindbrain (Fig. 130). The forebrain
consists almost entirely of a single part, known as

*The Cerebrum.*--The cerebrum comprises about seven eighths of the entire
brain, and occupies all the front, middle, back, and upper portions of the
cranial cavity, spreading over and concealing, to a large extent, the
parts beneath. The surface layer of the cerebrum is called the _cortex_.
This is made up largely of cell-bodies, and has a grayish appearance.(99)
The cortex is greatly increased in area by the presence everywhere of
ridge-like _convolutions_, between which are deep but narrow depressions,
called _fissures_. The interior of the cerebrum consists mainly of nerve
fibers, or axons, which give it a whitish appearance. These fibers connect
with the cell-bodies in the cortex (Fig. 131).

The cerebrum is a double organ, consisting of two similar divisions,
called the _cerebral hemispheres_. These are separated by a deep groove,
extending from the front to the back of the brain, known as the _median
fissure_. The hemispheres, however, are closely connected by a great band
of underlying nerve fibers, called the _corpus callosum_.

                                [Fig. 131]

Fig. 131--*Microscope drawing* of a neuron from cerebral cortex. _a._ Short
          segment of the axis cylinder with collateral branches.

At the base of the cerebrum three large masses of cell-bodies are to be
found. One of these, a double mass, occupies a central position between
the hemispheres, and is called the _optic thalami_. The other two occupy
front central positions at the base of either hemisphere, and are known as
the _corpora striata_, or the striate bodies.

*The Midbrain* is a short, rounded, and compact body that lies immediately
beneath the cerebrum, and connects it with the hindbrain. On account of
the great size of the cerebrum, the midbrain is entirely concealed from
view when the other parts occupy their normal positions. However, if the
cerebrum is pulled away from the hindbrain, it is brought into view
somewhat as in Fig. 130.

The midbrain carries upon its back and upper surface four small rounded
masses of cell-bodies, called the _corpora quadrigemina_. The upper two of
these bodies are connected with the eyes; the lower two appear to have
some connection with the organs of hearing. On the front and under
surface, the midbrain separates slightly as if to form two pillars, which
are called the _crura cerebri_, or cerebral peduncles. These contain the
great bundles of nerve fibers that connect the cerebrum with the parts of
the nervous system below.

*The Hindbrain* lies beneath the back portion of the cerebrum, and
occupies the enlargement at the base of the skull. It forms about one
eighth of the entire brain, and is composed of three parts--the cerebellum,
the pons, and the bulb.

*The Cerebellum* is a flat and somewhat triangular structure with its
upper surface fitting into the triangular under surface of the back of the
cerebrum. It is divided into three lobes--a central lobe and two lateral
lobes--and weighs about two and one half ounces. In its general form and
appearance, as well as in the arrangement of its cell-bodies and axons,
the cerebellum resembles the cerebrum. It differs from the cerebrum,
however, in being more compact, and in having its surface covered with
narrow, transverse ridges instead of the irregular and broader
convolutions (Fig. 132).

*The Pons*, or pons Varolii, named from its supposed resemblance to a
bridge, is situated in front of the cerebellum, and is readily recognized
as a circular expansion which extends forward from that body. It consists
largely of bands of nerve fibers, between which are several small masses
of cell-bodies. The fibers connect with different parts of the cerebellum
and with parts above.

                                [Fig. 132]

Fig. 132--*Human brain* viewed from below. _C._ Cerebrum. _Cb._ Cerebellum.
       _M._ Midbrain. _P._ Pons. _B._ Bulb. I-XII. Cranial nerves.

*The Bulb*, or medulla oblongata, is, properly speaking, an enlargement of
the spinal cord within the cranial cavity. It is somewhat triangular in
shape, and lies immediately below the cerebellum. It contains important
clusters of cell-bodies, as well as the nerve fibers that pass from the
spinal cord to the brain.

*The Spinal Cord.*--This division of the central nervous system is about
seventeen inches in length and two thirds of an inch in diameter. It does
not extend the entire length of the spinal cavity, as might be supposed,
but terminates at the lower margin of the first lumbar vertebra.(100) It
connects at the upper end with the bulb, and terminates at the lower
extremity in a number of large nerve roots, which are continuous with the
nerves of the hips and legs (Fig. 133). Two deep fissures, one in front
and the other at the back, extend the entire length of the cord, and
separate it into two similar divisions. These are connected, however,
along their entire length by a central band consisting of both gray and
white matter.

                                [Fig. 133]

 Fig. 133--*Spinal cord*, showing on one side the nerves and ganglia with
    which it is closely related in function. _A._ Bulb. _B._ Cervical
enlargement. _C._ Lumbar enlargement. _D._ Termination of cord. _E._ Nerve
  roots that occupy the spinal cavity below the cord. _P._ Pons. _D.G._
  Dorsal root ganglia. _S.G._ Sympathetic ganglia. _N._ Nerve trunks to
                       upper and lower extremities.

The arrangement of the neurons of the spinal cord is just the reverse of
that in the cerebrum--the center being occupied by a double column of
cell-bodies, which give it a grayish appearance, while the fibers occupy
the outer portion of the cord, giving it a whitish appearance.

The spinal cord is not uniform in thickness, but tapers slightly, though
not uniformly, from the upper toward the lower end. At the places where
the nerves from the arms and legs enter the cord two enlargements are to
be found, the upper being called the _cervical_ and the lower the _lumbar
enlargement_. These, on account of the difference in length between the
cord and the spinal cavity, are above--the lower one considerably above--the
places where the limbs which they supply join the trunk (Fig. 133).

*Arrangement of the Neurons of the Brain and Cord.*--The cell-bodies in the
brain and spinal cord are collected into groups, and their fibers extend
from these groups to places that may be near or remote. Guided by the
white and gray colors of the nervous tissue, and also by the structures
revealed by the microscope, physiologists have made out three general
schemes in the grouping of cell-bodies, as follows:

1. _That of surface distribution_, the cell-bodies forming a thin but
continuous layer over a given surface. This is the plan in the cerebrum
and cerebellum, and here are found devices for increasing the surface: the
cerebrum having convolutions, the cerebellum transverse ridges.

2. _That of collections of cell-bodies into rounded masses._ Such masses
are found in the bulb, the pons, the midbrain, and the base of the
cerebrum.

3. _That of arrangement in a continuous column._ This is the plan in the
spinal cord. It matters not at what place the spinal cord be cut, a
central area of gray matter, resembling in form the capital letter H, is
always found.

The fibers connecting with the cell-bodies in the brain and spinal cord
are gathered into bundles or tracts, and these pass through different
parts somewhat as follows:

1. _In the cerebrum_ they extend in three general directions, forming
three classes of fibers. The first connect different localities in the
same hemisphere, and are known as _association_ fibers (_A_, Fig. 134).
The second make connection between the two hemispheres, and form the
corpus callosum. These are known as _commissural_ fibers (_C_, Fig. 134).
The third connect the cerebrum with the parts of the nervous system below,
and are called _projection_ fibers (_P_, Fig. 134).

2. _In the cerebellum_ both association and commissural fibers are found.
Bands of fibers, passing upward toward the cerebrum and downward toward
the cord, connect this part of the brain with other parts of the nervous
system.

                                [Fig. 134]

Fig. 134--*Semi-diagrammatic representation of a section through the right
cerebral hemisphere*, showing fiber tracts. _A._ Association fibers. _C._
Commissural fibers. _P._ Projection fibers. The cell-bodies with which the
        fiber bundles connect are in the surface layer or cortex.

3. _In the midbrain, bulb, and spinal cord_ fibers are found: first, that
connect these parts with the cerebrum(101) and cerebellum above; second,
that pass into and become a part of the nerves of the body; and third,
that connect the opposite sides of these parts together.

*The Peripheral Division.*--The peripheral division of the nervous system
includes all the nervous structures found outside of the brain and spinal
cord. These consist of the cranial, spinal, and sympathetic nerves, and of
various small ganglia, all of which are closely connected with the central
system.

*Spinal Nerves and Dorsal-root Ganglia.*--The spinal nerves comprise a
group of thirty-one pairs, which connect the spinal cord with different
parts of the trunk, with the upper, and with the lower extremities. Each
nerve joins the cord by two roots, these being named from their positions
the _ventral_, or anterior, root and the _dorsal_, or posterior, root. The
two roots blend together within the spinal cavity to form a single nerve
trunk, which passes out between the vertebrae. On the dorsal root of each
spinal nerve is a small ganglion which is named, from its position, the
_dorsal-root ganglion_. (Consult Figs. 133 and 135, and also Fig. 125.)

*Double Nature of Spinal Nerves.*--Charles Bell, in 1811, made the
remarkable discovery that each spinal nerve is double in function. He
found the portion connecting with the cord by the dorsal root to be
concerned in the _production of feeling_ and the portion connecting by the
ventral root to be concerned in the _production of motion_. In keeping
with these functions, the two divisions of the nerve are made up of
different kinds of fibers, as follows:

1. The dorsal-root divisions, of the fibers of di-axonic neurons, the
cell-bodies of which form the dorsal-root ganglia (Fig. 135).

2. The ventral-root divisions, of the fibers of mon-axonic neurons, the
cell-bodies of which are in the gray matter of the cord.

The first convey impulses to the cord and are called _afferent_
neurons;(102) the second convey impulses from the cord and are known as
_efferent_ neurons. Thus, by forming a part of the nerve pathways between
the skin and the brain, the dorsal divisions of these nerves aid in the
production of feeling; and by completing pathways to the muscles, the
ventral divisions aid in the production of motion (Figs. 129, 135, and
141).

                                [Fig. 135]

  Fig. 135--*Connection of spinal nerves with the cord.* On the right is
  shown a nerve pathway from the skin to the muscle. A division of this
                        pathway reaches the brain.

*The Cranial Nerves.*--From the under front surface of the brain, twelve
pairs of nerves emerge and pass to the head, neck, and upper portions of
the trunk. These, the cranial nerves, have names suggestive of their
function or distribution and, in addition, are given numbers which
indicate the order in which they leave the brain (Fig. 136). Unlike the
spinal nerves, the cranial nerves present great variety among themselves,
scarcely any two of them being alike in function or in their connection
with different parts of the body. Several of them have to do with the
special senses, and are for this reason very important. They connect the
brain with the different parts of the head, neck, and trunk, as follows:

1. The first pair (_olfactory_ nerves; nerves of smell; afferent) connect
with the mucous membrane of the nostrils (Fig. 136).

2. The second pair (_optic_ nerves; nerves of sight; afferent) connect
with the retina of the eyes.

3. The third, fourth, and sixth pairs (_motores oculi;_ control muscles of
the eyes; efferent) connect with the internal and external muscles of the
eyeballs (Fig. 136).

                                [Fig. 136]

Fig. 136--*Diagram suggesting the distribution and functions of the cranial
                   nerves* (Colton). See also Fig. 132.

4. The fifth pair (_trigeminal_ nerves; nerves of feeling to the face, of
taste to the front of the tongue, and of control of muscles of
mastication; afferent and efferent) connect with the skin of the face, the
mucous membrane of the mouth, the teeth, and the muscles of mastication.

5. The seventh pair (_facial_ nerves; control muscles that give the facial
expressions; efferent) connect with the muscles just beneath the skin of
the face.

6. The eighth pair (_auditory_ nerves; nerves of hearing; afferent)
connect with the internal ear.

7. The ninth pair (_glossopharyngeal_ nerves; nerves of taste to back of
tongue and of muscular control of pharynx; afferent and efferent) connect
with the back surface of the tongue and with the muscles of the pharynx.

8. The tenth pair (_vagus_, or pneumogastric, nerves; nerves of feeling
and of muscular control; afferent and efferent) connect with the heart,
larynx, lungs, and stomach. They have the widest distribution of any of
the cranial nerves.

9. The eleventh pair (_spinal accessory_ nerves; control muscles of neck;
efferent) connect with the muscles of the neck.

10. The twelfth pair (_hypoglossal_ nerves; control muscles of the tongue;
efferent) connect with the muscles of the tongue.

*Sympathetic Ganglia and Nerves.*--The sympathetic ganglia are found in
different parts of the body, and vary in size from those which are half an
inch in diameter to those that are smaller than the heads of pins. The
largest and most important ones are found in two chains which lie in
front, and a little to either side, of the spinal column, and extend from
the neck to the region of the pelvis (Figs. 125 and 133). The number of
ganglia in each of these chains is about twenty-four. They are connected
on either side by the right and left sympathetic nerves which extend
vertically from ganglion to ganglion. In addition to the ganglia forming
these chains, important ones are found in the head (outside of the cranial
cavity) and in the plexuses of the thorax and the abdomen.

The sympathetic ganglia receive nerves from the central division of the
nervous system, but connect with glands, blood vessels, and the intestinal
walls through fibers from their own cell-bodies. Some of these latter
fibers join the spinal nerves, and some blend with each other to form
small sympathetic nerves.

*Protection of Brain and Spinal Cord.*--On account of their delicate
structure, the brain and spinal cord require the most complete protection.
In the first place, they are surrounded by the bones of the head and
spinal column; these not only shield them from the direct effects of
physical force, but by their peculiar construction prevent, to a large
degree, the passage of jars and shocks to the parts within. In the second
place, they are surrounded by three separate membranes, as follows:

1. The _dura_, or dura mater, a thick, dense, and tough membrane which
lines the bony cavities and forms supporting partitions.

2. The _pia_, or pia mater, a thin, delicate membrane, containing numerous
blood vessels, that covers the surface of the brain and cord.

3. The _arachnoid_, a membrane of loose texture, that lies between the
dura and the pin.

Finally, within the spaces of the arachnoid is a lymph-like liquid which
completely envelops the brain and the cord, and which, by serving as a
watery cushion, protects them from jars and shocks. Thus the brain and
cord are directly shielded by bones, by membranes, and by the liquid which
surrounds them. They are also protected from jars resulting from the
movements of the body by the general elasticity of the skeleton.

*Summary.*--The nervous system establishes connections between all parts of
the body, and provides a stimulus by means of which they are controlled.
It is made up of a special form of cells, called neurons. The neurons form
the different divisions of the nervous system, and also serve as the
active agents in carrying on its work. Through a side-by-side method of
joining they form the nerves, ganglia, spinal cord, and brain; and by a
method of end-to-end joining they connect places remote from each other,
and provide for nervous movements through the body. The nervous system,
may in some respects be compared to a complicated system of telephony, in
which the chains of neurons correspond to the wires, and the brain and
spinal cord to the central station.

Exercises.--1. Give the meaning of the term "cooerdination." Supply
illustrations.

2. What two general conditions are supplied in the body by the nervous
system?

3. Compare the skeleton outline of the nervous system with the bony
skeleton.

4. Sketch outlines of mon-axonic and di-axonic neurons.

5. Give two differences between the neurons and the other cells of the
body.

6. Describe the two general methods of connecting neurons in the body.
What purpose is accomplished by each method?

7. Name and locate the principal divisions of the nervous system.

8. Draw an outline of the brain (side view), locating each of its
principal divisions.

9. If a pencil were placed over the ear, what portions of the brain would
be above it and what below?

10. Describe briefly the cerebrum, the cerebellum, the midbrain, the pons,
and the bulb.

11. Locate and describe the cortex. State purpose of the convolutions.

12. State the general differences between the cranial and the spinal
nerves.

13. Locate and give the number of the dorsal-root ganglia. Locate and give
the approximate number of the sympathetic ganglia.

14. Show how the two portions of the spinal nerves are formed--the one from
the mon-axonic and the other from the di-axonic neurons.

15. Enumerate the different agencies through which the brain and spinal
cord are protected.

16. What cranial nerves contain afferent fibers? What ones contain
efferent fibers? What ones contain both afferent and efferent fibers?

17. In what respects is the nervous system similar to a system of
telephony? In what respects is it different?

PRACTICAL WORK

Examine a model of the brain, identifying the different divisions and
noting the position and relative size of the different parts (Fig. 137).
Observe the convolutions of the cerebrum and compare these with the
parallel ridges of the cerebellum. If the model is dissectible, study the
arrangement of the cell-bodies (gray matter) and the distribution of the
fiber bundles (white matter). Note the connection of the cranial nerves
with the under side.

                                [Fig. 137]

        Fig. 137--Model for demonstrating the brain (dissectible).

A prepared nervous system of a frog (such as may be obtained from supply
houses) should also be examined. Observe the appearance and general
distribution of the nerves and their connection with the brain and spinal
cord. If such a preparation is not at hand, some small animal may be
dissected to show the main divisions of the nervous system, as follows:

*Dissection of the Nervous System* (by the teacher).--For this purpose a
half-grown cat is generally the best available material. This should be
killed with chloroform and secured to a board as in the dissection of the
abdomen (page 169). Open the abdominal cavity and remove the contents,
tying the alimentary canal where it is cut, and washing out any blood
which may escape. Dissect for the nervous system in the following order:

1. Cut away the front of the chest, exposing the heart and lungs. Find on
each side of the heart a nerve which passes by the side of the pericardium
to the diaphragm. These nerves assist in controlling respiration and are
called the _phrenic_ nerves. Find other nerves going to different parts of
the thorax.

2. Remove the heart and lungs. Find in the back part of the thoracic
cavity, on each side of the spinal column, a number of small "knots" of
nervous matter joined together by a single nerve. These are sympathetic
ganglia. Where the neck joins the thorax, find two sympathetic ganglia
much larger than the others.

3. Cut away the skin from the shoulder and upper side of the fore leg. By
separating the muscles and connective tissue where the leg joins the
thorax, find several nerves of considerable size. These connect with each
other, forming a network called the _brachial plexus_. From here nerves
pass to the thorax and to the fore leg.

4. From the brachial plexus trace out the nerves which pass to different
parts of the fore leg. In doing this separate the muscles with the fingers
and use the knife only where it is necessary to expose the nerves. Note
that some of the branches pass into the muscles, while others connect with
the skin.

5. Remove the skin from the upper portion of one of the hind legs and
separate the muscles carefully until a large nerve is found. This is one
of the divisions of the _sciatic_ nerve. Carefully trace it to the spinal
cord, cutting away the bone where necessary, and find the connections of
its branches with the cord. Then trace it toward the foot, discovering its
branches to different muscles and to the skin.

6. Unjoint the neck and remove the head. Examine the spinal cord where
exposed. Cut away the bone sufficiently to show the connection between the
cord and one of the spinal nerves. On the dorsal root of one of the nerves
find a small ganglion. What is it called?

7. Fasten the head to a small board and remove the scalp. Saw through the
skull bones in several directions. Pry off the small pieces of bones,
exposing the upper surface of the brain. Study its membranes,
convolutions, and divisions.

8. With a pair of bone forceps, or nippers, break away the skull until the
entire brain can be removed from the cavity. Examine the different
divisions, noting the relative position and size of the parts.

9. With a sharp knife cut sections through the different parts, showing
the positions of the "gray matter" and of the "white matter."

NOTE.--If the entire class is to examine one specimen, it is generally
better to have the dissecting done beforehand and the parts separated and
tacked to small boards. This will permit of individual examination.
Sketches of the sciatic nerve, brachial plexus, and of sections through
the brain and spinal cord should be made.

*Location of Nerves in the Body.*--Several of the nerves of the body lie
sufficiently near the surface to be located by pressure and are easily
recognized as sensitive cords. Slight pressure from the fingers reveals
the presence of nerves in the grooves of the elbow (the crazy bone),
between the muscles on the inner side of the arm near the shoulder, and in
the hollow part of the leg back of the knee. These are all large nerves.
Small nerves may be located in the same manner in the face and neck.

CHAPTER XVIII - PHYSIOLOGY OF THE NERVOUS SYSTEM

In the preceding chapter was pointed out the method by which the different
parts of the body are brought into communication by the neurons or nerve
cells. We are now to study the means whereby the neurons are made to
control and cooerdinate the different parts of the body and bring about the
necessary adjustment of the body to its surroundings. This work of the
neurons naturally has some relation to their properties.

*Properties of Neurons.*--The work of the neurons seems to depend mainly
upon two properties--the property of irritability and the property of
conductivity. _Irritability_ was explained, in the study of the muscles
(page 243), as the ability to respond to a stimulus. It has the same
meaning here. The neurons, however, respond more readily to stimuli than
do the muscles and are therefore more irritable. Moreover, they are
stimulated by all the forces that induce muscular contraction and by many
others besides. They are by far the most irritable portions of the body.

_Conductivity_ is the property which enables the effect of a stimulus to
be transferred from one part of a neuron to another. On account of this
property, an excitation, or disturbance, in any part of a neuron is
conducted or carried to all the other parts. Thus a disturbance at the
distant ends of the dendrites causes a movement toward the cell-body and,
reaching the cell-body, the disturbance is passed through it into the
axon. This movement through the neuron is called the _nervous impulse_.

*Purpose of the Impulse. *--Though the nature of the nervous impulse is not
understood, (103) its purpose is quite apparent. It is the means employed
by the nervous system for controlling and cooerdinating the different parts
of the body. The arrangement of the neurons enables impulses to be started
in certain parts of the nervous system, and the property of conductivity
causes them to be passed _as stimuli _to other parts. This enables
excitation at one place to bring about action at another place.

Acting as stimuli, the impulses seem able to produce two distinct effects:
first, to throw resting organs into action and to increase the activity of
organs already at work; and second, to diminish the rate, or check
entirely, the activity of organs. Impulses producing the first effect are
called _excitant_ impulses; those producing the second effect,
_inhibitory_ impulses.

*Functions of the Parts of Neurons.*--The _cell-body_ serves as a nutritive
center from which the other parts derive nourishment. Proof of this is
found in the fact that when any part of the neuron is separated from the
cell-body, it dies, while the cell-body and the parts attached to the
cell-body may continue to live. In addition to this the cell-body probably
reenforces the nervous impulse.

The _dendrites_ serve two purposes: first, they extend the surface of the
cell-body, thereby enabling it to absorb a greater amount of nourishment
from the surrounding lymph; second, they act as _receivers of stimuli_
from other neurons. The same impulse does not pass from one neuron to
another. An impulse in one neuron, however, is able to excite the neuron
with which it makes an end-to-end connection, so that a series of impulses
is produced along a given nerve path (Fig. 129).

The special _function of the axon_ is to transmit the impulse. By its
length, structure, and property of conductivity it is especially adapted
to this purpose. The axis cylinder, however, is the only part of the axon
concerned in the transmission. The primitive sheath and the medullary
layer protect the axis cylinder, and, according to some authorities, serve
to insulate it. The medullary sheath may also aid in the nourishment of
the axis cylinder.

*Nerve Stimuli.*--While the properties of irritability and conductivity
supply a necessary cause for the production and transmission of nervous
impulses, these alone are not sufficient to account for their origin. An
additional cause is necessary--a force not found in the nerve protoplasm,
but one which, by its action on the protoplasm, makes it produce the
impulse. In this respect, the neuron does not differ essentially from the
cell of a muscle. Just as the muscle cell requires a stimulus to make it
contract, so does the neuron require a stimulus to start the impulse.
Hence, in accounting for the activities of the body, it is not sufficient
to say they are caused by nervous impulses. We must also investigate the
_nerve stimuli_--the means through which the nervous impulses are started.
Most of these are found outside of the body and are known as external
stimuli.

*Action of External Stimuli.*--In the arrangement of the nervous system the
most favorable conditions are provided for the reception of external
stimuli. Not only do vast numbers of neurons terminate at the surface of
the body,(104) but they connect there with delicate structures, called
_sense organs_. The purpose of the sense organs is to _sensitize_ (make
sensitive) the terminations of the neurons. This they do by supplying
special structures through which the stimuli can act to the best advantage
upon the nerve endings. Moreover, there are different kinds of sense
organs, and these cause the neurons to be sensitive to different kinds of
stimuli. Acting through the sense organs adapted for receiving them,
light, sound, heat, cold, and odors all act as stimuli for starting
impulses. Indeed, the arrangement is so complete that the nervous system
is subjected to the action of external stimuli in some form practically
all the time. The work of the sense organs is further considered in
Chapters XX, XXI, and XXII.

*How External Stimuli act on Internal Organs.*--For stimulating the neurons
not connected with the body surface we are dependent, so far as known,
upon the nervous impulses. An impulse started by the external stimulus
goes only so far as its neuron extends. But it serves as a stimulus for
the neuron with which the first connects and starts an impulse in this
connecting neuron, the point of stimulation being where the fiber
terminations of the first neuron make connection with the dendrites of the
second. This impulse in turn stimulates the next neuron, and so on,
producing a series of impulses along a given nerve path. In this way the
effect of an external stimulus may reach and bring about action in any
part of the body. This is in brief the general plan of inducing action in
the various organs of the body. This plan, however, is varied according to
circumstances, and at least three well-defined forms of action are easily
made out. These are known as _reflex action, voluntary action_, and
_secondary reflex action_.

*Reflex Action.*--When some sudden or strong stimulus acts upon the nerve
terminations at the surface of the body, an immediate response is
frequently observed in some quick movement. The jerking away of the hand
on accidentally touching a hot stove, the winking of the eyes on sudden
exposure to danger, and the quick movements from slight electrical shocks
are familiar examples. The explanation of reflex action is that external
stimuli start impulses in neurons terminating at the surface of the body
and these, in turn, excite impulses in neurons which pass from the spinal
cord or brain to the muscles (Fig. 138). Since there is an apparent
turning back of the impulses by the cord or brain, the resulting movements
are termed _reflex_.(105)

                                [Fig. 138]

   Fig. 138--*Diagram illustrating reflex action of an external organ.*

*Reflex Action and the Mind.*--If one carefully studies the reflex actions
of his own body, he will find that they occur at the time, or even a
little before the time, that he realizes what has happened. If a feather
is brought in contact with the more sensitive parts of the face of a
sleeping person, there is a twitching of the skin and sometimes a movement
of the hand to remove the offending substance. Surgeons operating upon
patients completely under the influence of chloroform, and therefore
completely unconscious, have observed strong reflex actions. These and
other similar cases indicate clearly that reflex action occurs
_independently_ of the mind--that the mind neither causes nor controls it.
If a further proof of this fact were needed, it is supplied by experiments
upon certain of the lower animals,(106) which live for a while after the
removal of the brain. These experiments show that the nervous impulses
that produce reflex action need only pass through the spinal cord and do
not reach the cerebrum, the organ of the mind.

*The Reflex Action Pathway.*--By study of the impulses that produce any
reflex action, a rather definite pathway may be made out, having the
following divisions:

1. _From the surface of the body to the central nervous system_ (usually
the spinal cord). This, the _afferent_ division, is made up of di-axonic
neurons, and these have (in the case of the spinal nerves) their
cell-bodies in the dorsal root ganglia (page 295). They are acted upon by
external stimuli, while their impulses in turn act on the neurons in the
spinal cord.

2. _Through the central system_ (spinal cord or base of brain). This, the
_intermediate_ division, may be composed of mon-axonic neurons, or it may
consist of branches from the afferent neurons. In the case of separate
neurons, these are acted upon by impulses from the afferent neurons, while
their impulses serve in turn as stimuli to other neurons within the cord
(Fig. 129).

3. _From the central nervous system to the muscles._ This, the _efferent_
division, is made up of mon-axonic neurons. Most of these have their
cell-bodies in the gray matter of the cord, while their fibers pass into
the spinal nerves by the ventral roots.(107) They may be stimulated by
impulses either from the intermediate neurons, or from branches of the
afferent neurons. Their impulses reach and stimulate the muscles.

*Reflex Action in Digestion.*--The flowing of the saliva, when food is
present in the mouth, is an example of reflex action. In this case,
however, the organ excited to activity is a gland instead of a muscle. The
food starts the impulses, and these, acting through the bulb, reach and
stimulate the salivary glands. In a similar manner food excites the glands
that empty their fluids into the stomach and intestines, and stimulates
the muscular coats of these organs to do their part in the digestive
process. To a considerable extent, neurons having their cell-bodies in the
sympathetic ganglia are concerned in these actions (Fig. 139).

                                [Fig. 139]

 Fig. 139--Diagram illustrating reflex action in its relation to the food
     canal. The nerve path in this case includes sympathetic neurons.

*Reflex Action in the Circulation of the Blood.*--On sudden exposure to
cold, the small arteries going to the skin quickly diminish in size, check
the flow of blood to the surface, and prevent too great a loss of heat. In
this case, impulses starting at the surface of the body are transmitted to
the bulb and then through the efferent neurons to the muscles in the walls
of the arteries. In a somewhat similar manner, heat leads to a relaxation
of the arterial walls and an increase in the blood supply to the skin.
Other changes in the blood supply to different parts of the body are also
of the nature of reflex actions. As in the work of digestion, neurons
having their cell-bodies in the sympathetic ganglia aid in the control of
the circulation.

*Purposes of Reflex Action.*--The examples of reflex action so far
considered illustrate its two main purposes--(1) protection, and (2) a
means of controlling important processes.

The pupil has but to study carefully the reflex actions of his own body
for a period, say of two or three weeks, in order to be convinced of their
protective value. He will observe that portions of his body have, on
exposure to danger, been moved to places of safety, while in some
instances, like falling, his entire body has been adjusted to new
conditions. He will also find that reflex action is quicker, and for that
reason offers in some cases better protection, than movements directed by
the mind. In digestion and circulation are found the best examples of the
control of important processes through reflex action.

*Voluntary Action.*--It is observed that reflex action, in the sense that
it has so far been considered, is not the usual mode of action of the
external organs, but is, instead, a kind of emergency action, due to
unusual conditions and excitation by strong stimuli. Voluntary actions, on
the other hand, represent the ordinary, or normal, action of these organs.
They comprise the movements of the body of which we are conscious and
which are _controlled by the mind_. But while they are of a higher order
than reflex actions and are under _intelligent_ direction, they are
brought about in much the same manner.

*Voluntary Action Pathways* differ in but one essential respect from those
of reflex action. They pass through the cerebrum, the organ of the mind
(Fig. 140). This is necessary in order that the mind may control the
action. From all portions of the body surface, afferent pathways may be
traced to the cerebrum; and from the cerebrum efferent pathways extend to
all the voluntary organs. A complex system of intermediate neurons, found
mostly in the brain, join the afferent with the efferent pathways. The
voluntary pathways are not distinct from, but include, reflex pathways, a
fact which explains why the same external stimulus may excite both reflex
and voluntary action (Fig. 141).

                                [Fig. 140]

            Fig. 140--*Diagram of a voluntary action pathway.*

*Choice in Voluntary Action.*--In reflex action a given stimulus, acting in
a certain way; produces each time the same result. This is not the case
with voluntary action, the difference being _due to the mind_. In these
actions the external stimulus first excites the mind, and the resulting
mental processes--perhaps as memory of previous experiences--supply a
variety of facts, any of which may act as stimuli to action. Before the
action takes place, however, some one fact must be singled out from among
the mental processes excited. This fact becomes the _exciting stimulus_
and leads to action. It follows, therefore, that the action which finally
occurs is not necessarily the result of an immediate external stimulus,
but of a _selected_ stimulus--one which is the result of choice.

                                [Fig. 141]

Fig. 141--*Diagram of voluntary action pathways* including reflex pathways.

Not only does the element of choice enter into the selection of the proper
stimulus, but it also enters into the time, nature, and intensity of the
action. For these reasons it is frequently impossible to trace voluntary
actions back to their actual stimuli. The pupil will recognize the element
of choice in such simple acts as picking up some object from the street,
complying with a request, and purchasing some article from a store.

*Reflex and Voluntary Action Compared.*--Certain likenesses and
differences, already suggested in these two forms of action, may now be
more fully pointed out. Reflex and voluntary action are alike in that the
primary cause of each is some outside force or condition which has
impressed itself upon the nervous system. They are also alike in the
general direction taken by the impulses in producing the action. The
impulses are, first, from the surface of the body to the central nervous
system; second, through the central system; and third, from the central
nervous system to the active tissues of the body.

Their chief differences are to be found, first, in the pathways followed
by the impulses, which are through the cerebrum (the organ of the mind) in
voluntary action, but in reflex action are only through the spinal cord or
the lower parts of the brain; and second, in the fact that voluntary
action is under the direction of the mind, while reflex action is not. It
would seem, therefore, that the statement sometimes made that "voluntary
action is reflex action plus the mind" is not far from correct. Mind,
however, is the important factor in this kind of action.

*Secondary Reflex Action.*--Everyday experience teaches that any voluntary
action becomes easier by repetition. A given act performed a number of
times under conscious direction establishes a condition in the nervous
system that enables it to occur without that direction and very much as
reflex actions occur. Actions of this kind are known as secondary reflex
actions, or as _acquired reflexes_. Walking, writing, and numerous other
movements pertaining to the occupation which one follows are examples of
such reflexes. These activities are at first entirely voluntary, but by
repetition they gradually become reflex, requiring only the stimulus to
start them.

The advantages to the body of its acquired reflexes are quite apparent.
The mind does not have to attend to the selection and direction of stimuli
and, to that extent, is left free for other work. A good example of this
is found in writing, where the mind apparently gives no heed to the
movements of the hand and is only concerned in what is being written. The
student will easily supply other illustrations of the advantages of
secondary reflex action.

The development of secondary reflexes probably consists in the
establishment of fixed pathways for impulses through the nervous system.
Through the branching of the nerve fibers many pathways are open to the
impulses. But in repeating the same kind of action the impulses are guided
into particular paths, or channels. In time these paths become so well
established that the impulses flow along them without conscious direction
and it is then simply necessary that some stimulus starts the impulses. By
following the established pathways, these reach the right destination and
produce the desired result. According to this view, secondary reflex
action is but a higher phase of ordinary reflex action--a kind of reflex
action, the conditions of which have been established by the mind through
repetition. (See functions of the cerebellum, page 317.)

*Habits.*--People are observed to act differently when exposed to the same
conditions, or when acted upon by the same stimuli. This is explained by
saying they have different habits. By _habits_ are meant certain general
modes of action that have been acquired by repetition. Certain acts
repeated again and again have established conditions in the nervous system
which enable definite forms of action to be excited, somewhat after the
manner of reflex action. On account of habits, therefore, the actions of
the individual are more or less _predisposed_. What he will do under
certain conditions may be foretold from his habits. Habits simply
represent, a higher order of secondary reflexes--those more closely
associated with the mental life and character than are the lower forms.

Habits, in common with other forms of secondary reflex action, serve the
important purpose of _economizing the nervous energy_. However, if
pernicious habits are formed instead of those that are useful, they are
detrimental from both a moral and physical standpoint. Youth is recognized
as the period in which fundamental habits are formed and character is
largely determined. Therefore parents and teachers do wisely when they
insist upon the formation of right habits by the young.

*Functions of Divisions of the Nervous System.*--The relationship between
the different parts of the nervous system is very close and one part does
not work independently of other parts. At the same time the general work
of the nervous system requires that its different divisions serve
different purposes:

1. The peripheral divisions of the nervous system are concerned in the
transmission of impulses between the surface of the body and the central
system and between the central system and the active tissues. The nerves
are the carriers of the impulses. The ganglia contain the cell-bodies
which serve as nutritive centers; and, in the case of the sympathetic
ganglia, these cell-bodies are the places where the fiber terminations of
one neuron connect with, and stimulate, other neurons.

2. The gray matter in the spinal cord, bulb, pons, and midbrain (through
the cell-bodies, fiber terminations, and short neurons which they contain)
completes the reflex action pathways between the surface of the body and
the voluntary muscles, and also between the surface of the body and the
organs of circulation and digestion.

3. The white matter of the spinal cord, bulb, pons, and midbrain (by means
of the fibers of which they are largely composed) forms connections with,
and passes impulses between, the various parts of the central nervous
system.

4. The bulb, because of certain special reflex-action pathways completed
through it, is the portion of the central nervous system concerned in the
control of respiration, circulation, and the secretion of liquids.

*Work of the Sympathetic Ganglia and Nerves.*--The neurons which form these
ganglia aid in controlling the vital processes, especially digestion and
circulation. These neurons are controlled for the most part by fibers from
the bulb and spinal cord, and cannot for this reason be looked upon as
forming an independent system. Their chief purpose seems to be that of
spreading the influence of neurons from the central system over a wider
area than they would otherwise reach. For example, a single neuron passing
out from the spinal cord may, by terminating in a sympathetic ganglion,
stimulate a large number of neurons, each of which will in turn stimulate
the cells of muscles or of glands. Because of this function, the
sympathetic neurons are sometimes called _distributing_ neurons.

*Functions of the Cerebellum.*--Efforts to discover some _special_ function
of the cerebellum have been in the main unsuccessful. Its removal from
animals, instead of producing definite results, usually interferes in a
mild way with a number of activities. The most noticeable results are a
general weakness of the muscles and an inability on the part of the animal
to balance itself. This and other facts, including the manner of its
connection with other parts of the nervous system, have led to the belief
that the cerebellum is the chief organ for the _reflex_ cooerdination of
muscular movements, especially those having to do with the balancing of
the body. In this connection it is subordinate to and under the control of
the cerebrum. Of the relations which the cerebellum sustains to the
cerebrum and to the different parts of the body, the following view is
quite generally held:

In the development of secondary reflexes, as already described, conditions
are established in the cerebellum, such that given stimuli may act
_reflexively_ through it and produce definite results in the way of
muscular contraction. After the establishment of these conditions,
afferent impulses from the eyes, ears, skin, and other places, under the
general direction of the cerebrum, may cause such actions as the balancing
of the body, walking, etc., as well as the delicate and varied movements
of the hand. This view of its functions makes of the cerebellum the great
center of secondary reflex action.

*Functions of the Cerebrum.*--While the work of the cerebrum is closely
related to that of the general nervous system, it, more than any other
part, exercises functions peculiar to itself. The cerebrum is the part of
the nervous system upon which our varied experiences leave their
impressions and through which these impressions are made to influence the
movements of the body. But the power to alter, postpone, or entirely
inhibit, nervous movements is but a part of the general work ascribed to
the cerebrum as _the organ of the mind_. Numerous experiments performed
upon the lower animals, together with observations on man, show the
cerebrum to be the seat of the mental activities, and to make possible, in
some way, the processes of consciousness, memory, volition, imagination,
emotion, thought, and sensation.

*Localization of Cerebral Functions.*--Many experiments have been performed
with a view to determining whether the entire cerebrum is concerned in
each of its several activities or whether special functions belong to its
different parts. These experiments have been made upon the lower animals
and the results thus obtained compared with observations made upon injured
and imperfectly developed brains in man. The results have led to the
conclusion that certain forms of the work of the cerebrum are _localized_
and that some of its parts are concerned in processes different from those
of others.

                                [Fig. 142]

 Fig. 142--*Location of cerebral functions.* Diagram of cerebrum, showing
               most of the areas whose functions are known.

The work of locating the functions of different parts of the cerebrum
forms one of the most interesting chapters in the history of brain
physiology. The portions having to do with sight, voluntary motion,
speech, and hearing have been rather accurately determined, while
considerable evidence as to the location of other functions has been
secured. Much of the cerebral surface, however, is still undetermined
(Fig. 142).

NERVOUS CONTROL OF IMPORTANT PROCESSES

*Circulation of the Blood.*--1. _Control of the Heart._--The ability to
contract at regular intervals has been shown to reside in the heart
muscle. Among other proofs is that furnished by cold-blooded animals, like
the frog, whose heart remains active for quite a while after its removal
from the body. These automatic contractions, however, are not sufficient
to meet all the demands made upon the circulation. The needs of the
tissues for the constituents of the blood vary with their activity, and it
is therefore necessary to vary frequently the force and rapidity of the
heart's contractions. Such changes the heart itself is unable to bring
about.

For the purpose of controlling the rate and force of its contractions, the
heart is connected with the central nervous system by two kinds of fibers:

_a._ Fibers that convey _excitant_ impulses to the heart to quicken its
movements.

_b._ Fibers that convey _inhibitory_ impulses to the heart to <DW44> its
movements.

The cell-bodies of the excitant fibers are found in the sympathetic
ganglia, but fibers from the bulb connect with and control them. The
cell-bodies of the inhibitory fibers are located in the bulb, from where
their fibers pass to the heart as a part of the vagus nerve.

In addition to the fibers above mentioned, are those that convey impulses
_from_ the heart to the bulb. These connect with neurons that in turn
connect with blood vessels and with them act reflexively, when the heart
is likely to be overstrained, to cause a dilation of the blood vessels.
This lessens the pressure which the heart must exert to empty itself of
blood. These fibers serve, in this way, as a kind of safety valve for the
heart.

2. _Control of Arteries._--Changes in the rate and force of the heart's
contractions can be made to correspond only to the _general_ needs of the
body. When the blood supply to a particular organ is to be increased or
diminished, this is accomplished through the muscular coat in the
arteries. The connection of the arterial muscle with the sympathetic
ganglia and the method by which they vary the flow of blood to different
organs has already been explained (pages 311 and 49), so that only the
location of the controlling neurons need be noted here. These, like the
controlling neurons of the heart, have their cell-bodies in the bulb. It
thus appears that the entire control of the circulation is effected in a
reflex manner through the nerve centers in the bulb. These centers are
stimulated by conditions that relate to the movement of the blood through
the body.

*Respiration.*--Efferent fibers connect the different muscles of
respiration with a cluster of cell-bodies in the bulb, called the
_respiratory center_. This center together with the nerves and muscles in
question form an automatic, or self-acting, mechanism similar in some
respects to that of the heart. Through the impulses passing from the
respiratory center to the muscles, a rhythmic action is maintained
sufficient to satisfy the usual needs of the body for oxygen. The demand
of the body for oxygen, however, varies with its activities, and to such
variations the respiratory center alone is unable to respond. The
regulating factor in the respiratory movements has been found to be the
condition of the blood with reference to the presence of oxygen and carbon
dioxide. If the blood contains much carbon dioxide and little oxygen, it
acts as a strong stimulus to the respiratory center, causing it, in turn,
to stimulate the respiratory muscles with greater intensity and frequency.
On the other hand, if the blood contains much oxygen and little carbon
dioxide, it acts only as a mild stimulus. This explains how physical
exercise increases the breathing, since the muscles at work consume more
oxygen than when resting and give more carbon dioxide and other wastes to
the blood.

The respiratory center is also connected by afferent nerves with the
mucous membrane of the air passages. Irritation of the nerve endings in
this membrane causes impulses to pass to the center, and this leads, by
reflex action, to such modifications of the respiratory acts as sneezing
and coughing. There is also a connection between the cerebrum and the
respiratory center. This is shown by the fact that one can voluntarily
change the rate and force of the respiratory movements, and further by the
fact that emotions affect the breathing.

*Regulation of the Body Temperature.*--As explained in the study of the
skin (page 270), the nervous system regulates the body temperature by
controlling the circulation of the blood through the skin and the internal
organs. This is accomplished by stimulating in a reflex manner the muscles
in the walls of certain arteries. To prevent the body from getting too
hot, muscles in the arteries going to the skin relax, thereby allowing
more blood to flow to the surface, where the heat can be disposed of
through radiation and through the evaporation of the perspiration. On the
other hand, if the body is in danger of losing too much heat, the muscles
in the walls of arteries going to the skin are made to contract and those
to internal organs relax, so that less blood flows to the skin and more to
the internal organs. In this way the nervous system adjusts the
circulation to suit the conditions of temperature outside of and within
the body and, in so doing, maintains the normal body temperature.

*Summary.*--The nervous system is able to control, cooerdinate, and adjust
the different organs of the body through its intimate connection with all
parts and through a stimulus (the nervous impulse) which it supplies and
transmits. Nervous impulses, excited by external stimuli, follow definite
paths and cause activity in the different parts of the body. All such
pathways are through the central nervous system. In reflex action the
impulses are mainly through the spinal cord, but to some extent through
the bulb, pons, and midbrain. In voluntary action they pass through the
cerebrum--a condition that leads to important modifications in the results.
The cerebrum, in addition to controlling the voluntary movements, is able
to establish the necessary conditions for secondary reflex actions, such
as walking, writing, etc. Although certain of the divisions of the nervous
system exercise special functions, all parts of it are closely related.

Exercises.--1. Give the function of each of the parts of a neuron.

2. State the purpose of the nervous impulse.

3. Show that the exciting cause of bodily action is outside of the nervous
system and, to a large extent, outside of the body.

4. Describe the arrangement that enables stimuli outside of the body to
cause action within the body.

5. Describe a reflex action and show how it is brought about.

6. Distinguish between afferent, efferent, and intermediate neurons.

7. Draw diagrams showing the impulse pathways in voluntary and in reflex
action.

8. What purposes are served by the sympathetic neurons?

9. Describe the method of control of the circulatory and digestive
processes. How do reflex actions protect the body?

10. Compare voluntary and reflex action. In what sense are all the
activities of the body reflex?

11. In what sense is walking voluntary? In what sense is it reflex?

12. How does secondary reflex action lessen the work of the nervous
system?

13. State the special functions of the nerves, ganglia, spinal cord, bulb,
cerebellum, and cerebrum.

14. State the importance of the formation of correct habits.

                                [Fig. 143]

          Fig. 143--Nerve board for demonstrating nerve pathways.

PRACTICAL WORK

*To demonstrate Nerve Pathways.*--A smooth board, 2x6 ft., is painted
black, and upon this is drawn in white a life-size outline of the body.
Pieces of cord of different colors and lengths are knotted to represent
mon-axonic and di-axonic neurons. These are then pinned or tacked to the
board in such a manner as to represent the connections in the different
kinds of nerve pathways. Fig. 143 shows such a board with connections for
a reflex action and a voluntary action of the same muscle.

*Study of the "Knee Jerk" Reflex.*--A boy is seated on a chair with the
legs crossed. With a small pointer he is given a light, quick blow on the
upper margin of the patella at the point of connection of the tendon. The
stroke will usually be followed by a reflex movement of the foot. Does
this take place independently of the mind? (The one upon whom the
experiment is being performed should assume a relaxed condition and make
no effort either to cause or prevent the movement.) Can the movement be
inhibited (prevented)? Repeat the experiment, effort being made to prevent
the movement, but not by contracting opposing muscles.

Other reflex actions adapted to class study are those of the eyes, such as
the closing of the lids on moving objects near them and the dilating of
the pupils when the eyes are shaded. The involuntary jerking of the head
on bringing the prongs of a vibrating tuning fork in contact with the end
of the nose is also a reflex action which can be studied to advantage.

*To determine the Reaction Time.*--Have several pupils join hands, facing
outwards, making a complete circle, excepting one gap. Give a signal by
touching the hand of one pupil at the end of the line. Let this pupil
communicate the signal, by pressure of the other hand, to the next pupil
and so on around, having the last pupil raise the free hand at close of
the experiment. Note carefully the time, preferably with a stop watch,
required to complete the experiment and divide this by the number of
pupils, to get the average reaction time. The experiment may be repeated
with boys only and then with girls, comparing their average reaction time.

*Reflex Action of the Salivary Glands.*--Place a small pinch of salt upon
the tongue and note the flow of saliva into the mouth. Try other
substances, as starch, bits of wood, and sugar. What appears to be the
natural stimulus for these glands? Compare with reflex actions of the
muscles.

CHAPTER XIX - HYGIENE OF THE NERVOUS SYSTEM

The far-reaching effects and serious nature of disorders of the nervous
system are sufficient reasons for considering carefully those conditions
that make or mar its efficiency. Controlling all the activities of the
body and affecting through its own condition the welfare of all the
organs, the hygiene of the nervous system is, in a large measure, the
hygiene of the entire body. Moreover, it is known that some of our worst
diseases, including paralysis and insanity, are disorders of the nervous
system and are prevented in many instances by a proper mode of living.

*The Main Problem.*--Many of our nervous disorders are undoubtedly due to
the age in which we live. Our modern civilization, with all its facilities
for human advancement and enjoyment, throws an extra strain upon the
nervous system. Educational and social standards are higher than ever
before and life in all its phases is more complex. Since we can hardly
change the conditions under which we live, and probably would not if we
could, we must learn to adapt or adjust ourselves to them so as to secure
for the nervous system such relief as it requires. This adjustment is
sometimes difficult, even when the actual needs of the nervous system are
known.

The healthful action of the nervous system requires, on the one hand,
exercise, but on the other hand, a certain condition of quietude, or
_poise_--a state which is directly opposed to that of restlessness. The
conditions of modern life seem able to force upon the nervous system all
the exercise that it needs, and more (whether it be of the right kind or
not), so that the main problem of to-day seems to be that of conserving,
or economizing, the nervous energy and of preventing nervous waste.

*Wasteful Forms of Nervous Activity.*--There are without doubt many forms
of activity that waste the vital forces of the body and lead to nervous
exhaustion. Take, for example, the rather common habit of worrying over
the trivial things of life. Certainly the nervous energy spent in this way
cannot be used in doing useful work, but must be counted as so much loss
to the body. One who would use his nervous system to the best advantage
must find some way of preventing waste of this kind.(108)

Undue excitement, as well as pleasurable dissipations, also tend toward
nervous exhaustion. And while the fact is recognized that pleasurable
activities supply a necessary mental exercise, the limit of healthful
endurance must be watched and _excesses of all kinds avoided_. Intense
emotional states are found to be exhausting in the extreme; and the
suppression of such undesirable feelings as anger, fear, jealousy, and
resentment are of immense value in the hygiene of the nervous system.

*The Habit of Self-control.*--Much of the needless waste of nervous energy,
including that of worrying over trivial matters, may be prevented through
the exercise of self-control. From the standpoint of the nervous system,
the present age differs from the past mainly in supplying a greater number
and variety of nerve stimuli. Self-control means the ability to suppress
activities that would result from undesirable stimuli and to direct the
bodily activities into channels that are profitable. Self-control,
therefore, is not only to be exercised on occasions of great emergency,
but in the everyday affairs of life as well. It is even more important
that the daily toiler at his task be able to keep the petty annoyances of
life from acting as irritants to his nervous system than that he keep cool
during some great calamity. The habit of self-control is acquired mainly
through the persistent effort to prevent any and all kinds of petty
annoyances from affecting the nerves or the temper.

*Nervousness.*--Self-control is much more easily practiced by some than by
others. This is due partly to habit, but is also due to an actual
difference in the degree of sensitiveness, or irritability, of the nervous
systems of different people. One whose nervous system tends to respond too
readily to any and all kinds of stimuli is said to be "nervous." This
condition is in some instances inherited, but is in most cases due to the
wasteful expenditure of nervous energy or to the action of some drug upon
the body. Excess of mental work, too much reading, long-continued anxiety,
eye strain, and the use of tea, coffee, alcohol, tobacco, or other drugs,
including many of those taken as medicines, are known to cause
nervousness. Nervousness is not only a source of great annoyance, both to
one's self and to others, but is a menace to the general health.

The first step toward securing relief from such a condition is the removal
of the cause. The habits should be inquired into and excesses of all kinds
discontinued. In some instances it may be necessary to _have the eyes
__examined_ and glasses fitted by a competent oculist.(109) The nervous
energy should be carefully economized and the habit of self-control
diligently cultivated. Special exercises that have for their purpose the
equalizing of the circulation and the strengthening of the blood vessels
of the neck and the brain also have beneficial effects.

*Nervous Overstrain.*--Both mental and physical overwork tends to weaken
the nervous system and to produce nervousness. Where hard mental work is
long continued, or where it is carried on under excitement, a tense
nervous condition is developed which is decidedly weakening in its
effects. The causes which lead to such a condition, and in fact overwork
of all kinds, should if possible be avoided. Where this is not possible,
and in many cases it is not, the period of overwork should be followed by
one of rest, recreation, and plenty of sleep. To the overworked in body or
in mind, nothing is more important from a hygienic, as well as moral,
standpoint, than the right use of the _one rest day in seven_. The best
interests of our modern civilization _require_ that the Sabbath be kept as
a quiet, rest-giving day.

*Disturbed Circulation of the Brain.*--Nervousness not infrequently is
accompanied by an increase in the circulation of the brain and disappears
when this condition is relieved. Though mental work and excitement tend
naturally to increase the circulation in the brain, this should subside
with rest and relief from excitement. When there is a tendency for this
condition to become permanent, effort should be made looking for relief.
Increasing the circulation in the lower extremities by hot or cold foot
baths, or by much walking, is found to be most beneficial. Special
exercises of the muscles of the neck are also recommended as a means of
relieving this condition.(110)

*Hygienic Value of Work.*--Within reasonable limits, both mental and
physical work are conducive to the vigor of the nervous system. Through
work the energies of the body find their natural outlet, and this prevents
dissipation and the formation of bad habits. Even hard work does not
injure the nervous system, and severe mental exertion may be undergone,
provided the proper hygienic conditions are observed. The nervous
disorders suffered by brain workers are not, as a rule, due to the work
which the brain does, but to violation of the laws of health, especially
the law of exercise. Such persons should observe the general laws of
hygiene and especially should they practice daily those forms of physical
exercise that tend to counteract the effects of mental work.

*Physical Exercise* properly taken is beneficial to the nervous system
through both direct and indirect effects. A large proportion of the nerve
cells have for their function the production of motion, and these are
called into play only through muscular activity. Then, as already
suggested, physical exercise counteracts the unpleasant effects of mental
work. Hard study causes an excess of blood to be sent to the brain and a
diminished amount to the arms and to the legs. Physical exercise
redistributes the blood and equalizes the circulation. Light exercise
should, therefore, follow hard study. The student before retiring at night
is greatly aided in getting to sleep and is put in a better condition for
the next day's work by ten to fifteen minutes of light gymnastics. A daily
walk of two or three miles is also an excellent means of counteracting the
effects of mental work. The brain worker should, however, avoid violent
exercise or the carrying of any kind of exercise to exhaustion.

*Sleep*, and plenty of it, is one of the first requirements of the nervous
system. It is during sleep that the exhausted brain cells are replenished.
To shorten the time for sleep is to weaken the brain and to lessen its
working force. No one should attempt to get along with less than eight
hours of sleep each day and most people require more. Children require
more sleep than adults. Those under six years should have from eleven to
twelve hours of sleep per day. Children between six and ten years should
have at least ten hours.

*Insomnia*, or sleeplessness, on account of its effects upon the nervous
system, is to be regarded as a serious condition, and hygienic means for
relieving it should be diligently sought. Having its cause in nervousness,
a disturbed circulation of the brain, or some form of nervous exhaustion,
it is benefited through relieving these conditions and in the manner
already described. Of course the external conditions for aiding sleep
should not be overlooked. The bed should be comfortable, and the room
should be cool, well ventilated, dark, and quiet. The inducing of sleep by
means of drugs is a dangerous practice and should never be resorted to
except under the direction of the physician.

*Effects of Heat and Cold.*--Heat and cold both have their effects upon the
nervous system. Heat increases the nervous irritability, while cold acts
as a natural sedative to the nerves. A nervous person is made more nervous
by an overheated atmosphere, but derives beneficial effects from exposing
the body freely to cold air and water. The tonic cold bath (page 273), if
taken with the usual precautions, can be used to good advantage in
diminishing nervousness. The taking of outdoor exercise in cold weather
is, for the same reason, an excellent practice.

*Effect of Emotional States.*--We have already noted the effect of certain
emotional states upon the digestion of the food (page 162). Emotional
states are also known to interfere with breathing and with the action of
the heart. Such effects are explained through the close relation of the
mind to the work of the nervous system in general. While certain emotional
states, such as fear, anger, melancholia, and the impulse to worry,
interfere seriously with the normal action of the nervous system, others,
such as contentment, cheerfulness, and joy, are decidedly beneficial in
their effects. How important, then, is the habit of suppressing the states
that are harmful and of cultivating those that are beneficial. From a
hygienic, as well as social, standpoint a cheerful, happy disposition is
worth all the effort necessary for its attainment.

*The Nervous Condition of Children* should be a matter of deep concern on
the part of both parents and teachers. In the home, as well as in the
school, the child may be "pushed" until the nervous system receives
permanent injury. Exhaustion of nerve cells is produced through too many
and too vivid impressions being made upon the immature brain. The child
should be protected from undue excitement. He should have the benefit of
outdoor exercise and should be early inured to cold. He should be shielded
from the poisoning effects of tea, coffee, tobacco, alcohol, and other
drugs. He should have impressed upon him the habit of self-control. He
should not be indulged in foolish caprices or whims, but should be taught
to be content with plain, wholesome food and with the simple forms of
enjoyment.

*Influences at School.*--School life is necessarily a great strain upon the
child. Night study added to the work of the day makes a heavy burden for
elementary pupils to bear. Though the legal school age is usually fixed at
six years, delicate children should be kept out of school until they are
seven or eight years old, provided they have good homes. In addition to
the excitation incident to studying and reciting lessons, conditions
frequently arise both in the schoolroom and upon the playground that
create a feeling of fear or dread in the minds of children. Quarrels and
feuds among the children and the bullying of big boys on the playground
may work untold harm. All conditions tending to develop fear, uneasiness,
or undue excitement on the part of children should receive the attention
of those in authority.

*Excessive Reading* is a frequent cause of injury to the nervous systems
of children. This has a bad effect, both on account of too many
impressions being made upon the mind and also on account of the strain to
the eyes. Then if the reading consists mostly of light fiction, the mind
is directed away from the really important things of life. The reading of
children should be thoughtfully controlled, both as to quality and
quantity. Exciting stories should, as a rule, be excluded, but a taste for
biography, historical and scientific writings, and for the great works of
literature should be cultivated. Simple fairy tales which have a
recognized value in developing the imagination of the child need not be
omitted, but it is of vital importance that the "story-reading habit" be
not formed.

*Effects of Drugs.*--Because of its delicacy of structure a number of
chemical compounds, or drugs, are able to produce injurious effects upon
the nervous system. Some of these are violent poisons, while others, in
small quantities, are mild in their action. Certain drugs, in addition to
their immediate effects, bring about changes in the nervous system which
cause an unnatural appetite, or craving, that leads to their continued
use. This is the case with alcohol, the intoxicating substance in the
usual saloon drinks, and with nicotine, the stimulating drug in tobacco.
The same is also true of morphine, chloral, and several other drugs used
as medicines. The _danger of becoming a slave_ to some useless and
pernicious habit should dissuade one from the use of drugs except in cases
of positive emergency.

*Alcohol and the Nervous System.*--Alcohol, as already shown, injures
practically all portions of the body; but it has its worst effects upon
the nervous system. Through its action on this system, it interferes with
the circulation of the blood, produces a condition of "temporary insanity"
called intoxication, weakens the will, and eventually dethrones the
reason. Worst of all, it produces a condition of "chronic poisoning" which
manifests itself in an unnatural craving, and this causes it to be used by
the victim even when he knows he is "drinking to his own destruction."
Though its use in small quantities does not, as a rule, produce such
marked effects upon the nervous system, it develops the "craving," and
this is apt in time to lead to its use in larger quantities. But even if
this does not occur, the practice is objectionable for its unhygienic
effects in general.(111) Tippling with such mild solutions of alcohol as
light wine, beer, and hard cider is, for these reasons, a dangerous
pastime.

*Alcohol and Crime.*--It is sometimes stated that no one who leaves alcohol
alone will be injured by it. This is true only of its direct effects; not
of its indirect effects. Whenever a crime is committed somebody is
injured, and alcohol is known to be a chief cause of crime. Alcohol causes
crime through the loss of self-control, seen especially in intoxication,
and also because of the moroseness and quarrelsomeness which it developes
in certain individuals. Indirectly it causes crime through the poverty
which it engenders and through its influence in bringing about social
conditions out of which crime develops. Everything considered, the free
use of alcohol is incompatible with the nervous health and moral tone of a
community.

*Nicotine and the Nervous System.*--Nicotine is an oily substance which is
extracted from the tobacco plant. Its action on the nervous system is in
general that of a poison. Taken in small quantities, it is a mild
stimulant and, if the doses are repeated, a habit is formed which is
difficult to break. Tobacco is used mainly for the stimulating effect of
this drug. While not so serious in its results as the alcohol and other
drug habits, the use of tobacco is of no benefit, is a continual and
useless expense, and, in many instances, causes a derangement of the
healthy action of the body.(112) With the bad effects of the nicotine must
be included those of questionable substances added to the tobacco by the
manufacturer, either for their agreeable flavor or for adulteration.

*Relation of Age to the Effects of Nicotine.*--The use of tobacco by the
young is especially to be deplored. In addition to the harmful effects
observed in those of mature years, nicotine interferes with the normal
development of the body and lays, in many instances, the foundation for
physical and mental weakness in later life. The cigarette is decidedly
harmful, especially when inhalation is practiced, its deadening effects
being in part due to the wrappers, some of which have been shown to
contain arsenic and other poisonous drugs. While dulling the intellect and
weakening the body, cigarette smoking also tends to make criminals of
boys.(113) Parents, teachers, school officers, and all who have the good
of mankind at heart should take every precaution, including that of
setting a good example, to prevent the formation of the tobacco habit by
those of immature years.

*Habit versus Self-control.*--The power of self-control, already emphasized
for its importance in the economical expenditure of the nervous energy, is
of vital importance in its relation to the habits of the body.
Self-control is the chief safeguard against the formation of bad habits
and is the only means of redemption from such habits after they have once
been formed. The persistent cultivation of the power to control the
appetites and the passions, as well as all forms of activity which tend to
injure the body or debase the character, gives a tone to the nervous
system which increases the self-respect and raises the individual to a
_higher plane of life_. The worst habits _can_ be broken and good ones
formed in their stead, if only there is sufficient determination to
accomplish these results. Failure comes from not having the mind
thoroughly "made up" and from not having, back of the desire to do better,
"the strong will of a righteous determination."

*Effects of External Conditions.*--While the inner life and habits have
most to do with the hygiene of the nervous system, a certain amount of
attention may properly be given to those conditions outside of the body
which affect directly or indirectly the state of this system. Noise,
disorder, and confusion act as nervous irritants, but quiet, order, and
system have the opposite effect. There is, therefore, much in the
management of the office, factory, schoolroom, or home that has to do with
the real hygiene of the nerves as well as with the efficiency of the work
that is being done. The suppression of distracting influences not only
enables the mind to be given fully to the work in hand, but actually
prevents waste of nervous energy. Although the responsibility for securing
the best conditions for work rests primarily with those in charge, it is
also true that each individual in every organization may contribute to the
order or disorder that prevails.

*Social Relations.*--In considering the external conditions that affect the
nervous system, the fact must not be overlooked that man is a social being
and has to adjust himself to an established social order. His relations to
his fellow-men, therefore, affect strongly his nervous condition and
theirs also. For this reason the best hygiene of the nervous system is
based upon _moral_ as well as physical right living. Along with the power
of self-control and the maintenance of a correct nervous poise, there
should be a proper regard for the welfare of others. On account of the
ease with which one individual may disturb the nervous state of another,
those social forms and customs which tend to establish harmonious
relations among men are truly hygienic in their effects, and may well be
carried out in spirit as well as "in letter."

It is also a fact that a given mental state in one person tends to excite
a like state in those with whom he associates. How important, then, that
each and all cultivate, as habits, the qualities of cheerfulness,
kindness, and good-will, instead of the opposite states of mind.
Especially in the family, and other groups of closely associated
individuals, should the nervous effect of one member upon the others be
considered and every effort made to secure and maintain harmonious
relations.

*The High Ideal.*--Everything considered, the conditions most favorable to
the healthfulness of the nervous system are in harmony with what our
greatest teachers have pointed to as the higher plane of living. On this
account a true conception of the value and meaning of life is of the
greatest importance. _An ever present, strong desire to live a vigorous,
but simple and noble, life_ will suggest the proper course to pursue when
in doubt and will stimulate the power of self-control. It will lead to the
stopping of "nerve leaks" and to the maintenance of harmonious relations
with one's fellows. It will cause one to recoil from the use of alcohol
and other nerve poisons, as from a deadly serpent, seeing the end in the
beginning, and will be the means eventually of leading the body into its
greatest accomplishments.

*Summary.*--The nervous system, on account of its delicate structure, is
liable to injury through wrong methods of using it and also through the
introduction of drugs, or poisons, into the body. There are also found in
our methods of living and systems of education conditions that tend to
waste the nervous energy. To protect the nervous system from all these
threatened dangers requires, among other things, the power of
self-control. This enables the individual to direct his life according to
his highest ideals and to free himself from habits known to be injurious.
Children must have their nervous systems safeguarded by parents and
teachers. Especially must they be kept from becoming enslaved to some
drug, such as alcohol or the nicotine of tobacco.

*Exercises.*--1. In what respect is the hygiene of the nervous system the
hygiene of the entire body?

2. Of what value in the hygiene of the nervous system is the power of
self-control? How is the habit of self-control formed?

3. Name several forms of activity that waste the nervous energy.

4. Name several influences that react unfavorably on the nervous systems
of children.

5. How may too much reading prove injurious to the nervous system?

6. What forms of physical exercise are beneficial to the brain worker?

7. Why is the use of alcohol even in small quantities to be regarded as a
dangerous practice?

8. Name several causes of nervousness.

9. What are the unanswerable arguments for preventing the use of tobacco
by the young?

10. Why do cigarettes have a more harmful effect upon the body than other
forms of tobacco?

11. Enumerate conditions in the schoolroom that dissipate the nervous
energy of pupils; that economize it.

CHAPTER XX - PRODUCTION OF SENSATIONS

Our study of the nervous system has shown that impulses arising at the
surface of the body are able, through connecting neurons, to bring about
various activities. Moving along definite pathways, they induce motion in
the muscles, and in the glands the secretion of liquids. It is now our
purpose to consider the effect produced by afferent impulses upon the
brain and, through the brain, upon the mind.(114) This effect is
manifested in a variety of similar forms, known as

*The Sensations.*--Sensations constitute the lowest forms of mental
activity. Roughly speaking, they are the states of mind experienced as the
_direct_ result of impulses reaching the brain. In a sense, just as
impulses passing to the muscles cause motion, impulses passing to the
brain cause sensations. The feeling which results from the hand's touching
a table is a sensation and so also is the pain which is caused by an
injury to the body. The mental action in each case is due to impulses
passing to the brain. Care must be exercised by the beginner, however, not
to confuse sensations with the nervous impulses, on the one hand, or with
_secondary_ mental effects, such as emotion or imagination, on the other.
Sensations are properly regarded as the first conscious effects of the
afferent impulses and as the _beginning stage_ in the series of mental
processes that may take place on account of them.

In some way, not understood, the mind associates the sensation with the
part of the body from which the impulses come. Pain, for example, is not
felt at the brain where the sensation is produced, but at the place where
the injury occurs. This association, by the mind, of the sensations with
different parts of the body, is known as "localizing the sensation."

*Sensation Stimuli.*--While the sensations are dependent upon the afferent
impulses, the afferent impulses are in turn dependent upon causes outside
of the nervous system. If these are removed, the sensations cease and they
do not start up again unless the exciting influences are again applied.
Any agency, such as heat or pressure, which, by acting on the neurons of
the body, is able to produce a sensation, may be called a _sensation
stimulus_. It has perhaps already been observed that the stimuli that lead
to voluntary action, as well as those that produce reflex action of the
muscles, cause sensations at the same time. From this we may conclude that
sensation stimuli are the same in character as those that excite motion.
On the other hand, it should be noted that sensations are constantly
resulting from stimuli that are of too mild a nature to cause motion.

*Classes of Sensations.*--Perhaps as many as twenty distinct sensations,
such as pain, hunger, touch, etc., are recognized. If these are studied
with reference to their origin, it will be seen that some of them result
from the action of definite forms of stimuli upon the neurons terminating
in sense organs; while the others, as a rule, arise from the action of
indefinite stimuli upon neurons in parts of the body that do not possess
sense organs. The members of the first class--and these include the
sensations of touch, temperature, taste, smell, hearing, and sight--are
known as the _special_ sensations. The others, including the sensations of
pain, hunger, thirst, nausea, fatigue, comfort, discomfort, and those of
disease, are known as _organic_, or general, sensations. These two classes
of sensations differ in their purpose in the body as well as in the manner
of their origin.

*Purposes of Sensations.*--Any given sensation is related to the stimulus
which excites it as an _effect_ to a _cause_. It starts up or stops,
increases in intensity or diminishes, according to the action of the
exciting stimulus. As the stimuli are outside of the nervous system, and
in the majority of cases outside of the body, the sensations indicate to
the mind what is taking place either in the body itself or in its
surroundings. They supply, in other words, the means through which the
mind acquires information. By means of the special sensations, a knowledge
of the physical surroundings of the body is gained, and through the
organic sensations the needs of the body and the state of the various
organs are indicated. In general, sensations are made to serve two great
purposes in the body, as follows:

1. They provide the necessary conditions for intelligent and purposeful
action on the part of the body.

2. They supply the basis for the higher mental activities, as perception,
memory, thought, imagination, and emotion.

Intelligent action is impossible without a knowledge both of the bodily
organs and of the body's surroundings. Protection and the regulation of
the work of an organ necessitate a knowledge of its condition, while the
adapting and adjusting of the body to its surroundings require a knowledge
of what those surroundings are. The dependence of all the higher forms of
mental activity upon sensations is recognized by psychologists and is
easily demonstrated by a study of the manner in which we acquire
knowledge. "Without sensation there can be no thought."

*Steps in the Production of Sensations.*--The steps in the production of
sensations are not essentially different from those in the production of
reflex action. First of all, external stimuli act upon the fiber
terminations in the sense organs, or elsewhere, starting impulses in the
neurons. These pass into the central nervous system and there excite
neurons which in turn discharge impulses into the cerebrum. The result is
to arouse an activity of the mind--a sensation. The steps in the production
of any _special_ sensation naturally involve the following parts:

1. A sense organ where the terminations of the neurons are acted upon by
the stimulus.

2. A chain of neurons which connect the sense organ with the brain.

3. The part of the cerebrum which produces the sensation.

*Sense Organs.*--The sense organs are not parts of the afferent neurons,
but are structures of various kinds, in which the neurons terminate. Their
function is to enable the sensation stimuli to start the impulses. By
directing, concentrating, or controlling the stimuli, the sense organs
enable them to act to the best advantage upon the neurons. When it is
recognized that such widely different forces as light waves, sound waves,
heat, pressure, and odors are enabled by them to stimulate neurons, the
importance of these organs becomes apparent. As would naturally be
inferred, the construction of any sense organ has particular reference to
the nature of the stimulus which it is to receive. This is most apparent
in the sense organs of sight and hearing.

*Simple Forms of Sense Organs.*--The simplest form of a sense organ (if
such it may be called) is one found among the various tissues. It consists
of the terminal branches of nerve fibers which spread over a small area of
cells, as a network or plexus. Such endings are numerous in the skin and
muscles.

Next in order of complexity are the so-called _end-bulbs_. These consist
of rounded, or elongated, connective tissue capsules, within which the
nerve fibers terminate. On the inside the fibers lose their sheaths and
divide into branches, which wind through the capsule. End-bulbs are
abundant in the lining membrane of the eye, and are found also in the skin
of the lips and in the tissues around the joints.

Slightly more complex than the end-bulbs are the _touch corpuscles_. These
are elongated bulb-like bodies, having a length of about one
three-hundredth of an inch, and occupying the papillae of the skin (Fig.
144). They are composed mainly of connective tissue. Each corpuscle
receives the termination of one or more nerve fibers. These, on entering,
lose the medullary sheath and separate into a number of branches that
penetrate the corpuscle in different directions.

                                [Fig. 144]

        Fig. 144--*A touch corpuscle* highly magnified. (See text.)

The largest of the simple forms of sense organs are bodies visible to the
naked eye and called, from their discoverer Pacini, the _Pacinian
corpuscles_. They lie along the course of nerves in many parts of the
body, and have the general form of grains of wheat. (See Practical Work.)
The Pacinian corpuscles are composed of connective tissue arranged in
separate layers around a narrow central cavity called the core (Fig. 145).
Within the core is the termination of a large nerve fiber. These
corpuscles are found in the connective tissue beneath the skin, along
tendons, around joints, and among the organs of the abdominal cavity.

                                [Fig. 145]

  Fig. 145--*Pacinian corpuscle*, magnified. _A._ Medullated nerve fiber.
   _B._ Axis cylinder terminating in small bulb at _C._ _D._ Concentric
              layers of connective tissue. _E._ Inner bulb.

The simple forms of sense organs have a more or less general distribution
over the body, and are concerned in the production of at least three
special sensations. These are _touch, temperature_, and the _muscular
sensation_.

*Touch*, or feeling, is perhaps the simplest of the sensations. The sense
organs employed are the touch corpuscles, and the external stimulus is
some form of pressure or impact. Pressure applied to the skin, by acting
on the fiber terminations in the corpuscles, starts the impulses that give
rise to the sensation. The touch corpuscles render the fiber terminations
so sensitive that the slightest pressure is able to arouse sensations of
touch. It is found that _a change of pressure_, rather than pressure that
is constant, is the active stimulus. That all parts of the skin are not
equally sensitive to pressure, and that the mind does not interpret
equally well the sensations from different parts, are facts easily
demonstrated by experiment. (See Practical Work.)

*The Temperature Sensation.*--Temperature sensations, like those of touch,
are limited almost entirely to the skin. They are of two kinds, and are
designated as _heat_ sensations and as _cold_ sensations. Whether the
sense organs for temperature are different from those of touch is not
known. It is known, however, that the same corpuscles do not respond alike
to heat, cold, and pressure.

_A Change of Temperature_, rather than any specific degree of heat or
cold, is the active temperature stimulus. The sensation of warmth is
obtained when the temperature of the skin is being raised, and of cold
when it is being lowered. This explains why in going into a hallway from a
heated room one receives a sensation of cold, while in coming into the
same hallway from the outside air he receives a sensation of warmth. It is
for the same reason that we are able to distinguish only the relative, not
the actual, temperature of bodies.

*Muscular Sensations.*--These are sensations produced by impulses arising
at the muscles. Such impulses originate at the fiber terminations which
are found in both the muscles and their tendons. By muscular sensations
one is conscious of the location of a contracting muscle and of the degree
of its tension. They also make it possible to judge of the weight of
objects.

                                [Fig. 146]

  Fig. 146--*Sense organs of taste.* _A._ Map of upper surface of tongue,
 showing on the left the different kinds of papillae, and on the right the
   areas of taste (after Hall). Area sensitive to bitter (----); to acid
    (....); to salt (--.--.--.--); to sweet (--------). _B._ Section through a
     papilla. _n._ Small nerve connecting with taste buds at _d. e._
  Epithelium. _C._ Single taste bud magnified. _n._ Nerve, the fibers of
which terminate between the spindle-shaped cells _a. e._ Epithelial cells.

*The Sensation of Taste.*--The sense organs of taste are found chiefly in
the mucous membrane covering the upper surface of the tongue. Scattered
over this surface are a number of rounded elevations, or large papillae (A,
Fig. 146). Toward the back of the tongue two rows of these, larger than
the others, converge to meet at an angle, where is located a papilla of
exceptional size. Surrounding each papilla is a narrow depression, within
which are found the sense organs of taste (B, Fig. 146). These are called,
from their shape, _taste buds_, and each bud contains a central cavity
which communicates with the surface by a small opening--_the gustatory
pore_. Within this cavity are many slender, spindle-shaped cells which
terminate in hair-like projections at the end nearest the pore, but in
short fibers at the other end. Nerve fibers enter at the inner ends of the
buds and spread out between the cells (_C_, Fig. 146). These fibers pass
to the brain as parts of two pairs of nerves--those from the front of the
tongue joining the trigeminal nerve, and those from the back of the
tongue, the glossopharyngeal nerve.

The gustatary, or _taste stimulus_, is some chemical or physical condition
of substances which is manifested only when they are in a liquid state.
For this reason _only liquid substances can be tasted_. Solids to be
tasted must first be dissolved.

The different taste sensations are described as bitter, sweet, sour, and
saline, and in the order named are recognized as the tastes of quinine,
sugar, vinegar, and salt. As to how these different tastes are produced,
little is known. Flavors such as vanilla and lemon, and the flavors of
meats and fruits, are really smelled and not tasted. Taste serves two main
purposes: it is an aid in the selection of food and it is a means of
stimulating the digestive glands (page 161).

                                [Fig. 147]

Fig. 147--*Sense organ of smell.* _A._ Distribution of nerves in outer wall
 of nasal cavity. 1. Turbinated bones. 2. Branch of fifth pair of nerves.
 3. Branches of olfactory nerve. 4. Olfactory bulb. _B._ Diagram showing
                connection of neurons concerned in smell.

*The Sensation of Smell.*--The sense organs of smell are found in the
mucous membrane lining the upper divisions of the nasal cavities. Here are
found two kinds of cells in great abundance--column-shaped epithelial cells
and the cells which are recognized as the sense organs of smell. These
olfactory cells are spindle-shaped, having at one end a slender,
thread-like projection which reaches the surface, and at the other end a
fiber which joins an olfactory nerve (B, Fig. 147). In fact, the olfactory
cells resemble closely the cell-bodies of neurons, and are thought to be
such. The divisions of the olfactory nerve pass through many small
openings in the ethmoid bone to connect with the olfactory bulbs, which in
turn connect with the cerebrum (A, Fig. 147).

*The Olfactory Stimulus.*--Only substances in the gaseous state can be
smelled. From this it is inferred that the stimulus is supplied by gas
particles. Solids and liquids are smelled because of the gas particles
which separate from them. The substance which is smelled must be kept
moving through the nostrils and made to come in direct contact with the
olfactory cells. There is practically no limit to the number of distinct
odors that may be recognized.

*Value of Smell.*--Although the sense of smell is not so acute in man as in
some of the lower animals, it is, nevertheless, a most important and
useful gift. It is the only sense that responds to matter in the gaseous
state, and is, for this reason, the only natural means of detecting
harmful constituents of the atmosphere. In this connection it has been
likened to a sentinel standing guard over the air passages. Many gases
are, however, without odor, and for this reason cannot be detected by the
nostrils. It is of especial importance that gases which are likely to
become mixed with the air supply to the body have odor, even though the
odor be disagreeable. The bad odors of illuminating gas and of various
compounds of the chemical laboratory, since they serve as danger signals
to put one exposed to them on his guard, are of great protective value.

*Sight and Hearing.*--The sense organs of sight and hearing are highly
complicated structures, and will be considered in the chapters following.

*Summary.*--Sensations are certain activities of the mind that result from
excitations within the body or at its surface. These cause the neurons to
discharge impulses which on reaching the cerebrum cause the sensations.
Sensations are necessary for intelligent and purposeful action and for
acquiring all kinds of knowledge. To enable the stimuli to act to the best
advantage in starting the impulses, special devices, called sense organs,
are employed. These receive the terminations of the neurons, and by their
special structure enable the most delicate stimuli to start impulses. The
simpler forms of sense organs are those of touch, temperature, taste, and
smell.

*Exercises.*--1. Compare sensations and reflex actions with reference to
their nature and cause. Give steps in the production of each.

2. Give examples of sensation stimuli. State the purpose of sense organs.

3. How do general sensations differ from special sensations?

4. Of what value is pain in the protection of the body?

5. Show that sensations lead to the higher forms of mental activity, such
as emotion and imagination.

6. Of what value to the body is the "localizing of the sensation"?

7. What kinds of sense organs are found in the skin? State the purpose of
each.

8. Through what sense avenues is one made aware of solids, of liquids, and
of gases?

9. Of what special protective value is the sense of smell?

PRACTICAL WORK

*To demonstrate the Pacinian Corpuscles.*--Spread out the mesentery from
the intestine of a cat and hold it between the eye and the light: Pacinian
corpuscles will appear as small translucent bodies having the general form
of grains of wheat. Secure a portion of the mesentery over a circular
opening in a thin piece of cork and examine it with a microscope of low
power. Follow the course of the nerve fiber to the nerve from which it
branches.

*To show Relative Sensitiveness of Different Parts of the Skin.*--Holding a
bristle between the fingers, bring the end in contact with the skin,
noting the amount of pressure necessary to cause a sensation of touch.
Test the lips, tongue, tips of fingers, and palm and back of hand, trying
different sizes of bristles. Has the degree of sensitiveness any relation
to the thickness of the cuticle?

*To show Perceptive Differences of Different Portions of the Skin.*--Place
the points of a pair of dividers on the back of the hand of one who looks
in the opposite direction. Is one point felt or two? Repeat several times,
changing the distance between the points until it is fully determined how
near the two points must be placed in order to be felt as one. In like
manner test other parts of the body, as the tips of the fingers and the
back of the neck. Compare results obtained at different places.

*To locate Warm and Cold Sensation Spots.*--Slowly and evenly draw a
blunt-pointed piece of metal over the back of the neck. If it be of the
same temperature as the skin, only touch sensations will be experienced.
If it be a little colder (the temperature of the room) sensations of cold
will be felt at certain spots. If slightly warmer than the body, heat
sensation spots will be found on other parts of the skin. If the heat and
cold sensation spots be marked and tested from day to day they will be
found to remain constant as to position. Inference.

CHAPTER XXI - THE LARYNX AND THE EAR

Man is a social being. His inclinations are not to live alone, but to be a
part of that great human organization known as society. For men to work
together, to be mutually helpful one to another, requires the ability to
exchange ideas and this in turn requires some means of communication.(115)
One means of communication is found in certain movements of the
atmosphere, known as _sound waves_. In the exchange of ideas by this means
there are employed two of the most interesting divisions of the body--the
larynx and the ear. The first is an instrument for the production of sound
waves; the second is the sense organ which enables the sound waves to act
as stimuli to the nervous system.

*Nature of Sound Waves.*--If some sonorous body, as a bell, be struck, it
is given a quivering, or vibratory, motion. This is not confined to the
bell, but is imparted to the air and other substances with which the bell
comes in contact. These take up the movements and pass them to objects
more remote, and they in turn give them to others, until a very
considerable distance is reached. Such progressive vibrations are known as
waves, and, since they act as stimuli to the organs of hearing, they are
called _sound waves_. Sound waves _always originate in vibrating
bodies_.(116) They are transmitted chiefly _by the air_, which, because of
its lightness, elasticity, and abundance, readily takes up the vibrations
and spreads them in all directions (Fig. 148).

While these vibratory movements of the atmosphere are correctly classified
as waves, they bear little resemblance to the waves on water. Instead of
being made of crests and troughs, as are the water waves, the sound waves
consist of alternating successions of slightly condensed and rarefied
layers of air. Then, while the general movement of the water waves is that
of ever widening circles _over a surface_, the sound waves spread as
enlarging spherical shells _through_ the air. In sound waves, as in all
other waves, however, it is only the form of the wave that moves forward.
The individual particles of air that make up the wave simply vibrate back
and forth.

                                [Fig. 148]

 Fig. 148--Diagram illustrating the spreading of sound waves through air.

*How Sound Waves act as Stimuli.*--Any sound wave represents a small but
definite amount of energy, this being a part of the original force that
acted on the vibrating body to set it in motion. The hammer, for instance,
in striking a bell imparts to it a measurable quantity of energy, which
the bell in turn imparts to the air. This energy is in the sound waves and
is communicated to the bodies against which they strike.(117) Though the
force exerted by most sound waves is, indeed, very slight, it is
sufficient to enable them to act as stimuli to the nervous system.

*How Sounds Differ.*--Three distinct effects are produced by sound waves
upon the nerves of hearing, and through them upon the mind. These are
known as _pitch, intensity_, and _quality_, and they are dependent upon
the vibrations of the sound-producing bodies.

_Pitch_, which has reference to the height, or degree of sharpness, of
tones, is determined by the rapidity of the vibrations of the vibrating
body. The more rapid the vibrations, the higher the pitch, the number of
vibrations doubling for each musical interval known as the octave.

_Intensity_ is the energy, or force, of the sound waves. This is
recognized by the strength of the sensation and is expressed by the term
_loudness_. Intensity is governed mainly by the width of the vibrations of
the vibrating body, and the width depends upon the force applied to the
body to make it vibrate.

_Quality_ is that peculiarity of sound that enables tones from different
instruments to sound differently, although they may have the same pitch
and intensity. Quality depends upon the fact that most tones are complex
in nature and result from the blending together of simple tones of
different pitch.

*Reenforcement of Sound Waves.*--The sound vibrations from small bodies are
not infrequently reenforced by surrounding conditions so that their
outgoing waves reach farther and are more effective than waves from larger
bodies. This is true of the sound waves produced by most musical
instruments and also those produced by the human larynx. Such
reenforcement is effected in two general ways--by sounding boards and by
inclosed columns of air. Stringed instruments--violin, guitar, piano,
etc.--employ sounding boards, while wind instruments, as the flute, pipe
organ, and the various kinds of horns, employ air columns for reenforcing
their vibrations. In the use of the sounding board, the vibrations are
communicated to a larger surface, and in the use of the air column the
vibrations are communicated to the inclosed air. (See Practical Work.)

*Value of Sound Waves to the Body.*--From a physiological standpoint, the
value of sound waves is not easily overestimated. In addition to the use
made of them in the communication of ideas, they serve the purpose of
protecting the body, and in the sphere of music provide one of the most
elevating forms of entertainment. Sounds from different animals, as well
as from inanimate objects, may also be the means of supplying needed
information. The existence of two kinds of sound instruments in the
body--the one for the production, the other for the detection, of sound--is
certainly suggestive of the ability of the body to adjust itself to, and
to make use of, its physical environment. Both the larynx and the ear are
constructed with special reference to the nature and properties of sound
waves.

THE LARYNX

*The Sound-producing Mechanism of the Body* consists of the following
parts:

1. Delicately arranged bodies that are easily set in vibration.

2. An arrangement for supplying the necessary force for making these
bodies vibrate.

3. Contrivances for modifying the vibrating parts so as to produce changes
in pitch and intensity.

4. Parts that reenforce the vibrations.

5. Organs by means of which the sounds are converted into the forms of
speech.

The central organ in this complex mechanism is

*The Larynx.*--The larynx forms a part of the air passages, being a short
tube at the upper end of the trachea. Mucous membrane lines the inside of
it and muscles cover most of the outer surface. The framework is made of
cartilage. At the top it is partly encircled by a small bone (the hyoid),
and its opening into the pharynx is guarded by a flexible lid, called the
_epiglottis_. The cartilage in its walls is in eight separate pieces, but
the greater portion of the structure is formed of two pieces only. These
are known as the _thyroid cartilage_ and the _cricoid cartilage_ (Fig.
149). Both can be felt in the throat--the thyroid as the projection known
as "Adam's apple," and the cricoid as a broad ring just below.

                                [Fig. 149]

  Fig. 149--The larynx.--_A._ Outside view. _B._ Vertical section through
  larynx, showing inside. 1. Thyroid cartilage. 2. Cricoid cartilage. 3.
Trachea. 4. Hyoid bone. 5. Epiglottis. 6. Vocal cord. 7. False vocal cord.
                      8. Lining of mucous membrane.

The _thyroid cartilage_ consists of two V-shaped pieces, one on either
side of the larynx, meeting at their points in front, and each terminating
at the back in an upward and a downward projection. Between the back
portions of the thyroid is a space equal to about one third of the
circumference of the larynx. This is occupied by the greater portion of
the _cricoid cartilage_. This cartilage has the general shape of a signet
ring and is so placed that the part corresponding to the signet fits into
the thyroid space, while the ring portion encircles the larynx just below
the thyroid. Muscles and connective tissue pass from the thyroid to the
cricoid cartilage at all places, save one on each side, where the downward
projections of the thyroid form hinge joints with the cricoid. These
joints permit of motion of either cartilage upon the other.

At the summit of the cricoid cartilage, on each side, is a small piece of
triangular shape, called the _arytenoid cartilage_. Each arytenoid is
movable on the cricoid and is connected with one end of a vocal cord.

                                [Fig. 150]

 Fig. 150--*Vocal cords* as seen from above. _A._ In producing sound, _B._
                         During quiet breathing.

*The Vocal Cords* are formed by two narrow strips of tissue which,
connecting with the thyroid cartilage in front and the arytenoid
cartilages behind, lie in folds of the mucous membrane. They have the
general appearance of ridge-like projections from the sides of the larynx,
but at their edges they are sharp and smooth. The open space between the
cords is called the _glottis_. When sound is not being produced, the
glottis is open and has a triangular form, due to the spreading apart of
the arytenoid cartilages and the attached cords. But when sound is being
produced, the glottis is almost completely closed by the cords. Above the
vocal cords, and resembling them in appearance, are two other folds of
membrane, called the _false vocal cords_ (B, Fig. 149). The false cords do
not produce sound, but they aid in the closing of the glottis.

*How the Voice is Produced.*--The voice is produced through the vibrations
of the vocal cords. A special set of muscles draws the arytenoid
cartilages toward each other, thereby bringing their edges very near and
parallel to each other in the passage. At the same time other muscles act
on the thyroid and cricoid cartilages to separate them at the top and give
the cords the necessary tension. With the glottis now almost closed,
blasts of air from the lungs strike the sharp edges of the cords and set
them in vibration (Fig. 150). The vocal cords do not vibrate as strings,
like the strings of a violin, but somewhat as reeds, similar to the reeds
of a French harp or reed organ.

The location of the vocal cords in the air passages enables the lungs and
the muscles of respiration to aid in the production of the voice. It is
their function to supply the necessary force for setting the cords in
vibration. The upper air passages (mouth, nostrils, and pharynx) supply
resonance chambers for reenforcing the vibrations from the vocal cords,
thereby greatly increasing their intensity. In ordinary breathing the
vocal cords are in a relaxed condition against the sides of the larynx and
are not acted upon by the air as it enters or leaves the lungs.

*Pitch and Intensity of the Voice.*--Changes in the pitch of the voice are
caused mainly by variations in the tension of the cords, due to the
movements of the thyroid and cricoid cartilages upon each other.(118) In
the production of tones of very high pitch, the vibrating portions of the
cords are thought to be actually shortened by their margins being drawn
into contact at the back. This raises the pitch in the same manner as does
the shortening of the vibrating portion of a violin string.

The _intensity_, or loudness, of the voice is governed by the force with
which the air is expelled from the lungs. The vibrations of the cords,
however, are greatly reenforced by the peculiar structure of the upper air
passages, as stated above.

*Production of Speech.*--The sounds that form our speech or language are
produced by modifying the vibrations from the vocal cords. This is
accomplished by "mouthing" the sounds from the larynx. The distinct
sounds, or words, are usually complex in nature, being made up of two or
more elementary sounds. These are classed either as _vowels_ or
_consonants_ and are represented by the different letters of the alphabet.
The vowel sounds are made with the mouth open and are more nearly the pure
vibrations of the vocal cords. The consonants are modifications of the
vocal cord vibrations produced by the tongue, teeth, lips, and throat.

*Words and their Significance.*--In the development of language certain
ideas have become associated with certain sounds so that the hearing of
these sounds suggests the ideas. Our words, therefore, consist of so many
sound signals, each capable of arousing a definite idea in the mind. To
talk is to express ideas through these signals, and to listen is to assume
an attitude of mind such that the signals may be interpreted. In learning
a language, both the sounds of the words and their associated ideas are
mastered, this being necessary to their practical use in exchanging ideas.
From spoken language man has advanced to written language, so that the
sight of the written or printed word also arouses in the mind the
associated idea.

THE EAR

*The Ear* is the sense organ which enables sound waves to so act upon
afferent neurons as to excite impulses in them. The effect upon the mind
which these impulses produce is known as the _sensation of hearing_. In
the performance of its function the ear receives and transmits sound waves
and also concentrates them upon a suitable exposure of nerve cells. It
includes three parts--the _external ear_, the _middle ear_, and the
_internal ear_.

*External Ear.*--The external ear consists of the part on the outside of
the head called the _pinna_, or auricle, and the tube leading into the
middle ear, called the _auditory canal_ (Fig. 151). The pinna by its
peculiar shape aids to some extent the entrance of sound waves into the
auditory canal.(119) It consists chiefly of cartilage. The auditory canal
is a little more than an inch in length and one fourth of an inch in
diameter, and is closed at its inner end by a thin, but important
membrane, called

*The Membrana Tympani.*--This membrane consists of three thin layers. The
outer layer is continuous with the lining of the auditory canal; the inner
is a part of the lining of the middle ear; and the middle is a fine layer
of connective tissue. Being thin and delicately poised, the membrana
tympani is easily made to vibrate by the sound waves that enter the
auditory canal. In this way it serves as a receiver of sound waves from
the air. It also protects

                                [Fig. 151]

 Fig. 151--*Diagram of section through the ear*, showing relations of its
                        various parts. (See text.)

*The Middle Ear.*--The middle ear, or tympanum,(120) consists of an
irregular cavity in the temporal bone which is lined with mucous membrane
and filled with air. It is connected with the pharynx by a slender canal
called the _Eustachian tube_. Extending across the middle ear and
connecting with the membrana tympani on one side, and with a membrane
closing a small passage to the internal ear on the other, is a tiny bridge
formed of three small bones. These bones, named in their order from the
membrana tympani, are the _malleus_, the _incus_, and the _stapes_ (Fig.
151). Where the malleus joins the membrane is a small muscle whose
contraction has the effect of tightening the membrane. The Eustachian tube
admits air freely to the middle ear, providing in this way for an equality
of atmospheric pressure on the two sides of the drum membrane. The bridge
of bones and the air in the middle ear receive vibrations from the
membrana tympani and communicate them to the membrane of the internal ear.

*Purposes of the Middle Ear. *--The middle ear serves two important
purposes. In the first place, it makes it possible for sound waves to set
the membrana tympani in vibration. This membrane could not be made to
vibrate by the more delicate of the sound waves if it were stretched over
a bone, or over some of the softer tissues, or over a liquid. Its
vibration is made possible by the presence of air on _both_ sides, and
this condition is supplied, on the inner side, by the middle ear. The
Eustachian tube, by providing for an _equality_ of pressure on the two
sides of the membrane, also aids in this purpose.

In the second place, the middle ear provides a means for _concentrating
the force of the sound waves_ as they pass from the membrana tympani to
the internal ear. This concentration is effected in the following manner:

1. The bridge of bones, being pivoted at one point to the walls of the
middle ear, forms a lever in which the malleus is the long arm, and the
incus and stapes the short arm, their ratio being about that of three to
two. This causes the incus to move through a shorter distance, but with
greater force than the end of the malleus.

2. The area of the membrana tympani is about twenty times as great as the
membrane of the internal ear which is acted upon by the stapes. The force
from the larger surface is, therefore, concentrated by the bridge of bones
upon the smaller surface. By the combination of these two devices, the
waves striking upon the membrane of the internal ear are rendered some
thirty times more effective than are the same waves entering the auditory
canal.

*The Internal Ear*, or labyrinth, occupies a series of irregular channels
in the petrous process of the temporal bone.(121) It is very complicated
in structure, and at the same time is very small. Its greatest length is
not more than three fourths of an inch and its greatest diameter not more
than one half of an inch. It is filled with a liquid which at one place is
called the _perilymph_, and at another place the _endolymph_. It is a
double organ, being made up of an outer portion which lies next to the
bone, and which surrounds an inner portion of the same general form. The
outer portion is surrounded by a membrane which serves as periosteum to
the bone and, at the same time, holds the liquid belonging to this part,
called the perilymph. The inner portion, called the _membranous
labyrinth_, consists essentially of a closed membranous sac, which is
filled with the endolymph. The auditory nerve terminates in this portion
of the internal ear. Three distinct divisions of the labyrinth have been
made out, known as the _vestibule_, the _semicircular canals_, and the
_cochlea_ (Fig. 152).

                                [Fig. 152]

Fig. 152--*General form, of internal ear.* The illustration represents the
  structures of the internal ear surrounded by a thin layer of bone. 1.
  Vestibule. 2. Cochlea. 3. Semicircular canals. 4. Fenestra ovalis. 5.
                            Fenestra rotunda.

*The Vestibule* forms the central portion of the internal ear and is
somewhat oval in shape. It is in communication with the middle ear through
a small opening in the bone, called the _fenestra ovalis_, at which place
it is separated from the middle ear only by a thin membrane. Sound waves
enter the liquids of the internal ear at this point, the foot of the
stapes being attached to the membrane. Six other openings lead off from
the vestibule at different places. One of these enters the cochlea. The
other five open into

*The Semicircular Canals.*--These canals, three in number, pass through the
bone in three different planes. One extends in a horizontal direction and
the other two vertically, but each plane is at right angles to the other
two. Both ends of each canal connect with the vestibule, though two of
them join by a common opening. The inner membranous labyrinth is
continuous through each canal, and is held in position by small strips of
connective tissue.

The purpose of the semicircular canals is not understood. It is known,
however, that they are not used in hearing. On the other hand, there is
evidence to the effect that they act as equilibrium sense organs, exciting
sensations necessary for balancing the body. Their removal or injury,
while having no effect upon the hearing, does interfere with the ability
to keep the body in an upright position.

                                [Fig. 153]

        Fig. 153--Diagram showing the divisions of cochlear canal.

*The Cochlea* is the part of the internal ear directly concerned in
hearing. It consists of a coiled tube which makes two and one half turns
around a central axis and bears a close resemblance to a snail shell
(Figs. 151 and 152). It differs in plan from a snail shell, however, in
that its interior space is divided into three distinct channels, or
canals. These lie side by side and are named, from their relations to
other parts, the _scala vestibula_, the _scala tympani_, and the _scala
media_. Any vertical section of the cochlea shows all three of these
channels (Fig. 153).

*The Scala Vestibula and the Scala Tympani* appear in cross section as the
larger of the canals. The former, so named from its connection with the
vestibule, occupies the upper position in all parts of the coil. The
latter lies below at all places, and is separated from the channels above
partly by a margin of bone and partly by a membrane. It receives its name
from its termination at the tympanum, or middle ear, from which it is
separated only by a thin membrane.(122) Both the scala vestibula and the
scala tympani belong to the outer portion of the internal ear and are, for
this reason, filled with the perilymph. At their upper ends they
communicate with each other by a small opening, making by this means one
continuous canal through the cochlea. This canal passes from the vestibule
to the tympanum and, in so doing, goes entirely around

*The Scala Media.*--This division of the cochlea lies parallel to and
between the other two divisions. It is above the scala tympani and below
the scala vestibula, and is separated from each by a membrane. The scala
media belongs to the membranous portion of the internal ear and is,
therefore, filled with the endolymph. It receives the terminations of
fibers from the auditory nerve and may be regarded as the true sense organ
of hearing. The nerve fibers terminate upon the membrane known as the
_basilar membrane_, which separates it from the scala tympani. This
membrane extends the length of the cochlear canals, and is stretched
between a projecting shelf of bone on one side and the outer wall of the
cochlea on the other. It is covered with a layer of epithelial cells, some
of which have small, hair-like projections and are known as the _hair
cells_. Above the membrane, and resting partly upon it, are two rows of
rod-like bodies, called the _rods of Corti_. These, by leaning toward each
other, form a kind of tunnel beneath. They are exceedingly numerous,
numbering more than 6000, and form a continuous series along the margin of
the membrane.

                                [Fig. 154]

 Fig. 154--*Diagram* illustrating passage of sound waves through the ear.

*How We Hear.*--The sound waves which originate in vibrating bodies are
transmitted by the air to the external ear. Passing through the auditory
canal, the waves strike against the membrana tympani, setting it into
vibration. By the bridge of bones and the air within the middle ear the
vibrations are carried to and concentrated upon the liquid in the internal
ear (Fig. 154). From here the vibrations pass through the channels of the
cochlea and set into vibration the contents of the scala media and
different portions of the basilar membrane. This serves as a stimulus to
the fibers of the auditory nerve, causing them to transmit impulses which,
on passing to the brain, produce the sensation of hearing.

Much of the peculiar structure of the cochlea is not understood. Its
minute size and its location in the temporal bone make its study extremely
difficult. The connection of the scala vestibula with the scala tympani,
and this with the middle ear, is necessary for the passage of vibrations
through the internal ear. Its liquids, being practically incompressible
and surrounded on all sides by bones, could not otherwise yield to the
movements of the stapes. (See Practical Work.) The rods of Corti are
thought to act as dampers on the basilar membrane, to prevent the
continuance of vibrations when once they are started.

*Detection of Pitch.*--The method of detecting tones of different pitch is
not understood. Several theories have been advanced with reference to its
explanation, one of the most interesting being that proposed by Helmholtz.
This theory is based on our knowledge of sympathetic vibrations. The
basilar membrane, while continuous throughout, may be regarded as made up
of many separate cords of different lengths stretched side by side. A tone
of a given pitch will set into vibration only certain of these cords,
while tones of different pitch will set others into vibration.

Another theory is that the basilar membrane responds to all kinds of
vibrations and the analysis of sound takes place in the brain.

A third view is that the filaments from the hair cells, rather than the
basilar membrane, respond to the vibrations and in turn stimulate the
terminations of the nerve fibers.

                                [Fig. 155]

 Fig. 155--*Diagram* showing how wax may plug the auditory canal and cause
                                deafness.

*Hygiene of the Ear.*--The ear, being a delicate organ, is frequently
injured by careless or rough treatment. The removal of the ear wax by the
insertion of pointed instruments has been found to interfere with the
natural method of discharge and to irritate the membrane. It should never
be practiced. It is unnecessary in the healthy ear thus to cleanse the
auditory canal, as the wax is passed by a natural process to where it is
easily removed by a damp cloth. If the natural process is obstructed,
clean warm water and a soft linen cloth may be employed in cleansing the
canal, without likelihood of injury. Clean warm water may also be
introduced into the auditory canal as a harmless remedy in relieving
inflammation of the auditory canal and of the middle ear. Children's ears
are easily injured, and it goes without saying that they should never be
pulled nor boxed.

It frequently happens that a mass of wax collects in the auditory canal
and closes the passage so completely as to cause deafness (Fig. 155). This
may come about without pain and so gradually that one does not think of
seeking medical aid. Such masses are easily removed by the physician, the
hearing being then restored. Both for painful disturbances of the ear and
for the gradual loss of hearing, the physician should be consulted.

*The Hearing of School Children.*--School children not infrequently have
defective hearing and for this reason are slow to learn. The hearing is
easily tested with a watch, the normal ear being able to hear the watch
tick at a distance of at least two feet. Pupils with defective hearing
should, of course, have medical attention, and in the classroom should be
seated where they can hear to the best advantage.

*Summary.*--Sound waves constitute the external stimuli for the sensation
of hearing. They consist of progressive vibratory movements of the air
that originate in vibrating bodies. Through the larynx and the ear, sound
waves are utilized by the body in different ways, but chiefly as a means
of communication. The larynx produces sound waves which are reenforced and
modified by the air passages. The ear supplies suitable conditions for the
action of sound waves upon nerve cells. Both the ear and the larynx are
constructed with special reference to the nature and properties of sound
waves, and they illustrate the body's ability to adjust itself to, and to
make use of, its physical environment.

*Exercises.*--1. For what different purposes are sound waves employed in
the body?

2. How do sound waves originate? How are they transmitted? How do they
differ from the waves on water?

3. How are sound waves able to act as nerve stimuli?

4. Describe two methods of reenforcing sound waves. Which method is
employed in the body?

5. Name all the parts of the body that are directly or indirectly
concerned in the production of sound.

6. Describe the larynx.

7. Describe the condition of the vocal cords in speaking and in ordinary
breathing.

8. How are sounds differing in pitch and intensity produced by the larynx?

9. How is the sound produced by the vocal cords changed into speech?

10. What parts of the ear are concerned in transmitting sound waves?

11. Give the purposes of the middle ear.

12. Trace a sound wave from a bell to the basilar membrane, and trace the
impulse that it causes from there to the brain.

13. Give the purpose of the Eustachian tubes; of the rods of Corti; of the
semicircular canals.

14. Give directions for the proper care of the ear.

PRACTICAL WORK

*To illustrate the Origin of Sound.*--1. Strike a bell an easy blow and
hold some light substance, as a pith ball attached to a thread, against
the side, noting the result. 2. Sound a tuning fork by striking it against
the table. Test it for vibrations as above, or by letting the vibrating
prongs touch the surface of water. 3. Pluck a string of a guitar or
violin, and find proof that it is vibrating while giving out sound.

*To show the Transmission of Sound.*--1. Vibrate a tuning fork and press
the stem against a table or desk. The vibrations which are reenforced in
this way will be heard in all parts of the room. Now press one end of a
wooden rod, as a broom handle, against the table, and bring the stem of
the vibrating fork against the other end. The vibrations now move down the
stick to the table, from whence they are communicated to the air. Observe
that the sound waves, to reach the ear, must pass through the rod, the
table, and the air. 2. Fasten the tuning fork to a flat piece of cork by
pressing the stem into a small hole in the center. Vibrate the fork and
let the cork rest on the surface of water in a half-filled tumbler on the
table. The sound will, as before, pass to the table and then to the air.
Observe that in this case the vibrations are transmitted by a liquid, a
solid, and by the air. Compare this action with the transmission of sound
waves by different portions of the ear.

*To show Effects of Sound Waves.*--1. Place two large tuning forks of the
same pitch, and mounted on thin boxes for reenforcing their vibrations,
near each other on a table. Vibrate one of the forks for a moment and then
stop it by means of the hand. Observe that the other fork has been set in
vibration. (This experiment does not work with forks of different pitch.)
2. While holding a thin piece of paper against a comb with the open lips,
produce musical tones with the vocal cords. These will set the paper in
vibration, producing the so-called "comb music." 3. Examine the disk in a
telephone which is set in vibration by the voice. Observe that it is a
thin disk and, like the membrane of the ear, has air on both sides of it.

*To show the Reenforcement of Sound.*--1. Vibrate a tuning fork in the air,
noting the feebleness of the tone produced. Then hold the stem against a
door or the top of a table, noting the difference. 2. Hold a vibrating
tuning fork over a tall jar, or bottle, and gradually add water. If the
vessel is sufficiently tall, a depth will be reached where the air in the
vessel reenforces the sound from the fork. 3. Hold a vibrating fork over
the mouth of a small fruit jar, partly covered with a piece of cardboard.
By varying the size of the opening, a position will be found where the
sound is reenforced. If not successful at first, try bottles and jars of
different sizes.

*To illustrate the Manner of Vibration of the Liquid in the Internal
Ear.*--Tie a piece of dental rubber over the end of a glass or wooden tube
about half an inch in diameter and six inches in length. Fill the tube
entirely full of water and, without spilling, tie a piece of thin rubber
tightly over the other end. Holding the tube horizontally, press the
rubber in at one end and note that it is pushed out at the other end. Make
an imitation of a vibration with the finger against the rubber at one end
of the tube and note the effect at the other end. To what do the tube and
the rubber on the ends of the tube correspond in the internal ear?

                                [Fig. 156]

        Fig. 156--*Simple apparatus* for demonstrating the larynx.

*To show the Plan of the Larynx.*--Cut from stiff paper four pieces of
different shapes as indicated in Fig. 156. (The piece to the left should
have a length of about six inches, the others proportionally large.) The
largest represents the thyroid cartilage, the next in size the cricoid,
and the two smallest the arytenoid cartilages. By means of pins, or
threads, connect these with each other according to the description of the
larynx on page 253. With this simple model the movements of the different
cartilages and their effect upon the vocal cords may be illustrated.

*To show the Relation of the Movements of the Vocal Organs to the
Production of Different Sounds.*--1. Lightly grasp the larynx with the
fingers while talking. Observe the changes, both in the position and shape
of the larynx, in the production of sounds of different pitch. 2. Observe
the difference in the action of the muscles of respiration in the
production of loud and faint sounds. 3. Pronounce slowly the vowels, A, E,
I, O, U, and the consonants C, F, K, M, R, S, T, and V, noting the shape
of the mouth, the position of the tongue, and the action of the lips in
each case.

*To demonstrate the Ear.*--Examine a dissectible model of the ear, locating
and naming the different parts. Trace as far as possible the path of the
sound waves and find the termination of the auditory nerve. Note also the
relative size of the parts, and calculate the number of times the model is
larger than the natural ear. _Suggestion_: The greatest diameter of the
internal ear is about three fourths of an inch.

In an extended course it is a profitable exercise to dissect the ear of a
sheep or calf, observing the auditory canal, middle ear, bridge of bones,
and the tympanic membrane with attached malleus and tensor tympanic
muscle. Pass a probe from the nasal pharynx through the Eustachian tube
into the middle ear. With bone forceps or a fine saw, split open the
petrous portion of the temporal bone and observe the cochlea and the
semicircular canals. By a careful dissection other parts of interest may
also be shown.

CHAPTER XXII - THE EYE

Sight is considered the most important of the sensations. It is the chief
means of bringing the body into proper relations with its surroundings
and, even more than the sensation of hearing, is an avenue for the
reception of ideas. The sense organs for the production of sight are the
eyes; the external stimulus is

*Light.*--Light, like sound, consists of certain vibrating movements, or
waves. They differ from sound waves, however, in form, velocity, and in
method of origin and transmission. Light waves are able to pass through a
vacuum, thus showing that they are not dependent upon air for their
transmission. They are supposed to be transmitted by what the physicist
calls ether--a highly elastic and exceedingly thin substance which fills
all space and penetrates all matter. As a rule, light waves originate in
bodies that are highly heated, being started by the vibrations of the
minute particles of matter.

Light is influenced in its movements by various conditions. In a substance
of uniform density it moves with an unchanging velocity and in a straight
line. If it enters a less dense, or rarer, substance, its velocity
increases; if one more dense, its velocity diminishes; and if it enters
either the rarer or denser substance in any direction other than
perpendicularly, it is bent out of its course, or _refracted_. If it
strikes against a body lying in its course, it may be thrown off
(_reflected_), or it may enter the body and either be passed on through
(_transmitted_) or _absorbed_ (Fig. 157). Light which is absorbed is
transformed into heat.

*Kinds of Reflection.*--Waves of light striking against the smooth surface
of a mirror are thrown off in definite directions, depending on the angle
at which they strike. (Illustrate by holding a mirror in the direct rays
of the sun.) But light waves that strike rough surfaces are reflected in
practically all directions and apparently without reference to the angle
at which they strike. (Illustrate by placing a piece of white paper in the
direct rays of the sun. It matters not from what direction it is viewed,
waves of light strike the eye.) This kind of reflection is called
_diffusion_, and it serves the important purpose of making objects
visible. The light waves passing out in all directions from objects which
have received light from the sun, or some other luminous body, enable them
to be seen.

                                [Fig. 157]

 Fig. 157--*Diagram illustrating passage of light waves.*On the right the
 light is transmitted by the glass, reflected by the mirror, refracted by
the prism, and absorbed by the black cloth. On the left the light from the
 candle forms an image by passing through a small hole in a cardboard and
                          falling upon a screen.

*Formation of Images.*--Another principle necessary to seeing is that of
refraction. _Refraction_ means the bending, or turning, of light from a
straight course. One of the most interesting effects of refraction is the
formation of images of objects, such as may be accomplished by light from
them passing in a certain manner through convex lenses. If, for example, a
convex lens be moved back and forth between a candle and a screen in a
dimly lighted room, a position will be found where a picture of the candle
falls upon the screen. This picture, called the _image_, results from the
refraction of the candle light in passing through the lens.

                                [Fig. 158]

  Fig. 158--*Diagram illustrating formation of images.* On the right the
image is formed by a double convex lens; on the left by the lenses of the
 eye. The candle flame represents a luminous, or light-giving, body; but
       light passes from the large arrow by reflection. (See text.)

In order to form an image, the light waves spreading out from the object
must be brought together, or focused. Focusing means literally the
bringing of light to a point, but it is evident in the formation of an
image that all the waves are not brought to a single point. If they were,
there would be no image. In the example of the candle given above, the
explanation is as follows:

The light from the candle comes from a great number of separate and
distinct points in the candle flame. The lens, by its peculiar shape,
bends the waves coming from any single point so that they are brought to a
corresponding point on the screen. Furthermore, the points of focused
light are made to occupy the same relative positions on the screen as the
points from which they emanate in the candle flame (Fig. 158). This is why
the area of light on the screen has the same form as the candle, or makes
an image of it. The same explanation applies if, instead of the luminous
candle, a body that simply reflects light, as a book, is used.

*The Problem of Seeing.*--What we call _seeing_ is vastly more than the
stimulation of the brain through the action of light upon afferent
neurons. It is the _perceiving _of all the different things that make up
our surroundings. If one looks toward the clear sky, he receives a
_sensation of light_, but sees no object. He may also get a sensation of
light with the eyelids closed, if he turn the eyes toward the window or
some bright light. But how different when the light from various objects
enters the eyes. There is apparently no consciousness of light, but
instead a consciousness of the size, form, color, and position of the
objects. _Seeing is perceiving objects._ Stimulation by the light waves is
only the means toward this end. The chief problem in the study of sight is
that of determining _how light waves enable us to become conscious of
objects._

*Sense Organs of Sight.*--The sense organs of sight consist mainly of the
two eyeballs. Each of these is located in a cavity of the skull bones,
called the _orbit_, where it is held in position by suitable tissues and
turned in different directions by a special set of muscles. A cup-shaped
receptacle is provided within the orbit, by layers of fat, and a smooth
surface is supplied by a double membrane that lies between the fat and the
eyeball. In front the eyeballs are provided with movable coverings, called
the _eyelids_. These are composed of dense layers of connective tissue,
covered on the outside by the skin and lined within by a sensitive
membrane, called the _conjunctiva_. At the base of the lids the
conjunctiva passes to the eyeball and forms a firmly attached covering
over its front surface. This membrane prevents the passage of foreign
materials back of the eyeball, and by its sensitiveness stimulates effort
for the removal of irritating substances from beneath the lids. The
eyelashes and the eyebrows are also a means of protecting the eyeballs.

*The Eyeball*, or globe of the eye, is a device for _focusing_ light upon
a sensitized nervous surface which it incloses and protects. In shape it
is nearly spherical, being about an inch in diameter from right to left
and nine tenths of an inch both in its vertical diameter and from front to
back. It has the appearance of having been formed by the union of two
spherical segments of different size. The smaller segment, which forms
about one sixth of the whole, is set upon the larger and forms the
projecting transparent portion in front. The walls of the eyeballs are
made up of three separate layers, or coats--an _outer coat_, a _middle
coat_, and an _inner coat_ (Fig. 159).

                                [Fig. 159]

 Fig. 159--*Diagram of the eyeball in position.* 1. Yellow spot. 2. Blind
spot. 3. Retina. 4. Choroid coat. 5. Sclerotic coat. 6. Crystalline lens.
 7. Suspensory ligament. 8. Ciliary processes and ciliary muscle. 9. Iris
  containing the pupil. 10. Cornea. 11. Lymph duct. 12. Conjunctiva. 13.
Inferior and superior recti muscles. 14. Optic nerve. 15. Elevator muscle
of eyelid. 16. Bone. _A._ Posterior chamber containing the vitreous humor.
           _B._ Anterior chamber containing the aqueous humor.

*The Outer Coat* surrounds the entire globe of the eye and consists of two
parts--the sclerotic coat and the cornea. The _sclerotic coat_ covers the
greater portion of the larger spherical segment and is recognized in front
as "the white of the eye." It is composed mainly of fibrous connective
tissue and is dense, opaque, and tough. It preserves the form of the
eyeball and protects the portions within. It is pierced at the back by a
small opening which admits the optic nerve, and in front it becomes
changed into the peculiar tissue that makes up the cornea.

The _cornea_ forms the transparent covering over the lesser spherical
segment of the eyeball, shading into the sclerotic coat at its edges. It
has a complex structure, consisting in the main of a transparent form of
connective tissue. It serves the purpose of admitting light into the
eyeball.

*The Middle Coat* consists of three connected portions--the _choroid coat_,
the _ciliary processes_, and the _iris_. These surround the larger
spherical segment. All three parts are rich in blood vessels, containing
the blood supply to the greater portion of the eyeball.

The _choroid coat_ lies immediately beneath the sclerotic coat at all
places except a small margin toward the front of the eyeball. It is
composed chiefly of blood vessels and a delicate form of connective tissue
that holds them in place. It contains numerous pigment cells which give it
a dark appearance and serve to absorb surplus light. Near where the
sclerotic coat joins the cornea, the choroid coat separates from the outer
wall and, by folding, forms many slight projections into the interior
space. These are known as the _ciliary processes_. The effect of these
folds is to collect a large number of capillaries into a small space and
to give this part of the eyeball an extra supply of blood. Between the
ciliary processes and the sclerotic coat is a small muscle, containing
both circular and longitudinal fibers, called the _ciliary muscle_.

The _iris_ is a continuation of the choroid coat across the front of the
eyeball. It forms a dividing curtain between the two spherical segments
and gives the color to the eye. At its center is a circular opening,
called the _pupil_, which admits light to the back of the eyeball. By
varying the size of the pupil, the iris is able to regulate the amount of
light which passes through and it employs for this purpose two sets of
muscular fibers. One set of fibers forms a thin band which encircles the
pupil and serves as a sphincter to diminish the opening. Opposing this are
radiating fibers which are attached between the inner and outer margins of
the iris. By their contraction the size of the opening is increased. Both
sets of fibers act reflexively and are stimulated by variations in the
light falling upon the retina.

                                [Fig. 160]

  Fig. 160--*Diagram showing main nervous elements in the retina.* Light
waves stimulate the rods and cones at back surface of the retina, starting
impulses which excite the ganglion cells at the front surface. Fibers from
              the ganglion cells pass into the optic nerve.

*The Inner Coat, or Retina.*--This is a delicate membrane containing the
expanded termination of the optic nerve. It rests upon the choroid coat
and spreads over about two thirds of the back surface of the eyeball.
Although not more than one fiftieth of an inch in thickness, it presents a
very complex structure, essentially nervous, and is made up of several
distinct layers. Of chief importance in the outer layer are the cells
which are acted upon directly by the light and are named, from their
shape, the _rods_ and _cones_. In contact with these, but occupying a
separate layer, are the ends of small afferent nerve cells. These in turn
communicate with nerve cells in a third layer, known as the ganglion
cells, that send their fibers into the optic nerve (Fig. 160).

In the center of the retina is a slight oval depression having a faint
yellowish color, and called, on that account, the _yellow spot_. This is
the part of the retina which is most sensitive to light. Directly over the
place of entrance of the optic nerve is a small area from which the rods
and cones are absent and which, therefore, is not sensitive to light. This
is called the _blind spot_. (See Practical Work.)

*The Crystalline Lens.*--Immediately back of the iris and touching it is a
transparent, rounded body, called the crystalline lens. This is about one
fourth of an inch thick and one third of an inch through its long
diameter, and is more curved on the back than on the front surface. It is
inclosed in a thin sheath, called the _membranous capsule_, which connects
with a divided sheath from the sides of the eyeball, called the
_suspensory ligament_ (Fig. 159). Both the lens and the capsule are highly
elastic.

*Chambers and "Humors" of the Eyeball.*--The crystalline lens together with
the suspensory ligament and the ciliary processes form a partition across
the eyeball. This divides the eye space into two separate compartments,
which are filled with the so-called "humors" of the eye. The front cavity
of the eyeball, which is again divided in part by the iris, is filled with
the _aqueous_ humor. This is a clear, lymph-like liquid which contains an
occasional white corpuscle. It has a feeble motion and is slowly added to
and withdrawn from the eye. It is supplied mainly by the blood vessels in
the ciliary processes and finds a place of exit through a small lymph duct
at the edge of the cornea (Fig. 159).

The back portion of the eyeball is filled with a soft, transparent,
jelly-like substance, called the _vitreous_ humor. It is in contact with
the surface of the retina at the back and with the attachments of the lens
in front, being surrounded by a thin covering of its own, called the
_hyaloid membrane_. The aqueous and vitreous humors aid in keeping the
eyeball in shape and also in focusing.

*How we see Objects.*--To see an object at least four things must happen:

1. Light must pass from the object into the eye. Objects cannot be seen
where there is no light or where, for some reason, it is kept from
entering the eye.

2. The light from the object must be focused (made to form an image) on
the retina. In forming the image, an area of the retina is stimulated
which corresponds to _the form of the object_.

3. Impulses must pass from the retina to the brain, stimulating it to
produce the sensations.

4. The sensations must be so interpreted by the mind as to give an
impression of the object.

*Focusing Power of the Eyeball.*--The eyeball is essentially a device for
focusing light. All of its transparent portions are directly concerned in
this work, and the portions that are not transparent serve to protect and
operate these parts and hold them in place. Of chief importance are the
crystalline lens and the cornea. Both of these are lenses. The cornea with
its inclosed liquid is a plano-convex lens, while the crystalline lens is
double convex.(123) Because of the great difference in density between the
air on the outside and the aqueous humor within, the cornea is the more
powerful of the two. The crystalline lens, however, performs a special
work in focusing which is of great importance. The iris also aids in
focusing since it, through the pupil, regulates the amount of light
entering the back chamber of the eyeball and causes it to fall in the
center of the crystalline lens, the part which focuses most accurately.

                                [Fig. 161]

 Fig. 161--*Diagram showing changes in shape of crystalline lens* to adapt
                      it to near and distant vision.

*Accommodation.*--A difficulty in focusing arises from the fact that the
degree of divergence of the light waves entering the eye from different
objects, varies according to their distance. Since the waves from any
given point on an object pass out in straight lines in all directions, the
waves that enter the eye from distant objects are at a different angle
from those that enter from near objects. In reality waves from distant
objects are practically parallel, while those from very near objects
diverge to a considerable degree. To adjust the eye to different distances
requires some change in the focusing parts that corresponds to the
differences in the divergence of the light. This change, called
_accommodation_, occurs in the crystalline lens.(124) In the process of
accommodation, changes occur in the shape of the crystalline lens, as
follows:

1. In looking from a distant to a near object, the lens becomes more
convex, _i.e._, rounder and thicker (Fig. 161). This change is necessary
because the greater divergence of the light from the near objects requires
a greater converging power on the part of the lens.(125)

2. In looking from near to distant objects, the lens becomes flatter and
thinner (Fig. 161). This change is necessary because the less divergent
waves from the distant objects require less converging power on the part
of the lens.

The method employed in changing the shape of the lens is difficult to
determine and different theories have been advanced to account for it. The
following, proposed by Helmholtz, is the theory most generally accepted:

The lens is held in place back of the pupil by the suspensory ligament.
This is attached at its inner margin to the membranous capsule, and at its
outer margin to the sides of the eyeball, and entirely surrounds the lens.
It is drawn perfectly tight so that the sides of the eyeball exert a
continuous tension, or pull, on the membranous capsule, which, in its
turn, exerts pressure on the sides of the lens, tending to flatten it.
This arrangement brings the elastic force of the eyeball into opposition
to the elastic force of the lens. The ciliary muscle plays between these
opposing forces in the following manner:

_To thicken the lens_, the ciliary muscle contracts, pulling forward the
suspensory ligament and releasing its tension on the membranous capsule.
This enables the lens to thicken on account of its own elastic force. _To
flatten the lens_, the ciliary muscle relaxes, the elastic force of the
eyeball resumes its tension on the suspensory ligament, and the membranous
capsule resumes its pressure on the sides of the lens. This pressure,
overcoming the elastic force of the lens, flattens it.

*Movements of the Eyeballs.*--In order that the light may enter the
eyeballs to the best advantage, they must be moved in various directions.
These movements are brought about through the action of six small muscles
attached to each eyeball. Four of these, named, from their positions, the
superior, inferior, internal, and external recti muscles, are attached at
one end to the sides of the eyeball and at the other end to the back of
the orbit (Fig. 162). These, in the order named, turn the eyes upward,
downward, inward, and outward. The other two, the superior and inferior
oblique muscles, aid in certain movements of the recti muscles and, in
addition, serve to rotate the eyes slightly. The movements of the eyeballs
are similar to those of ball and socket joints.

                                [Fig. 162]

                 Fig. 162--*Exterior muscles of eyeball.*

*Binocular Vision.*--In addition to directing the eyeballs so that light
may enter them to the best advantage from different objects, the muscles
also enable two eyes to be used as one. Whenever the eyes are directed
toward the same object, an image of this object is formed on the retina of
each. Double vision is prevented only by having the images fall on
corresponding places in the two eyes. This is accomplished by the muscles.
In each act of seeing, it becomes the task of the superior and inferior
recti muscles to keep the eyes in the same plane, and of the external and
internal recti muscles to give just the right amount of convergence. If
slight pressure is exerted against one of the eyes, the action of the
muscles is interfered with and, as a consequence, one sees double. The
advantages of two eyes over one in seeing lie in the greater distinctness
and broader range of vision and in the greater correctness of judgments of
distance.

*Visual Sensations.*--The visual sensations include those of _color_ and
those of a _general sensibility to light_. Proof of the existence of these
types of sensation is found in color blindness, a defect which renders the
individual unable to distinguish certain colors when he is still able to
see objects. Color sensations are the results of light waves of different
lengths acting on the retina. While the method by which waves of one
length produce one kind of sensation and those of another length a
different sensation is not understood, the cones appear to be the portions
of the retina acted on to produce the color. On the other hand, the rods
are sensitive to all wave lengths and give general sensibility to light.

*Visual Perceptions.*--"Seeing" is very largely the mental interpretation
of the primary sensations and the conditions under which they occur. For
example, our ability to see objects in their natural positions when their
images are inverted on the retina is explained by the fact that we are not
conscious of the retinal image, but of the mind's interpretation of it
through experience. Experience has also taught us to locate objects in the
direction toward which it is necessary to turn the eyes in order to see
them. In other words, we see objects in the direction from which the light
enters the eyes. That the object is not always in that direction is shown
by the image in the mirror. The apparent size and form of objects are
inferences, and they are based in part upon the size and form of the area
of the retina stimulated. We judge of distance by the effort required to
converge the eyes upon the objects, by the amount of divergence of the
waves entering the pupil, and also by the apparent size of the object.

*The Lachrymal Apparatus.*--Seeing requires that the light penetrate to the
retina. For this reason all the structures in front of the retina are
transparent. One of these structures, the cornea, on account of its
exposure to the air, is liable to become dry, like the skin, and to lose
its transparency. To preserve the transparency of the cornea, and also to
lubricate the eyelids and aid in the removal of foreign bodies, a
secretion, called _tears_, is constantly supplied.

                                [Fig. 163]

  Fig. 163--*Diagram of irrigating system of the eye.* After wetting the
      eyeball the tears may also moisten the air entering the lungs.

The lachrymal, or tear, glands are situated at the upper and outer margins
of the orbits. They have the general structure of the salivary glands and
discharge their liquid by small ducts beneath the upper lids. From here
the tears spread over the surfaces of the eyeballs and find their way in
each eye to two small canals whose openings may be seen on the edges of
the lids near the inner corner (Fig. 163). These canals unite to form the
_nasal duct_, which conveys the tears to the nasal cavity on the same side
of the nose. When by evaporation the eyeball becomes too dry, the lids
close reflexively and spread a fresh layer of tears over the surface. Any
excess is passed into the nostrils, where it aids in moistening the air
entering the lungs.

HYGIENE OF THE EYE

*Defects in Focusing.*--The delicacy and complexity of the sense organs of
sight render them liable to a number of imperfections, or defects, the
most frequent and important being those of focusing. Such defects not only
result in the imperfect vision of objects, but they throw an extra strain
upon the nervous system and may render the process of seeing exceedingly
painful.

A normal eye is able, when relaxed, to focus light accurately from objects
which are twenty feet or more away and to accommodate itself to objects as
near as five inches. An eye is said to be _myopic_, or _short-sighted_,
when it is unable to focus light waves from distant objects, but can only
distinguish the objects which are near at hand. In such an eye the ball is
too long for the converging power of the lenses, and the image is formed
in front of the retina (_C_, Fig. 164).

                                [Fig. 164]

 Fig. 164--*Diagrams illustrating long-sightedness and short-sightedness*,
  and method of remedying these defects by lenses. _A._ Normal eye. _B._
                Long-sighted eye. _C._ Short-sighted eye.

A _long-sighted_, or _hypermetropic_, eye is one which can focus light
from distant objects, but not from near objects. In such an eye the ball
is too short for the converging power of the lenses and the image tends to
form back of the retina (_B_, Fig. 164). These defects in focusing are
remedied by wearing glasses with lenses so shaped as to counteract them.
Short-sightedness is corrected by concave lenses and long-sightedness by
convex lenses, as shown in diagrams above.

_Astigmatism_ is another defect in the focusing power of the eye. In
astigmatism the parts of the eye fail to form the image in the same plane,
so that all portions of the object do not appear equally distinct. Certain
parts of it are indistinct, or blurred. The cause is found in some
difference in curvature of the surfaces of the cornea or crystalline lens.
It is corrected by lenses so ground as to correct the particular defects
present in a given eye.

Whenever defects in focusing are present, particularly in astigmatism,
extra work is thrown on the ciliary muscle as well as the muscles that
move the eyeballs. The result is frequently to induce a condition, known
as _muscle weakness_, which renders it difficult to use the eyes. Even
after the defect in focusing has been remedied, the muscles recover slowly
and must be used with care. For this reason glasses should be fitted by a
competent oculist(126) as soon as a defect is known to exist. When one is
unduly nervous, or suffers from headache, the eyes should be examined for
defects in focusing (page 326).

*Eye Strain and Disease.*--The extra work thrown upon the nervous system
through seeing with defective eyes, especially in reading and other close
work, is now recognized as an important cause of disease. Through the tax
made upon the nervous system by the eyes, there may be left an
insufficient amount of nervous energy for the proper running of the vital
processes. As a result there is a decline of the health. Ample proof that
eye strain interferes with the vital processes and causes ill health, is
found in the improvements that result when, by means of glasses, this is
relieved.

*The Eyes of School Children.*--School children often suffer from defects
of vision which render close work burdensome, and cause headache, general
nervousness, and disease. Furthermore, the visual defects may be unknown
both to themselves and to their parents. Pupils showing indications of
eye-strain should be examined by an oculist, and fitted with glasses
should defects be discovered.(127) The precaution, adopted by many
schools, of having the eyes of all children examined by a competent
physician employed for the purpose, is most excellent and worthy of
imitation.

*Reading Glasses.*--Many people whose eyes are weak, because slightly
defective, find great relief in the use of special glasses for reading and
other close work. By using such glasses they may postpone the time when
they are compelled to wear glasses constantly. It is in the close work
that the extra strain comes upon the eyes, and if this is relieved, one
can much better withstand the work of distant vision. The reading glasses
should be fitted by a competent oculist, and used only for the purpose for
which they are intended.

*General Precautions in the Use of the Eyes.*--If proper care is exercised
in the use of the eyes, many of their common ailments and defects may be
avoided. Any one, whether his eyes are weak or strong, will do well to
observe the following precautions:

1. Never read in light that is very intense or very dim. 2. When the eyes
hurt from reading, stop using them. 3. Never hold a book so that the
smooth page reflects light into the eyes. The best way is to sit or stand
so that the light passes over the shoulder to the book. 4. Never study by
a lamp that is not shaded. 5. Practice cleanliness in the care of the
eyes. Avoid rubbing the eyes with the fingers unless sure the fingers are
clean.

If the eyes are weak, use them less and avoid, if possible, reading by
artificial light. Weak eyes are sometimes benefited by bathing them in
warm water, or with water containing enough salt to make them smart
slightly. Boracic acid dissolved in water (40 grains to 4 ounces of
distilled water) is also highly recommended as a wash for weak eyes.

                                [Fig. 165]

       Fig. 165--*Method of procedure in lifting the eyelid* (Pyle).

*Removal of Foreign Bodies from the Eyes.*--Foreign bodies embedded in the
eyeball should be removed by the oculist or physician. Small particles of
dust or cinder may be removed without the aid of the physician, by
exercising proper care. First let the tears, if possible, wash the
offending substance to the corner of the eye, or edge of the lid, where it
can be removed with a soft cloth. If it sticks to the ball or the under
surface of the lid, it will be necessary to find where it is located, and
then dislodge it from its position. Begin by examining the lower lid. Pull
it down sufficiently to expose the inner surface, and, if the foreign
substance be there, wipe it off with the hem of a clean handkerchief. If
it is not under the lower lid, it will be necessary to fold back the upper
lid. "The patient is told to look down, the edge of the lid and the lashes
are seized with the forefinger and thumb of the right hand (Fig. 165), and
the lid is drawn at first downward and forward away from the globe; then
upward and backward over the point of the thumb or forefinger of the left
hand, which is held stationary on the lid, and acts as a fulcrum."(128)
The foreign body is now removed in the same manner as from the lower lid.
A large lens may be used to good advantage in finding the irritating
substance.

*Strong Chemicals in the Eyes.*--Students in the laboratory frequently,
through accident, get strong chemicals, as acids and bases, in the eyes.
The first thing to do in such cases is quickly and thoroughly to _flood
the eyes with water_. Any of the chemical which remains may then be
counteracted by the proper reagent, care being taken to use a very dilute
solution. To counteract an acid, use sodium bicarbonate (cooking soda),
and for bases use a very dilute solution of acetic acid (vinegar). To
guard against getting the counteractive agent too strong for the inflamed
eye, it should first be tried on an eye that has not been injured.

*Summary.*--The nervous impulses that cause the sensation of sight are
started by light waves falling upon a sensitized nervous surface, called
the retina. By means of refractive agents, forming a part of the eyeball
in front of the retina, light from different objects is focused and made
to form images of the objects upon the surface. In this way the light is
made to stimulate a portion of the retina corresponding to the form of the
object. This, _the image method of stimulation_, enables the mind to
recognize objects and to locate them in their various positions. While the
greater portion of the eyeball is concerned in the focusing of light, the
crystalline lens, operated by the ciliary muscle, serves as the special
instrument of accommodation. Muscles attached to the eyeballs turn them in
different directions, and so adjust them with reference to each other that
double vision is avoided.

*Exercises.*--1. Under what conditions are light waves reflected,
refracted, and absorbed?

2. Why does the body not need a light-producing apparatus, corresponding
to the larynx in the production of sound?

3. How is the light from a candle made to form an image?

4. What different things must happen in order that one may see an object?

5. Make a sectional drawing of the eyeball, locating and naming all the
parts.

6. Of what parts are the outer, middle, and inner coats of the eyeball
made up?

7. What portions of the eyeball reflect light? What absorb light? What
transmit light? What refract light?

8. Show how the iris, the crystalline lens, the retina, the ciliary
muscle, and the cornea aid in seeing.

9. Trace a wave of light from a visible object to the retina.

10. Why does not the inverted image on the retina cause us to see objects
upside down?

11. What change occurs in the shape of the crystalline lens when we look
from distant to near objects? From near to distant objects? Why are these
changes necessary? How are they brought about?

12. How does the method of adjustment, or accommodation, of the eyeball
differ from that of a telescope or a photographer's camera?

13. With two eyes how are we kept from seeing double?

14. What different purposes are served by the tears. Trace them from the
lachrymal glands to the nostrils.

15. Show how the proper lenses remedy short- and long-sightedness.

16. Describe the conjunctiva and give its functions. Why should it be so
sensitive?

17. How may eye strain cause disease in parts of the body remote from the
eyes?

18. How does "image stimulation" differ from light stimulation in general?

PRACTICAL WORK

*To illustrate Simple Properties of Light.*--1. Heat an iron or platinum
wire in a clear gas flame. Observe that when a high temperature is reached
it gives out light or becomes luminous.

2. Cover one hand with a white and the other with a black piece of cloth,
and hold both for a short time in the direct rays of the sun. Note and
account for the difference in temperature which is felt.

3. Stand a book or a block of wood by the side of an empty pan in the
sunlight, so that the end of the shadow falls on the bottom of the pan.
Mark the place where the shadow terminates and fill the pan with water.
Account for the shadow's becoming shorter.

4. Place a coin in the center of an empty pan and let the members of the
class stand where the coin is barely out of sight over the edges of the
pan. Fill the pan with water and account for the coin's coming into view.
Show by a drawing how light, in passing from the water into the air, is so
bent as to enter the eye.

5. With a convex lens, in a darkened room, focus the light from a candle
flame so that it falls on a white screen and forms an image of the candle.
Observe that the image is inverted. In a well-lighted room focus the light
from a window upon a white screen. Show that, as the distance from the
window to the screen is changed, the position of the lens must also be
changed. (Accommodation.)

6. Hold a piece of cardboard, about eight inches square and having a
smooth, round hole an eighth of an inch in diameter in the center, in
front of a lighted candle in a darkened room. Back of the opening place a
muslin or paper screen (Fig. 157). Observe that a dim image is formed.
Account for the fact that it is inverted. Hold a lens between the
cardboard and the screen so that the light passes through it also. The
image should now appear smaller and more distinct.

                                [Fig. 166]

        Fig. 166--*Diagram* for proving presence of the blind spot.

*To prove the Presence of the Blind Spot.*--Close the left eye and with the
right gaze steadily at the spot on the left side of this page (Fig. 166).
Then starting with the book a foot or more from the face, move it slowly
toward the eye. A place will be found where the spot on the right entirely
disappears. On bringing it nearer, however, it is again seen. As the book
is moved forward or backward, the position of the image of this spot
changes on the retina. When the spot cannot be seen, it is because the
image falls on the blind spot.

*Dissection of the Eyeball.*--Procure from the butcher two or three
eyeballs obtained from cattle. After separating the fat, connective
tissue, and muscle, place them in a shallow vessel and cover with water.
Insert the blade of a pair of sharp scissors at the junction of the
sclerotic rotic coat with the cornea and cut from this point nearly around
the entire circumference of the eyeball, passing near the optic nerve.
Spread open in the water and identify the different parts from the
description in the text. Open the second eyeball in water by cutting away
the cornea. Examine the parts in front of the lens.

                                [Fig. 167]

             Fig. 167--*Model* for demonstrating the eyeball.

*To illustrate Accommodation.*--Paste together the ends of a strip of stiff
writing paper (two by five inches) making a ring a little less than three
inches in diameter. This is to represent the crystalline lens. Now paste a
piece of thin paper (two by seven inches) upon a second strip of the same
size, leaving an open place in the middle for the insertion of the paper
lens. A flexible piece of cardboard (three by twelve inches) is now bent
into the form of a half circle and to its ends are fastened the strips of
paper containing the ring. Make a small hole in each of the four corners
of the bent cardboard. Through these holes pass two loops of thread, or
fine string, in opposite directions, letting the ends hang loose from the
cardboard.

When everything is in position, the tension from the cardboard flattens
the paper lens, while pulling the strings releases this tension and
permits the lens to become more rounded. With this simple device the
changes in the curvature of the lens for near and distant vision are
easily shown.

CHAPTER XXIII - THE GENERAL PROBLEM OF KEEPING WELL

    "To cure was the voice of the Past: to prevent is the divine
    whispering of To-day."

As stated in the introduction to our study, the fundamental law of hygiene
is the law of harmony: _Habits of living must harmonize with the plan of
the body._ Having acquainted ourselves with the plan of the body, we may
now review briefly those conditions that help or hinder its various
activities. The hygiene already presented in connection with the study of
the various organs may be condensed into general rules, or laws, as
follows:

1. Of exercise: Exercise daily the important groups of muscles.

2. Of form: Preserve the natural form of the body.

3. Of energy: Observe regular periods of rest and exercise and avoid
exhaustion.

4. Of nutriment: Eat moderately of a well-cooked and well-balanced diet
and drink freely of pure water.

5. Of respiration: Breathe freely and deeply of pure air and spend a part
of each day out of doors.

6. Of nervous poise: Suppress wasteful and useless forms of nervous
activity, avoid nervous strain, and practice cheerfulness.

7. Of cleanliness: Keep the body and its immediate surroundings clean.

8. Of restraint: Abstain from the unnecessary use of drugs as well as from
the practice of any form of activity known to be harmful to the body.

9. Of elimination: Observe all the conditions that favor the regular
discharge of waste materials from the body.

Obedience to these laws is of vast importance in the proper management of
the body. They should, indeed, be so thoroughly impressed upon the mind as
to become fixed habits. There are, however, other conditions that relate
to this problem, and it is to these that we now turn. These conditions
have reference more specifically to

*The Prevention of Disease.*--While the average length of life is not far
from thirty-five years, the length of time which the average individual is
capable of living is, according to some of the lowest estimates, not less
than seventy years. This difference is due to disease. People do not, as a
rule, die on account of the wearing out of the body as seen in extreme old
age, but on account of the various ills to which flesh is heir. It is true
that many people meet death by accident and not a few are killed in wars,
but these numbers are small in comparison with those that die of bodily
disorders. The prevention of disease is the greatest of all human
problems. Though the fighting of disease is left largely to the physician,
much is to be gained through a more general knowledge of its causes and
the methods of its prevention.

*Causes of Disease.*--Disease, which is some _derangement of the vital
functions_, may be due to a variety of causes. Some of these causes, such
as hereditary defects, are remote and beyond the control of the
individual. Others are the result of negligence in the observance of
well-recognized hygienic laws. Others still are of the nature of
influences, such as climate, the house in which one lives, or one's method
of gaining a livelihood, that produce changes in the body, imperceptible
at the time, but, in the long run, laying the foundations of disease. And
last, and most potent, are the minute living organisms, called microbes or
germs, that find their way into the body. Although there are two general
kinds of germs, known as _bacteria_ (one-celled plants) and _protozoa_
(one-celled animals), most of our germ diseases are caused by bacteria.

*Effects of Germs.*--While there are many kinds of germs that have no ill
effect upon the body and others that are thought to aid it in its work,
there are many well-known varieties that produce effects decidedly
harmful. They gain an entrance through the lungs, food canal, or skin,
and, living upon the fluids and tissues, multiply with great rapidity
until they permeate the entire body. Not only do they destroy the
protoplasm, but they form waste products, called _toxins_, which act as
poisons. Diseases caused by germs are known as infectious, or contagious,
diseases.(129) The list is a long one and includes smallpox, measles,
diphtheria, scarlet fever, typhoid fever, tuberculosis, la grippe,
malaria, yellow fever, and others of common occurrence. In addition to the
diseases that are well pronounced, it is probable that germs are
responsible also for certain bodily ailments of a milder character.(130)

*Avoidance of Germ Diseases.*--The problem of preventing diseases caused by
germs is an exceedingly difficult one and no solution for all diseases has
yet been found. One's chances of avoiding such diseases, however, may be
greatly enhanced:

1. By strengthening the body through hygienic living so that it offers
greater resistance to the invasions of germs.

2. By living as far as possible under conditions that are unfavorable to
germ life.

3. By understanding the agencies through which disease germs are spread
from person to person.

*Conditions Favorable and Unfavorable for Germs.*--Conditions favorable for
germ life are supplied by animal and vegetable matter, moisture, and a
moderate degree of warmth. Hence disease germs may be kept alive in damp
cellars and places of filth. Even living rooms that are poorly lighted or
ventilated may harbor them. Water may, if it contain a small per cent of
organic matter, support such dangerous germs as those of typhoid fever.
Fresh air, sunlight, dryness, cleanliness, and a high temperature, on the
other hand, are destructive of germs. The germs in impure water, as
already noted (page 165), are destroyed by boiling.

*How Germs are Spread.*--Some of the more common methods by which the germs
of disease are spread, and by so doing find new victims, are as follows:

1. _By Means of Foods._--Foods, on account of the locality in which they
are produced or the method of gathering or of handling-them, may become
contaminated with germs, which are then transported with the foods to the
consumer.

2. _By Means of Dust._--Material containing germs, _e.g._, discharges from
the throat and lungs, will on drying form dust. This is lifted with other
fine particles by the air and may be carried quite a distance. The dust
from public halls and other places where people congregate is the kind
most likely to contain disease germs. Dust should be breathed as little as
possible and only through the nostrils. Where one is compelled, as in
sweeping, to breathe dust-laden air for some time, he should inhale
through a moistened sponge, or cloth, tied in front of the nostrils.

3. _By Means of Domestic Pets and Different Kinds of Household
Vermin._--Germs sticking to the bodies of small animals are carried about
and may be easily communicated to people. By this means, rats, mice,
bedbugs, etc., where such exist, are frequently the means of spreading
disease; and particularly dangerous, on this account, is the common house
fly. Feeding as it does on filth of all kinds, it is easy for it to
transfer the bacteria that may stick to its body to the food which is
supplied to the table. The proper screening of houses and the destruction
of material in which flies may develop, such as the refuse from stables,
are necessary precautions.

Germs are spread also by the clothing of people, by railroad and steamship
lines, by the mails, and by the natural elements. In fact, any kind of
carrier, in or upon which germs can live, may serve as a means of
spreading those of certain kinds.

*Public Sanitation.*--The general conditions under which germs may thrive
and some of the means by which they are scattered, emphasize the practical
value of measures which have for their purpose the making of one's
surroundings more wholesome and hygienic. Such measures may be directed
both toward one's immediate surroundings--the home--and toward the
neighborhood, town, or city in which one lives. The hygienic conditions of
primary importance in every city or town are as follows:

1. An adequate public supply of pure water.

2. An efficient system of underground pipes for the removal of sewage.

3. An efficient system for removing from the streets and alleys everything
of the nature of waste.

4. Prevention, by enforcement of ordinances, of spitting upon sidewalks
and the floors of public halls and conveyances.

5. A hospital or sanitarium in which people can be cared for when sick
with infectious diseases.

In the larger cities other hygienic measures demand attention, such as
provisions for parks and playgrounds, the proper housing of the poor of
the city, and the suppression of the smoke and dust nuisances. Crowded
together as people are in the cities, the welfare of each individual
depends in a large measure upon the welfare of all. Hence the problems of
public sanitation are matters in which all are vitally concerned.

*Sanitary Conditions of the Home.*--The home, being the feeding and resting
place for the entire family, is the most important factor in one's
physical, as well as moral, environment. For this reason there is no place
where careful attention to hygienic requirements will yield better
results. Much of the danger from germs may be prevented by instituting and
maintaining proper sanitary conditions in and about the home.

One of the first requisites of the home is a suitable location for the
house. The house should be built upon ground that is well drained, and if
natural drainage be lacking, artificial drainage must be supplied. It
should not be situated nearer than a quarter of a mile to any marsh or
swamp and, if so near as that, it ought to be on the side from which the
wind usually blows. A stone foundation should be provided, and at least
eighteen inches of ventilated air space should be left between the ground
and the floor. Ample provisions must be made for pure air and sunlight in
all the rooms. The cellar, if one is desired, needs to be constructed with
special care. It should be perfectly dry and provided with windows for
light and ventilation. Adequate means must also be provided, by sewage
pipes and other methods, for the disposal of all waste. Where drainage
pipes are provided, care must be taken to prevent the entrance of sewer
gas into the house and also the passage of material from these pipes into
the water supply. The placing and connecting of sewer pipes should, of
course, be under the direction of a plumber.

*The Water Supply.*--Since water readily takes up and holds the impurities
with which it comes in contact, it should be exposed as little as possible
in the process of collecting. Where cistern water is used, care must be
taken to prevent filth from the roof (Fig. 168), water pipes, or soil from
getting into the reservoir. Water should be collected from the roof only
after it has rained long enough for the roof and pipes to have been
thoroughly cleaned. The cistern should have no leaks (Fig. 169), and the
top should be tightly closed to prevent the entrance of small animals and
rubbish.

                                [Fig. 168]

 Fig. 168--*Contamination of cistern water* by birds nesting in the gutter
                                 trough.

Shallow wells are to be condemned, as a rule, because of the likelihood of
surface drainage (Fig. 169), and water from springs should, for the same
reason, be used with caution. Deep wells that are kept clean usually may
be relied on to furnish water free from organic impurities, but such water
often holds in solution so much of mineral impurities as to render it
unfit for drinking. The presence in water of any considerable quantity of
the compounds of iron or calcium makes it objectionable for regular use.

                                [Fig. 169]

      Fig. 169--*Sources of contamination of cistern and well water.*
 Illustration shows liability of contamination from surface drainage and
                      from entrance of filth at top.

*Hygienic Housekeeping.*--However carefully a house has been constructed
from a sanitary standpoint, the constant care of an intelligent
housekeeper is required to keep it a healthful place in which to live.
Daily cleaning and airing of all living rooms are necessary, while such
places as the kitchen, the cellar, and the closets need extra
thoughtfulness and, at times, hard work. Moreover, the problem is not all
indoors. The immediate premises must be kept clean and sightly, and all
decaying vegetable and animal matter should be removed. Home sanitation
consists, not of one, but of many, problems, all more or less complex.
None of these can be slighted or turned over to a novice.

*Destruction of Infectious Material.*--At times the housekeeping has to be
directed especially toward hygienic requirements, such an occasion being
the sickness of one of the inmates with some contagious disease. Unless
special precautions are taken, the disease will spread to other members of
the household and may reach people in the neighborhood. Not only must
great care be exercised that nothing used in connection with the sick
shall serve as a carrier of disease, but germs passing from the patient
should, as far as possible, be actually destroyed. All discharges from the
body likely to contain bacteria, should be burned or treated with
disinfectants and buried deeply at a remote distance from the water supply
to the house.

After recovery all clothing, bedding, and furniture used in connection
with the sick should be disinfected or burned. The room also in which the
sick was cared for should be thoroughly disinfected and cleaned; in some
instances the woodwork ought to be repainted and the walls repapered or
calcimined. The purpose is, of course, to destroy all germs and prevent,
by this means, a recurrence of the disease.

*Fumigation.*--To destroy germs in the air or adhering to the walls of
rooms, furniture, clothing, etc., fumigation is employed. This is
accomplished by saturating the air of rooms with some vapor or gas which
will destroy the germs. Fumigation is quite generally employed in the
general cleaning after the patient leaves his room. This, to be effective,
must be thorough. Formaldehyde is considered the best disinfectant for
this purpose, and it should be evaporated with heat in the proportion of
one half pint of the 40 per cent solution to 1000 cu. ft. of space. Since
formaldehyde is inflammable and easily boils over, it has to be evaporated
with care. It should be boiled in a tall vessel (a tin or copper vessel
which holds about four times the quantity to be evaporated) over a quick
fire, the room being tightly closed (openings around windows and doors
plugged with cotton or cloth). After three or four hours the room may be
opened and thoroughly aired. Since formaldehyde is most disagreeable to
breathe, one should not attempt to occupy the room until it is free from
the gas. This will require a day or more of thorough ventilation.

*Facts Relating to the Spread of Certain Diseases.*--The problem of
preventing disease in general often resolves itself into the problem of
preventing the spread of some particular disease. It is then of vital
importance to know the special method by which the germs of this disease
leave the body of the patient and are conveyed to the bodies of others.
Some of these methods are novel in the extreme, and are not at all in
accord with prevailing notions. Particularly is this true of that disease
known as

*Malaria, or Malarial Fever.*--This disease, so common in warm climates and
also prevalent to a large extent in the temperate zones, is due to animal
germs (protozoa), which attack and destroy the red corpuscles of the
blood. These germs, it is found, pass from malarial patients to others
through the agency of a variety of mosquitoes known as _Anopheles_. In
sucking the blood of a malarial patient, the mosquito first infects her
own body.(131) In the body of the mosquito the germs undergo an essential
stage of their development, after which they are injected beneath the skin
of whomsoever the mosquito feeds upon. For the spreading of malaria, then,
two conditions are necessary: first, there must be people who have the
disease; and second, there must be in the neighborhood the special variety
of mosquito that spreads the disease. If either condition be lacking, the
disease is not spread. The malarial mosquito (_Anopheles_) may be
distinguished from the harmless variety (_Culex_) by the position which it
assumes in resting, as shown in Fig. 170.

                                [Fig. 170]

 Fig. 170--*Mosquitoes* in resting position. (From Howard's _Mosquitoes_.)
  On left the malarial mosquito (_Anopheles_); on the right the harmless
                           mosquito (_Culex_).

*Remedies against Mosquitoes.*--The natural method of preventing the spread
of malaria is, of course, the destruction of mosquitoes. This is
accomplished by draining pools of water where they are likely to breed,
and by covering pools of water that cannot be drained with crude petroleum
or kerosene. The kerosene, by destroying the larvae, prevents the
development of the young. In communities where such measures have been
diligently carried out, the mosquito pest has been practically eliminated.
Other methods are also under investigation, such as the stocking of
shallow bodies of water with varieties of fish that feed upon the mosquito
larvae.

                                [Fig. 171]

     Fig. 171--*Stegomyia*, the yellow-fever mosquito (after Howard).

*Yellow Fever.*--This scourge of the tropics is, like malaria, caused by
animal germs. It is also propagated in the same manner as malaria, but by
a different variety of mosquito (_Stegomyia_, Fig. 171). The stamping out
of yellow fever in Havana, the Panama Canal Zone, and other places,
through the destruction of this variety of mosquito, affords ample proof
of the correctness of the "mosquito theory."

                                [Fig. 172]

  Fig. 172--*Consumption germs* from the spit of one having the disease.
 Highly magnified and stained. (Huber's _Consumption and Civilization_.)

*Consumption*, or tuberculosis of the lungs, spoken of as the "white
plague," was among the first diseases shown to be due to bacteria.
Consumption is now recognized as an infectious disease, though not so
readily communicated as some other diseases. Several methods are
recognized by which the germs are passed from the sick to the well, the
most important being as follows:

1. By personal contact of the sick with the well, especially in kissing.

2. By the sputum, or spit, which, if allowed to dry, is blown about as
dust and breathed into the lungs(132) (Fig. 172).

3. By means of objects (drinking cups, tableware, etc.) that have been
handled by consumptives.

4. By infectious material associated with houses or rooms in which
consumptives have lived.

These methods of spreading consumption suggest the necessity for the
greatest care, on the part of both the patient and those having him in
charge.(133) The material coughed up from the lungs and throat should be
collected on cloths or paper handkerchiefs and afterwards burned. The
house where a consumptive has lived should be disinfected, repapered or
calcimined, and thoroughly cleaned before it is again occupied. The inside
woodwork should also be repainted. The approaches to the house where the
patient may have expectorated should be disinfected and cleaned. Since the
germs are able to live in the soil, fresh lime or wood ashes should be
spread around the doorsteps and along the walks.

*Typhoid Fever*, one of our most dangerous diseases, is caused by germs
(bacteria) that enter the body through the food canal. They attack certain
glands in the walls of the small intestine, where they produce toxins that
pass with the germs to all parts of the body. Typhoid fever germs spread
from those having the disease to others, chiefly through the discharges
from the bowels and the kidneys. The germs contained in these, if not
destroyed by disinfectants, find their way into the soil, or into sewage,
where they may be picked up by water and widely distributed. Finding
suitable places, such as those containing decaying material, the germs may
rapidly increase in number, and from these sources find their way into the
bodies of new victims. They are likely, on account of manures, to get on
vegetables; on account of uncleanly methods of milking, to get into the
milk supply; and from sewerage outlets, to get into the oysters that grow
in bays and harbors near seaboard cities; but they are most frequently
introduced into the body through the drinking of impure water.

*Diphtheria*, also known as "membranous croup," is caused by germs that
attack the membranes of the throat. This most dangerous of children's
diseases is spread chiefly by discharges from the mouth and throat. These
should be collected on cloths and burned, or rendered harmless with
disinfectants. The disease may be spread also by objects brought into
contact with the mouth, such as cups, toys, pencils, etc. Children are
known to have diphtheria germs in the mouth for some time after recovering
from the disease, and should, for this reason, be kept away from other
children until pronounced safe by the physician.

The _antitoxin method_ of treating diphtheria has robbed this disease of
much of its terror, yet it not infrequently happens that the physician is
called too late to administer this remedy to the best advantage. Since
certain cases of diphtheria are likely to be mistaken for croup, the
parent frequently does not realize the serious condition of the child. A
croupy cough _that lasts through the day_, or a sore throat which shows
small white patches, are indications of diphtheria.

*Scarlet Fever, Measles, Chicken Pox, and Smallpox*, on account of the
eruptions of the skin which attend them, are classed as eruptive diseases.
As the eruptions heal, scales separate from the skin, and these are
supposed to be the chief means of spreading the germs. Attention must be
given to the destruction of these scales by burning or thoroughly
disinfecting all objects, such as clothing, bedding, etc., that may serve
as carriers of them. Those having eruptive diseases should be confined to
their rooms as long as the scales continue to separate from the body.

*Vaccination.*--The method of preventing smallpox known as vaccination,
which has been practiced since its discovery in 1796 by Jenner, has always
proved effective. In some instances the sore arm causes considerable
inconvenience, but this generally results from neglect to cleanse the arm
thoroughly before applying the virus, or from contact of the sore with the
clothing later. The virus should be applied by a physician and the wound
should be protected after the operation. If discomfort is felt when it
"takes," medical advice should be sought.

*Isolation*, or quarantining, is a most important method of combating
contagious diseases. By removing the sick from the well many outbreaks of
disease are quickly checked. Isolation of individual patients, and
sometimes of infected neighborhoods, is absolutely necessary; and while
this works a hardship to the few, it is frequently the only safeguard of
the many. The community, on the other hand, should make ample provision
for the care of the afflicted in the way of hospitals, or sanitaria, and
if it is deemed necessary to remove people from their homes, they should
not be subjected to unnecessary hardship.

Where one is sick from some contagious disease in the home and there is
liability of communicating it to the other members of the family, _room
isolation_ should be practiced. Infection cannot spread through solid
walls, and where the doors, and the cracks around the doors, are kept
completely closed and the usual precautions are observed by those
attending the patient, the other inmates of the house can be protected
from the disease.

*The Physician and His Work.*--In combating disease the services of the
physician are a prime necessity. The special knowledge which he has at his
command enables the conflict to be carried on according to scientific
requirements and vastly increases the chances for recovery. He should be
called early and his directions should be carefully followed. Everything,
however, must not be left to the physician, for recovery depends as much
upon proper nursing and feeding as upon the drugs that are administered.
Of great importance is _the saving of the energy of the patient_, and to
accomplish this visitors should, as a rule, be excluded from the sick
room.

*Precautions in Recovery from Disease.*--Many diseases, if severe, not only
leave the body in a weakened condition, but may, through the toxins which
the germs deposit, cause untold harm if the patient leaves his bed or
resumes his usual activities too soon. Especially is this true of typhoid
fever,(134) diphtheria, scarlet fever, and measles. Rheumatism and
affections of the heart, lungs, kidneys, and other bodily organs
frequently follow these diseases, as the result of slight exposure or
exertion before the body has sufficiently recovered from the effects of
the toxins. To guard against such results, certain physicians require
their patients to keep their beds for a week, or longer, after apparent
recovery from diseases like typhoid fever, diphtheria, and scarlet fever.

*Relation of Vocation to Disease.*--With a few exceptions, the pursuit of
one's vocation, or calling in life, does not supply either the quantity or
the kind of activity that is most in harmony with the plan of the body.
Especially is this true of work that requires most of the time to be spent
indoors, or which exercises but a small portion of the body. The effect of
such vocations, if not counteracted, is to weaken certain organs, thereby
disturbing the functional equilibrium of the body--a result that may be
brought about either by the overwork of particular organs or by lack of
exercise of others. Herein lies the explanation of the observed fact that
people of the same calling in life have similar diseases.

*A Special Problem for the Brain Worker.*--Farthest removed from those
forms of activity which harmonize with the plan of the body, and which
therefore are most hygienic, is that class of workers known as the
professional class, or the "brain workers." This class includes not only
the members of the learned professions--law, medicine, and the ministry--but
a vast army of business men, engineers, teachers, stenographers, office
clerks, etc., a class that is ever increasing as our civilization
advances. It is this class in particular that must give attention to those
conditions that indirectly, but profoundly, influence the bodily
well-being and must seek to obviate if possible such weaknesses as the
occupation induces.

*The Remedy* lies in two directions--that of spending sufficient time away
from one's work to allow the body to recover its normal condition, and
that of counteracting the effect of the work by special exercise or other
means. In many cases the first symptoms of weakness indicate a suitable
remedy. Thus exhaustion from overwork suggests rest and recreation. The
diverting of too much blood from other parts of the body to the brain
suggests some form of exercise which will equalize the circulation. If
feebleness of the digestive organs is being induced, some natural method
of increasing the blood supply to these organs is to be looked for. And
effects arising from lack of fresh air and sunlight are counteracted by
spending more time out of doors.

*Exercise as a Counteractive Agent.*--In counteracting tendencies to
disease and in the maintenance of the functional equilibrium of the body,
no agent has yet been discovered of greater importance than physical
exercise, when applied systematically and persistently. This may consist
of exercises that call into play all the muscles of the body, or which are
concentrated upon special parts. When general tonic effects are desired,
the exercise should be well distributed; but when counteractive or
remedial effects are wanted, it must be applied chiefly to the parts that
are weak or that have not been called into action by the regular work.
Unfortunately, health is sometimes confused with physical strength and
exercise is directed toward the stronger parts of the body with the effect
of making them still stronger. Not only is health not to be measured by
the pounds that one can lift or by some gymnastic feat that one can
perform, but the possession of great muscular power may, if the heart and
other vital organs be not proportionally strong, prove a menace to the
health. This being true, one having his health primarily in view will use
physical exercise, in part at least, as a means of building up organs that
are weak. Since the body, like a chain, can be no stronger than its
weakest part, this is clearly the logical method of fortifying it against
disease.

*Value of Work.*--Although there may exist in one's vocation certain
tendencies to disease, it must not be inferred that work in itself is
detrimental to health. Health demands activity, and those forms of
activity that provide a regular and systematic outlet for one's surplus
energy and compel the formation of correct habits of eating, sleeping, and
recreating best serve the purpose. Work furnishes activity of this kind
and serves also as a safeguard against the unhealthful and immoral habits
contracted so often from idleness. Even physical exercise which has for
its purpose the reenforcement of the body against disease may frequently
consist of useful work without diminishing its hygienic effects.

*The Mental Attitude.*--While a proper thoughtfulness and care for the body
is both desirable and necessary, it is also true that over-anxiety about,
or an unnatural attention to, the needs of the body reacts unfavorably
upon the nervous system. Observance of the laws of health, therefore,
should be natural and without special effort--a matter of habit. The
attention should never be turned with anxiety upon any organ or process,
but the mental attitude should at all times be that of _confidence in the
power of the body organization to do its work_. Fear and morbidity, which
are disturbing and paralyzing factors, should be supplanted by courage,
cheerfulness, and hopefulness.

Let it be borne in mind that hygienic living requires nothing more than
the application of the same intelligence and practical common sense to the
care of the body that the skillful mechanic applies to an efficient, but
delicate, machine. And, just as in the case of the machine, care of the
body keeps its efficiency at the maximum and lengthens the period that it
may be used. This end and aim of hygienic living is best attained by
cultivating that attitude of mind toward the body that avoids interference
in the vital processes and permits the natural appetites, sensations, and
desires to indicate very largely the body's needs.

*Attitude toward Habit-forming Drugs.*--Among the different substances
introduced into the body, either as foods or as medicines, are a number
which have the effect of developing an artificial appetite or craving
which leads to their continued use. Since the effect of such substances is
usually harmful and since they tend to engraft themselves upon communities
as social customs, they present a twofold relation to the general problem
of keeping well. The individual may be injured through the personal use
which he makes of them, or he may be injured through the effect which they
have upon relatives or friends or upon society at large. Since our social
environment is a factor in health little less important than our physical
environment, the conditions that make for their continuance should be more
generally understood.

*How Social Agencies perpetuate the Use of Habit-forming Drugs.*--When the
use of some habit-forming drug has risen to the importance of a general
custom, a number of conditions arise which tend to continue its use, even
though the fact may be quite generally known that the substance does harm.
In the first place, those who have formed the habit suffer inconvenience
and distress when deprived of its use. In the second place, a number of
people will have become interested in the production and sale of the
substance, and these will lose financially if it is discontinued. In the
third place, those of the rising generation will, from imitation or
persuasion, be constantly acquiring the habit before they are sufficiently
mature to decide what is best for them. Thus may the use of a substance
most harmful, such as the opium of the Chinese, be indefinitely
continued--a species of slavery from which the individual finds it hard to
escape.

Such is human nature and such are the forces and influences of human
society, that the freeing of a people from the bondage of some
habit-forming drug cannot be accomplished without strenuous and persistent
effort. Education, persuasion, the good example of abstainers, and legal
restrictions must be pitted against the forces that make for its
continuance. Such a struggle is now in progress in all civilized countries
relative to the use of alcoholic beverages.(135)

*How the Use of Alcohol became a Social Custom.*--The general use of
alcohol as a beverage may be accounted for by three facts. Alcohol is a
habit-forming drug; it has a stimulating effect which many have found
agreeable; and being a product of the fermentation of fruit juices and
other liquids containing sugar, it is easily obtained. Through the
operation of these causes the human family became habituated very early to
the use of alcohol. The "wine" of primitive man, however, did little harm
as compared with the alcoholic liquors of modern times. It was a weak
solution and on account of the crude methods of manufacture and storage
could only be produced in limited quantities. Perhaps the worst effect of
its early use was the establishment of a general belief in its power to
benefit, since this laid the foundation for excess in its use when the
developments of a later period made it possible.

During the eleventh century the method of making alcoholic drinks from
starch-producing substances, such as wheat, barley, and potatoes, became
quite generally known, and also the method of concentrating them by
distillation. This knowledge made possible the manufacture of alcoholic
drinks in large quantities and in considerable variety. Alcoholic
indulgence was now no longer the pastime of the few, but the privilege of
all. Its evil effects followed as a matter of course; and as these became
more and more apparent, there began the struggle to restrict the
consumption of alcohol which has continued with varying success to the
present time.

*Counts against Alcohol.*--The statements found in different parts of this
book relative to the effects of alcohol upon the body may here be
summarized as follows:--

1. Alcohol has an injurious effect upon the white corpuscles of the blood
and lessens the power of the body to resist attacks of disease (pages 35,
98).

2. Alcohol injures the heart and the blood vessels (page 56).

3. Alcohol causes diseases of the liver and kidneys and interferes with
the discharge of waste through these organs (pages 210, 212).

4. Alcohol interferes seriously with the regulation of the body
temperature (page 271).

5. Alcohol is one of the worst enemies to the nervous system (pages 326,
332-334. 336, 337).

6. Through its effect upon the nervous system and through its interference
with the production of bodily energy (page 195), alcohol greatly
diminishes the efficiency of the individual.

7. The taking of alcohol in amounts that apparently do not harm the
tissues is, nevertheless, liable to produce a habit which leads to its use
in amounts that are decidedly harmful.

*Alcohol and the Social Environment.*--Our social environment includes the
people with whom we are directly or indirectly associated. The presence in
any community of those who are immoral, inefficient, or defective, places
a burden upon those who are mentally and physically capable and renders
them liable to results which are the outgrowth of weakness or viciousness.
The fact that alcohol causes pauperism, crime, and general inefficiency,
thereby rendering the social environment less conducive to what is best in
life, is plainly evident. To realize how alcohol harms the individual
through its effects upon society in general, one has only to take into
account his dependence upon society for intellectual and moral stimuli,
for industrial and economic opportunity, for protection, and for general
conditions that make for health and happiness. As we strive to improve our
physical environment, so should we also strive for the betterment of
social conditions.

*Industrial Use of Alcohol.*--Interesting and instructive in this
connection is the fact that alcohol is, after all, a substance capable of
rendering great service to humanity. The injury which it causes is the
result of its misuse. Though unfit for introduction into the human body,
except in the most guarded manner, it is adapted to a great variety of
uses outside of the body. A combustible substance which is readily
convertible into a gas, it may be substituted for gasoline in the cooking
of food, lighting of dwellings, and the running of machinery. As a solvent
for gums, resins, essential oils, etc., it is used in the preparation of
varnishes, extracts, perfumes, medicines, and numerous other substances of
everyday use. Through its chemical interactions, it is used in the
manufacture of ether, chloroform, explosives, collodion, celluloid,
dyestuffs, and artificial silk. In fact, alcohol is stated by one
authority to be, next to water, the most valuable liquid known.(136)

Opposed to an extensive use of alcohol for industrial purposes is the
guard which the government must keep over its manufacture on account of
its use in beverages. Though alcohol may be profitably manufactured and
sold at thirty cents per gallon, the government revenue stamp of $2.08 per
gallon practically prohibits its use for many purposes. A step toward a
wider application to industrial purposes has been taken by the law
permitting the sale of so-called "denatured"(137) alcohol without the tax
for revenue. This law has proved beneficial to some extent, though the
practical solution of the problem is still remote.

*Nicotine and Social Custom.*--The influences which brought about a general
use of tobacco are similar to, though not identical with, those that
engrafted alcohol upon society. The drug nicotine is a habit-forming
substance and the plant producing it is easily cultivated.(138) Its
immediate effect upon the user is generally agreeable, acting as a
stimulant to some, but having a soothing effect upon the nerves of others.
Moreover, a strong deterring factor in its use is lacking, since its
harmful effects are not readily discernible and by many are avoided
through moderation in its use.

As with alcohol, tobacco is conveniently used to promote sociability among
men, a fact which has much to do with its very general use. If it could be
limited to social purposes, it would likely do little harm, but the habit,
once started, is continued without reference to sociability--a matter of
selfish indulgence. In fact, one effect of tobacco is to cause the user to
become less sensitive to the rights of others, this being evidenced by
smokers who do not hesitate to make rooms and public halls almost
unbearable to those unaccustomed to tobacco.

*Counts against Nicotine.*--The physiological objections to the use of
tobacco, as already stated (pages 56, 92, 326, 333, 336), are the
following:--

1. The use of tobacco before one reaches maturity stunts the growth. The
boy who uses it cannot develop into so strong and capable a man as he
would by leaving it alone.

2. Tobacco injures the heart.

3. Tobacco injures the air passages, especially when inhalation is
practiced.

4. Tobacco injures the nervous system and by this means interferes in a
general way with the bodily processes. For the same reason it interferes
with mental and moral development, the cigarette being a chief cause of
criminal tendencies in boys.

5. In some cases tobacco injures the vision.

6. The tobacco habit is expensive and is productive of no good results.

*Tobacco and the Rising Generation.*--The problem of limiting the use of
tobacco to the point where it would do slight harm, in comparison to what
it now does, would be solved if those under twenty years of age could be
kept from using it. But few would then acquire the habit, and those who
did would not be so seriously injured. In our own country it lies within
the province of the home and the school to bring about this result. The
fact that parents use tobacco is no reason why the boys should also
indulge. The decided difference in effects upon the young and upon the
mature makes this point very clear. Laws protecting boys from the evil
effects of tobacco, not only cigarettes, but other forms as well, are both
just and necessary.

*Social Custom and the Caffeine Habit.*--By suitable processes a white,
crystalline solid, easily soluble in water, can be separated from the
leaves of tea, and from the berry of the coffee plant. This is the drug
caffeine, the substance which gives to tea and coffee their stimulating
properties, but not their agreeable flavors. Less injurious, on the whole,
than either alcohol or tobacco, caffeine has come into general use in much
the same way as these substances. In a sense, however, caffeine is more
deceptive than either alcohol or nicotine, because the usual mode of
preparing tea and coffee gives them the appearance of real foods. The
housewife who would feel condemned in purchasing caffeine put up as a drug
somehow feels justified when she extracts it from plant products in the
regular preparation of the meal.

*Counts against Caffeine.*--People of vigorous constitutions and of active
outdoor habits are injured but slightly, if at all, by either tea or
coffee when these are used in moderation. As already stated (pages 56,
167, 326, 329), they do harm when used to excess and, in special cases, in
very small amounts, in one of the following ways:--

1. By stimulating the nervous system, thereby causing nervousness and
insomnia and interfering with vital organs.

2. By introducing a waste which forms uric acid into the body, thereby
throwing an extra burden upon the organs of elimination.

In this connection it may also be stated that there appears to be little,
if any, real advantage to the healthy body from the use of either tea or
coffee, beyond that of temporary stimulation and the gratification of an
appetite artificially acquired. Hence the large sums of money expended for
these substances in this country yield no adequate returns.

*Caffeine Restrictions Necessary.*--Though with many the cup of tea or
coffee at breakfast does no harm, but gives an added pleasure to the meal,
there is no question but that the use of caffeine beverages should be
greatly curtailed. Children should not be permitted to drink either tea or
coffee. Brain workers and indoor dwellers generally should use these
substances very sparingly, and people having a tendency to indigestion,
nervousness, constipation, rheumatism, or diseases of the heart, kidneys,
or liver frequently find it best to omit them altogether.

*Caffeine and "Soft" Drinks.*--Recently the practice has sprung up of using
caffeine as a constituent of certain drinks supplied at the soda-water
fountains. Such drinks usually purport to be made from the kola nut, which
contains caffeine, or to consist of extracts from the plants which yield
cocoa and chocolate, when in reality they consist of artificial mixtures
to which caffeine has been added. Those using these beverages are
stimulated as they would be by tea or coffee and soon acquire the habit
which makes them regular customers. Chief harm comes to the children who
frequent the soda fountains and to those who, on account of constitutional
tendencies, should avoid caffeine in all of its forms. It is generally
understood that the so-called "soft" drinks are harmless. If this
reputation is to be maintained, those containing caffeine must be
excluded.

*Danger from Certain Medicinal Agents.*--Among the most valuable drugs used
by the physician in the treatment of disease are several, such as
morphine, chloral, and cocaine, which possess the habit-forming
characteristic. Sad indeed are the cases in which some pernicious drug
habit has been formed through the reckless administration of such
medicines. Even the taking of such a drug as quinine as a "tonic" tends to
develop a dependence upon stimulation which is equivalent to a habit. In
the same list come also the drugs that are taken to relieve a frequently
recurring indisposition, such as headache. The so-called headache powders
are most harmful in their effects upon the nervous system and should be
carefully avoided.(139)

*Stimulants in Health Unnecessary.*--Stimulants have been aptly styled "the
whips of the nervous system." The healthy nervous system, however, like
the well-disposed and well-fed horse, needs no whip, but is irritated and
harmed through its use. Even in periods of weakness and depression,
stimulants are usually not called for, but a more perfect provision for
hygienic needs. Rest, relaxation, sleep, proper food, and avoidance of
irritation, not stimulants, are the great restorers of the nervous system.
A surplus of nervous energy gained through natural means is more conducive
to health and effective work than any result that can possibly be secured
through drugs. Then withal comes the satisfaction of knowing that one has
the expression of his real self in the way in which he feels and in what
he accomplishes--not a "whipped-up" condition that must be paid for by
weakness or suffering later on.

*Summary.*--To solve the problem of keeping well, one must live the life
which is in closest harmony with the plan of the body. Such a life,
because of differences in physical organization, as well as differences in
environment and occupation, cannot be the same for all. All, however, may
observe the conditions under which the body can be used without injuring
it and the special hygienic laws relative to the care of different organs.
Causes of disease, whether they be in one's environment, vocation, in his
use of foods or drugs, or in his mode of recreation, must either be
avoided or counteracted.

While the problem is beset with such difficulties as lack of sufficient
knowledge, inherited weakness, and time and opportunity for doing what is
known to be best for the body, yet study and work that have for their aim
the preservation or improvement of the health are always worth while.
_Health is its own reward._ The expression of the poet,

    "Each morn to feel a fresh delight to wake to life,
  To rise with bounding pulse to meet whate'er of work, of care, of
strife,
    day brings to me,"

suggests the _joy_ of being well. But the ultimate realization of one's
aims and ambitions in life and the actual prolongation of one's period of
usefulness are _higher and more enduring rewards_.

*Exercises.*--1. Summarize the different laws of hygiene. Upon what one
fundamental law are these based?

2. State the important differences between a condition of health and one
of disease.

3. In what general ways may disease originate in the body?

4. Describe a model sanitary home. With what special hygienic problems has
the housekeeper to deal?

5. Describe a method of collecting a wholesome supply of cistern water.
State possible objections to well and spring water.

6. What means may be employed in preventing the spread of contagious
diseases?

7. By what means are malaria, typhoid fever, diphtheria, and tuberculosis
spread from one individual to another?

8. Why are extra precautions necessary in the recovery from certain
diseases, as typhoid fever, diphtheria, and scarlet fever?

9. How may one's vocation become a cause of disease? What conditions in
the life of a student may, if uncounteracted, lead to poor health?

10. Of what special value are the parks and pleasure grounds in a city to
the health of its inhabitants?

11. Discuss the hygienic value of work.

12. What conditions lead to the continuance of habit-forming substances
after their use has become general?

13. How is it possible for one not using alcohol to be injured by this
substance?

14. Discuss the effect of alcoholic abuse upon social environment.

15. Summarize the rewards of hygienic living.

SUMMARY OF PART II

For the maintenance of life the needs of the cells must be supplied and
_the body as a whole must be brought into proper relations with its
surroundings_. The last-named condition requires that the body be moved
from place to place; that its parts be controlled and cooerdinated; and
that it be adjusted in its various activities to external physical
conditions. To accomplish these results there are employed:

1. The skeleton, or bony framework, which preserves the form of the body
and supplies a number of mechanical devices, or machines, for causing a
variety of special movements.

2. The muscular system, which supplies the energy necessary for executing
the movements of the body.

3. The nervous system, which (_a_) controls and cooerdinates the various
activities and (_b_) provides for the _intelligent_ adjustment of the body
to its environment. (Review Summary of Part I, page 215, and consult Fig.
92, page 214.)

APPENDIX

*Equipment.*--Nearly all of the apparatus and materials called for in this
book may be found in the physical, chemical, and biological laboratories
of the average high school. There should be ready, however, for frequent
and convenient use, the following: One or more compound microscopes with
two-thirds and one-fifth inch objectives; a set of prepared and mounted
slides of the various tissues of the body; a set of dissecting
instruments, including bone forceps; a mounted human skeleton and a
manikin or a set of physiological charts; a set of simple chemical
apparatus including bottles, flasks, test tubes, and evaporating dishes;
and a Bunsen burner or some other means of supplying heat.

The few chemicals required may be obtained from a drug store or from the
chemical laboratory. Access to a work bench having a set of carpenter's
tools will enable one to prepare many simple pieces of apparatus as they
are needed.

*Physiological Charts* are easily prepared by teachers or pupils by
carefully enlarging the more important illustrations found in text-books
or by working out original sketches and diagrams. These, if drawn on heavy
Manila paper, may be hung on the wall as needed and preserved
indefinitely. By the use of colors, necessary contrasts are drawn and
emphasis placed on parts as desired. The author has for a number of years
used such home-made charts in his teaching and has found them quite
satisfactory. His plan has been to draw on heavy Manila paper, cut in
sizes of two by three feet, the general outline in pencil and then to mark
over this with the desired colors. There is of course an opportunity for
producing results that are artistic as well as practical, and if one has
time and artistic skill, better results can be obtained. Many of the cuts
in this book are excellently suited to enlargement and, if properly
executed, will provide a good set for general class purposes.

*Models.*--The use of prepared models of the different bodily organs is
strongly urged. These may be so used in elementary courses as to obviate
much of the dissections upon lower animals. Although the actual tissues
cannot be so well portrayed, the general form and construction of organs
are much better shown. Models well adapted to class or laboratory work are
easily obtained through supply houses. Illustrations of several of these
are shown in connection with the "Practical Work."

INDEX

Abdomen; dissection of, 169.

Abdominal cavity, 7, 138, 152.

Absorption, 173-186.
  Defined, 18, 173.

Accommodation, 379.
  To illustrate, 391.

Acid reactions, 171.

Acquired reflexes, 314.

Adipose tissue, 5, 178.

Afferent neurons, 296.

Air, 76.
  Changes it undergoes in lungs, 101.
  Complemental, 89, 103.
  Reserve, 89, 103.
  Residual, 89, 103.
  Tidal, 88, 103.

Air passages, 80.

Albuminoids, 119.
  Purpose served by, 121.

Alcohol,
  A cause of crime, 333.
  Effects on circulation, 55, 56.
  Effects on digestion, 167.
  Effects on energy supply, 195.
  Effects on respiratory organs, 98.
  Effects on social environment, 413.
  Effect on temperature regulation, 271.
  Effects on waste elimination, 212.
  General considerations, 412-415.

Alimentary canal, coats of, 138.

Alimentary muscles, work of, 159.

Alkaline reactions, 171.

Alveoli, 82.

Amylopsin, 155, 156.

Anatomy, defined, 1.

Animal heat, 192.

Anopheles, 401.

Antiseptic ointment, 275.

Antitoxin, 405.

Appetite, natural, 163.

Aqueous humor, 377.

Arachnoid, 299.

Arteries, 47.
  Bronchial, 84.
  Functions of, 51.
  Pulmonary, 84.
  Renal, 202.
  To illustrate elasticity of, 62.
  Why elastic, 48.

Articulations, 230-232.
  Kinds of, 230.

Assimilation, 18, 182.

Astigmatism, 384.

Atlas, 223.

Atoms, defined, 105.

Attraction sphere, 15.

Auditory canal, 358.

Auricles, 42.

Axis, 223.

Axis cylinder, 284.

Axon, 283.
  Form and length of, 284.
  Function of, 306.
  Structure of, 284.

Bacteria, 394.

Ball-and-socket joint, 231.

Basement membrane, 197.

Basilar membrane, 363.

Bathing, 272, 274.

Biceps muscle, action of, 263.

Bicuspids, 143.

Bile, 154, 155.

Binocular vision, 381.

Blind spot, 377.
  To prove presence of, 390.

Blood, 24-39.
  Changes in, 34.
  Checking flow from wounds, 58.
  Coagulation of, 31.
  Experiments with, 37-39.
  Flow of, how regulated, 50.
  Functions of, 33.
  Hygiene of, 34-36.
  Physical properties of, 24.
  Quantity of, 33.
  Supply to lungs, 82.
  Velocity of, 54.
  Where found, 24.

Blood platelets, 25.

Blood pressure, 52, 70.

Blood pressure and velocity, 52.

Blood vessels, to strengthen, 57.

Body, organization of, 19.

Bone groups, 223-229.

Bones, 216-242.
  Adaptation of, 228.
  Composition, 217.
  Gross structure of, 218.
  Minute structure of, 219.
  Observation on gross structure, 241.
  Properties of, 217.
  Table of, 229.
  To show composition of, 241.
  To show minute structure of, 242.

Bowels, rules for care of, 166.

Brachial plexus, 302.

Brain, 280, 288-291.
  Disturbed circulation, 327.
  Protection of, 299.

Brain workers, 408.

Breathing, _see_ Respiration.
  Causes of shallow, 92.
  Illustrated, 87.
  To prevent shallow, 92.

Breathing exercises, 93.

Bronchus, 80.

Bulb, 291.

Caecum, 151, 158.

Calcium carbonate, 122.

Calcium phosphate, 122.

Calorie, defined, 126.

Cane sugar, 120.

Canines, 143.

Capillaries, 50, 64, 249.
  Blood pressure at, 70.
  Functions of, 51.
  Work of, 174.

Carbohydrates, 119, 125.
  Purpose served by, 121.
  Storage of, 177.
  Tests for, 135.

Carbon, 134.

Carbon dioxide,
  Final disposition of, 111.
  Preparation, 115.
  Pressure, 110.
  Properties, 110, 115.

Cardiac cycle, 46.

Cardiac orifice, 147.

Carpals, 227.

Carpus, 228.

Cell body, 283.
  Functions of, 305.

Cell-division, 16.

Cell nucleus, 14.

Cell reproduction, 16.

Cell structure, 14.

Cell surroundings, 17.

Cell wall, 15.

Cells, 13-23.
  Bone, how nourished, 220.
  Ciliated epithelial, 81.
  Food supply to, 180.
  General work of, 17.
  Importance of, 15.
  Passage of materials to, 183.
  Relation to nutrient fluid, 20.
  Specialized, 197.
  Special work of, 18.
  Striated muscle, 244.

Cerebellum, 290.
  Functions of, 317.

Cerebral functions, localization of, 318.

Cerebral hemispheres, 289.

Cerebral peduncles, 290.

Cerebrum, 288.
  Functions of, 317.

Chlorine, 135.

Cholesterine, 155.

Chordae tendineae, 43.

Choroid coat, 375.

Chyme, 150.

Cigarettes, 333.

Cilia, 81.
  To observe, 101.

Ciliary muscle, 375.

Ciliary processes, 375.

Circulation of blood, 40-64.
  Causes of, 54.
  Discovery of, by Harvey, 40.
  Divisions of, 51, 52.
  Effects of exercise upon, 63.
  Effects of gravity upon, 64.
  In a frog's foot, 64.
  Organs of, 40-54.
  Routes to, 174.

Coagulation,
  Causes of, 31.
  Purpose of, 32.
  Time required for, 33.

Cochlea, 362.

Coffee,
  Effects on complexion, 274.
  Effects on digestion, 167.
  Effects on heart, 56.

Colds, 193.
  Serious nature of, 94.
  To cure, 94.

Colon, parts of, 158.

Complexion, care of, 273.

Compound, defined, 104.

Conduction pathways, 286.

Conductivity, 304.

Condyloid joint, 232.

Conjunctiva, 373.

Consumption, _see_ Tuberculosis.

Control of arteries, 319.

Convolutions, 289.

Cooerdination, defined, 279.

Cornea, 375.

Corpora quadrigemina, 290.

Corpora striata, 289.

Corpus callosum, 289, 293.

Cortex, 288, 294.

Coughing, 81.

Cranial cavity, 7, 225.

Cranial nerves, 296.

Crura cerebri, 290.

Crystalline lens, 380.

Culex, 402.

Cytoplasm, 15.

Defects in focusing, 383.

Deformities of skeleton, 233-236.
  Correction of, 236.
  Prevention of, 235.

Deglutition, 145.
  Steps in, 146.

Dendrites, 283, 306.

Dentine, 143.

Dermis, 264.

Dextrose, 30, 120, 150.

Diaphragm, 88.
  To illustrate action of, 102.

Diastole, 46.

Diaxonic neuron, 283.

Diet, one-sided, 124.

Diffusion, 371.

Digestion, 130-172.
  Hygiene of, 160.
  Nature of, 130.
  Not a simple process, 131.
  Of fat, 156.
  Purpose of, 177.
  Stomach, 148.

Digestive fluids, 132.

Digestive organs, 160.
  Table of, 138.

Digestive processes, 130, 141.
  Illustrated, 137.

Diphtheria, 94, 405.
  Care after, 211.

Disaccharides, 120.

Disease, 392-412.
  Causes of, 393.
  Eruptive, 405.
  Precautions in recovery from, 407.
  Prevention of, 393.

Dislocations, 239.

Dorsal-root ganglia, 295.

Drill, "setting up," 237.

Drugs, effects of, 35, 55, 129, 332.

Duodenum, 151.

Dura, 299.

Ear, 358.
  Hygiene of, 365.
  To demonstrate, 369.

Ear drum, 359.

Efferent neurons, 296.

Element, defined, 104.

Elevators of the ribs, 87.

Emetics, 151.

Emotional states, effects of, 330.

End bulbs, 342.

Endocardium, 42.

Endolymph, 361.

End-plate, 244.

End-to-end connections, 286.

Energy, 107, 186-196.
  Bodily control of, 192.
  From sun to cells, 191.
  How plants store sun's, 189.
  Increasing one's bodily, 194.
  In food and oxygen, 190.
  Kinds of, 186.
  Methods of storing, 187, 188.
  Transformation of, in muscle, 248, 249.

Enzymes, 132, 155.
  Of the tissues, 184.

Epidermis, 264, 266.

Epiglottis, 80, 354.

Epithelium, 139.

Eruptive diseases, 405.

Esophagus, 146.

Eustachian tube, 359.

Excessive reading, 331.

Excitant impulse, 305.

Excretion, 197-213.
  Defined, 18.
  Necessity for, 201.

Exercise, 256, 257, 328, 409.
  General rules for, 259.
  Results of, 257.

Exhaustion, nervous, 211.
  Results of, 195.

External ear, 358.

External stimuli, action of, 307.

Eye, 370-391.

Eyeball, 373.
  Chambers of, 377.
  Focusing power of, 378.
  Movements of, 381.

Eyelids, 373.

Eyes,
  Care of, 386.
  Removal of foreign bodies from, 387,
  Strong chemicals in, 388.

Eye strain, 211.
  And disease, 385.

Fat, 30, 149, 162.
  Digestion of, 156.
  Emulsification of, 157.
  Purpose served by, 121.
  Route taken by, 175.
  Tests for, 137.
  Where stored, 178.

Fatty acid, 156.

Fenestra ovalis, 361.

Fenestra rotunda, 363.

Ferments, _see_ Enzymes.

Fibrin, 31.

Fibrin ferment, 32.

Fibrinogen, 30, 31.

Fissures, 289.

Food, 117-137.
  Advantages of coarse, 167.
  Classes of, 118, 119.
  Composition of, 124.
  Dangers from impure, 165.
  Defined, 117.
  Elements supplied by, 134.
  Excess of proteid, 208.
  Frequency of taking, 165.
  Materials, table of, 126, 126.
  Nitrogenous, 119.
  Order of taking, 161.
  Preparation of, 164.
  Purity of, 128.
  Quantity of, 164.
  Simple, 118.
  Variety, 128.
  With reference to digestive changes, 132.

Foot lever, diagram of, 253.

Foot-pound, 196.

Foot-wear, hygienic, 238.

Fractures, treatment of, 239.

Fumigation, 400.

Furniture, school, 236.

Gall bladder, 154.

Ganglia, 281.
  Dorsal-root, 295.
  Sympathetic, 298.

Gastric glands, 147.

Gastric juice, to illustrate action of, 172.

Gelatine, 218.

Germ diseases, avoidance of, 394.

Germs, 29, 394, 395.
  How spread, 395.

Glands, 197-213.
  Digestive, 140.
  Ductless, 208.
  Excretory, work of, 201.
  Gastric, 147.
  Kinds of, 197, 198.
  Lymphatic, 68, 208.
  Perspiratory, 206.
  Salivary, 144.
  Structure of, 197.
  Thymus, 208.
  Thyroid, 208.

Gliding joint, 232.

Glottis, 355.

Glycogen, 120, 177.

Grape sugar, tests for, 120, 136.

Gross anatomy, defined, 1.

Gullet, 146.

Gustatory pore, 345.

Gustatory stimulus, 345.

Habits, 315, 334.

Hair, 267.
  Care of, 276.

Hair cells, 363.

Hair follicle, 267.

Haversian canals, 219.

Hearing, defective, 366.

Heart, 41.
  Care of, 55.
  Connection with arteries and veins, 45.
  Difference in parts of, 44.
  How it does its work, 45.
  Observations on, 60, 61, 62.
  Sounds of the, 47.
  Valves of, 43.

Heart muscle, structure of, 247.

Heat and cold, effects of, 330.

Hemoglobin, 26.

Hepatic artery, 154.

Hepatic veins, 154.

Hindbrain, 290.

Hinge joint, 231.

Histology, defined, 1.

Humerus, 227.

Hyaloid membrane, 378.

Hydrochloric acid, 149, 150.

Hydrogen, 134.

Hygiene,
  Defined, 2.
  General aim of, 2.
  General laws of, 2, 392.
  Of digestion, 160.
  Of skeleton, 238.
  Relation of physiology and anatomy to, 3.

Hygienic housekeeping, 399.

Hypoglossal nerves, 298.

Ileo-caecal valve, 151.

Ileum, 151.

Images,
  Diagram illustrating, 372.
  Formation of, 371.

Incisors, 143.

Incus, 359.

Infectious diseases, 394.

Infundibula, 80, 84.

Inhibitory impulse, 305.

Insomnia, 329.

Inspiratory force, 70.

Intercellular material, production of, 13, 18.

Internal ear, 360.

Intestinal juice, 152, 157.

Iris, 375.

Iron, 135.

Irritability, 6, 243, 304.

Isolation, 406.

Jejunum, 151.

Joints, 230-232, 242.

Kidneys, 201.
  Blood supply to, 204.
  Cortex of, 204.
  Inflammation of, 211.
  Pelvis of, 202.
  Structure, 202.
  Symptoms of diseased, 211.
  Work of, 205.

Knee jerk reflex, 322.

Lachrymal glands, 383.

Lacteals, work of, 174.

Lacunae, 220.

Laminae, 220.

Large intestine, 157.
  Division of, 158.
  Work of, 159.

Larynx, 80, 353-357.
  To show plan of, 368.

Lever, 251.
  Application to the body, 251.
  Classes of, in body, 251.
  Producing motion, diagram of, 252.
  To show action of, 252.

Leucocytes, 27.

Levulose, 120, 150.

Life, maintenance of, 20.

Light, 370, 371.
  Simple properties, illustrated, 389.

Light waves, diagram illustrating passage of, 370.

Lime water, to prepare, 101.

Liver, 52, 152-155, 178.
  Protection of, 210.
  Work of, 206.

Lockjaw, 276.

Longsightedness, 384.

Lung capacity, diagram illustrating, 89.

Lung diseases, out-door cure for, 98.

Lungs, 77-103.
  Capacity of, 88.
  Changes air undergoes in, 101.
  Excretory work of, 207.
  Interchange of gases in, 88.
  Observations of, 100.
  Supply of blood to, 82.
  To estimate capacity of, 103.
  Weakest portions of, 92.

Lymph, 65-75.
  Composition, 66.
  Movements at the cells, 71.
  Origin of, 65.
  Physical properties, 66.
  Where it enters the blood, 70.

Lymph movements, causes of, 69.

Lymph spaces, 66.

Lymph vessels, 66.
  Variable pressure on the walls of, 70.

Magnesium, 135.

Malarial fever, 401.

Malleus, 359.

Malpighian capsules, 203.

Maltose, 120.

Massage, 259.

Mastication,
  Muscles of, 144.
  Slow, 145.
  Thorough, 160.
  To show importance of, 171.

Matrix, 267.

Measles, 94.
  Care after, 211.

Median fissures, 289.

Medulla oblongata, 291.

Medullary sheath, 284.

Membrana tympani, 358.

Membrane,
  Active, 173.
  Basement, 197.
  Basilar, 363.

Membranous capsule, 377.

Membranous labyrinth, 361.

Mesentery, 152.

Metacarpals, 227.

Midbrain, 289.

Middle ear, 359.
  Purposes of, 360.

Milk sugar, 120.

Mineral salts, 30.
  Uses, 121.

Moderate drinkers, 333.

Molars, 143.

Molecules, defined, 105.

Mon-axonic neuron, diagram of, 282.

Mono-saccharides, 120.

Mosquitoes, 401-403.
  Remedies against, 402.

Mouth, 141.

Movable joints,
  Kinds of, 231.
  Structure of, 230.

Mucous membrane, 80, 264.

Mucus, 139.

Muscle organ, 245.

Muscles, 243-263.
  Alimentary, 189.
  Important, 254-256.
  Intercostal, 87.
  Of mastication, 144.
  Properties of, 243.

Muscular force, plan of using, 249.

Muscular sensations, 344.

Muscular stimulus, 248.

Muscular stimulus and contraction, to illustrate, 261.

Muscular tissue, kinds of, 243, 244.

Nails, 267.
  Care of, 276.

Nasal duct, 383.

Neck exercise, 328.

Nerve cells, 281, 282.

Nerve fibers, 282, 293, 294.

Nerve path, diagram of, 286.

Nerve pathways, to demonstrate, 322.

Nerves, 281.

Nerve skeleton, 280.
  Diagram of, 281.

Nerve stimuli, 306.

Nerve trunks, 281.

Nervous activity, wasteful forms of, 325.

Nervous control of,
  Body temperature, 320.
  Circulation of blood, 318.
  Respiration, 320.

Nervous energy, economizing of, 315.

Nervous impulse, 248, 305.

Nervousness, 326.

Nervous system, 279-337.
  Diagram of, 287.
  Dissection of, 302.
  Divisions of, 287.
  Hygiene of, 324-337.
  Nature of, 287.
  Physiology of, 304-323.
  Work of, 280.

Neural arch, 224.

Neurilemma, 284.

Neurons, 281, 282.
  Arrangement of, 284, 293.
  Diagram, illustrating, 285.
  Properties of, 304.

Nicotine,
  Effects of, 333.
  Relation of age to effects, 333.

Nitrogen, 134.

Non-striated cells, to show, 261.

Non-striated muscles,
  Purpose of, 246.
  Structure of, 246.
  Work of, 247.

Normal temperature, 269.

Nosebleed, 58.

Nucleoplasm, 14.

Nutrients (_see_ Foods),
  Composition of, 135.
  Relative quantity needed, 123.
  Routes taken by, 175.
  Tests for, 136.

Nutriment, storage of, 177-180.

Olfactory stimulus, 347.

Opsonins, 34.

Optic thalami, 289.

Orbit, 373.

Organ, defined, 7.

Organism, defined, 19.

Organization, defined, 10.

Osmosis, 72.
  At the cells, 72.
  To illustrate, 75.

Ossein, 218.

Overstudy, 211.

Oxidation, defined, 106.

Oxygen, 104-117.
  Combined, 105, 113.
  Free, 105, 113.
  How it unites, 105.
  Main uses of, 108.
  Movement a necessity, 106, 108, 115.
  Movement in body, 106, 108, 115.
  Nature of, 104.
  Passage of, from cells, 110.
  Passage of, through blood, 109.
  Passage of, toward cells, 109.
  Preparation of, 113.
  Pressure, 109.
  Properties of, 113.
  Purpose of, in the body, 106.

Oxyhemoglobin, 27.

Pacinian corpuscles, 342, 343.
  To demonstrate, 348.

Pancreas, 155.

Pancreatic juice, 155.

Papillae, 266.

Patent medicines, 166.

Pelvic girdle, 226.

Pepsin, 149.

Peptones, 149, 176.

Pericardium, 41.

Perilymph, 361.

Perimysium, 245.

Periosteum, 218.

Peritoneum, 180.

Perspiration, 207.

Pharynx, 145.
  Openings into, 145, 146.

Phosphorus, 135.

Phrenic nerve, 302.

Physiological salt solution, 38.

Physiology, defined, 2.

Pia, 299.

Pigment granules, 266.

Pinna, 358.

Pitch, detection of, 365.

Pivot joint, 232.

Plasma, 25, 29.

Pleura, 84.

Plexus, 281.

Pneumonia, 94.

Pons, 290.

Pons Varolii, 290.

Portal vein, 154.

Primitive sheath, 284.

Proteids, 161.
  Circulating, 179.
  Kinds of, 118.
  Purposes of, 119.
  Supplied by, 125.
  Tests for, 135, 136.
  Tissue, 179.

Proteoses, 149, 176.

Protoplasm, 14.

Protozoa, 394.

Ptyalin, 145.

Public sanitation, 396.

Pulp cavity, 143.

Pupil, 375.

Pure food law, 128.

Pus, 28, 29.

Pyloric orifice, 147.

Pyramids, 202.

Quarantine, 406.

Radius, 227.

Reaction time, to determine, 323.

Reading glasses, 386.

Receptacle of the chyle, 68, 170.

Rectum, 158.

Red corpuscles, 25.
  Disappearance of, 27.
  Function of, 26.
  Origin of, 27.
  To examine, 38.
  To prepare models of, 39.

Red marrow, 219.

Reenforcement of sound, 352, 356, 368.

Reflection, kinds of, 371.

Reflex action, 308.
  Diagram illustrating, 310.
  In circulation of blood, 311.
  In digestion, 310.
  Purposes of, 311.

Reflex action and mind, 308.

Reflex action pathway, 309.

Refraction, 371.

Rennin, 149.

Respiration, 76-103.
  Artificial, 97.
  Internal, 89.
  Lung, 76.

Retina, 376.

Retinitis, 333.

Rheumatism,
  Effects on the heart, 56.
  Sequel to other diseases, 407.

Right lymphatic duct, 67.

Rods and cones, 377.

Rods of Corti, 364.

Sacrum, 224.

Saliva, 145.
  Composition of, 145.
  Uses of, 145.
  To show action on starch, 171.

Salivary glands, 144.
  Kinds of, 144.
  Reflex action of, 323.

Sanitation, defined, 2.

Sarcolemma, 244.

Sarcoplasm, 244.

Scala media, 363.

Scala tympani, 363.

Scala vestibula, 363.

Scarlet fever, care after, 211.

Sciatic nerve, 302.

Sclerotic coat, 374.

Secondary reflex action, 314.

Secretions, 197.
  Kinds of, 200.

Secretory process, nature of, 199.

Seeing, problem of, 372.

Self-control, 326, 334.
  Habit of, 325.

Semicircular canals, 362.

Semilunar valves, 44.

Sensations, 338-349.
  Classes of, 339.
  Production of, 338, 349.
  Purposes of, 340.
  Special, 340.

Sensations (_continued_).
  Steps in production of, 341.

Sensation stimuli, 339.

Sense organs, simple forms of, 341, 342

Serous coat, 140, 148.

Serous membrane, 264.

Serum albumin, 30.

Serum globulin, 30.

Shortsightedness, 384.

Shoulder girdle, 226.

Sight, organs of, 373.

Sigmoid flexure, 158.

Simple life, 410.

Skeleton, 216-243.
  How deformed, 234.
  Hygiene of, 233.
  Plan of, 221.
  Purpose of, 221.

Skin, 264-277.
  As regulator of temperature, 270.
  Experiments on, 349.
  Functions of, 267, 268.
  Observations on skin, 278.

Skin wounds, treatment of, 275.

Skull, 225.

Sleep, 329.

Small intestine, 151.
  Mucous membrane of, 151.
  Muscular coat of, 152.
  As organ of absorption, 173.
  Parts of, 151.
  Serous coat of, 152.
  Work of, 157.

Smell,
  Sensation of, 346.
  Value of, 347.

Sneezing, 81.

Sodium, 135.

Sodium carbonate, 155.

Sodium chloride, 122.

Soft palate, 141.

Solution, 131.
  Kinds of, 73.

Solution theory, 156.

Solvents, 131.

Sound,
  To illustrate origin of, 367.
  To show transmission of, 367.

Sound waves,
  As stimuli, 331.
  Nature of, 350.
  Reenforcement of, 352.
  To show effects of, 368.
  Value of, 353.

Speech, production of, 357.

Spinal column, 223-225.
  Hygiene of, 233.

Spinal cord, 280.
  Protection of, 299.

Spinal nerves, 295.
  Double nature of, 295.

Spitting, 403.

Spleen, 208.

Sprains, 239, 240.

Stapes, 359.

Starch, 162.
  Action of, on saliva, 171.
  Animal, 120.
  Tests for, 136.

Steapsin, 155, 156.

Stegomyia, 403.

Sternum, 225.

Stomach, 147.
  Mucous membrane of, 147.
  Muscular action of, 150.
  Muscular coat, 148.

Serous coat, 148.

Storage of nutriment, 177-179.

"Strenuous life," 410.

Striated fibers, to show, 261.

Striated muscles, to show, 261.

Stroma, 25.

Sugars, kinds, 120.

Sulphur, 135.

Supra-renal bodies, 208.

Suspensory ligament, 377.

Sutures, 230.

Sympathetic ganglia and nerves, 298.
  Work of, 316.

Synovial fluid, 231.

Synovial membrane, 231.

System, defined, 20.

Systole, 46.

Taste buds, 345.

Tea,
  Effects on digestion, 167.
  Effects on heart, 56.

Tears, 383.

Teeth, 142.
  Care of 163.
  Kinds of, 143.

Temperature,
  Body, 207.
  Corpuscles, 271, 345.
  Sensation, 343.

Tendon of Achilles, 256.

Tendons, 246.

Tests for foods, 136, 137.

Tetanus, 262, 275.

Thoracic cavity, 7, 85, 100, 102.

Thoracic duct, 67, 170.

Thorax, 85.
  Bones of, 225.

Tissue enzymes, 182.

Tissues, 4.
  Complex nature of, 13.
  Defined, 20.
  General purposes of, 5.
  Kinds of, 5, 6.
  Observations on, 12.
  Properties of, 6.

Tobacco, effect on heart, 56.

"Tobacco heart," 56, 333.

Tongue, 143.

Tonic bath, 273.

Touch, 343.

Touch corpuscles, 342.

Toxins, 394.

Trachea, 80.

Trypsin, 155, 156.

Tuberculosis, 90, 92, 94, 98.
  How communicated, 403.
  Outdoor treatment, 98.
  To prevent, 404.

Tympanum, 359.

Typhoid fever, 404, 407.

Ulna, 227.

Urea, 110, 205, 207, 210.

Ureters, 170.

Uriniferous tubules, 203.

Vaccination, 406.

Valves,
  Advantages of, in veins, 49, 63.
  Mitral, 43.
  Position of, in veins, 63.
  Purposes of, 49, 63.
  Tricuspid, 43.

Veins, 47.
  Functions of, 51.
  Renal, 202.

Ventilation, 94.
  Rules for, 95, 96.

Ventricles, 42.
  To illustrate action of, 62.

Vermiform appendix, 158.

Vertebrae, 223-225.
  Interlocking of, 225.
  Joining of, 224.
  Kinds, 223.

Vestibule, 361.

Villi, 152.
  Parts of, 173, 174.

Visual perceptions, 382.

Visual sensations, 382.

Vitreous humor, 378.

Vocal cords, 355.

Voice, 353-357.
  How produced, 356.
  Pitch and intensity, 356.

Voluntary action, 311, 312.

Voluntary action pathways, 312.

Vomiting, 151, 152.

Waste material, passage from body, 210.

Wastes, 30.

Water,
  Importance of, 123.
  Supply of, 398.
  Value of, 210.

Water-vapor, 208.

White corpuscles, 27, 28.
  Functions of, 29.
  To examine, 39.

Work,
  Hygienic value of, 328, 409.

Worry, 211.

Yellow fever, 403.

Yellow marrow, 218.

Yellow spot, 377.

FOOTNOTES

    1 The body is affected by what it does (exercise, work, sleep), by
      things taken into it (food, air, drugs), and by things outside of it
      (the house in which one lives, climate, etc.). That phase of hygiene
      which has for its object the making of the surroundings of the body
      healthful is known as _sanitation_.

    2 When classified according to their essential structure, the tissues
      fall into four main groups: epithelial and glandular tissue,
      muscular tissue, nervous tissue, and connective tissue. According to
      this system the osseous, cartilaginous, and adipose tissues are
      classed as varieties of connective tissue. See page 18.

    3 The properties of substances are the qualities or characteristics
      (color, weight, etc.) by means of which they are recognized.

    4 Certain of these cells also form deposits of fat, giving rise to the
      adipose, or fatty, tissue.

    5 Any organized structure, such as the body, whose parts are pervaded
      by a common life, is known as an _organism_. The term "organism" is
      frequently applied to the body.

    6 In birds, reptiles, amphibians, and fishes the red corpuscles have
      nuclei (Fig. 9).

    7 The micron is the unit of microscopical measurements. It is equal to
      1/1000 of a millimeter and is indicated by the symbol {~GREEK SMALL LETTER MU~}.

    8 The peculiar shape of the red corpuscle has no doubt some relation
      to its work. Its circular form is of advantage in getting through
      the small blood vessels, while its extreme thinness brings all of
      its contents very near the surface--a condition which aids the
      hemoglobin in taking up oxygen. If the corpuscles were spherical in
      shape, some of the hemoglobin could not, on account of the distance
      from the surface, so readily unite with the oxygen.

    9 The coloring matter of the bile consists of compounds formed by the
      breaking down of the hemoglobin; the spleen contains many large
      cells that seem to have the power first of "engulfing" and later of
      decomposing red corpuscles. A further evidence that the spleen aids
      in the removal of worn-out corpuscles is found in the fact that
      during diseases that cause a destruction of the red corpuscles, such
      as the different forms of malaria, the spleen becomes enlarged.

   10 An infected part of the body, such as a boil or abscess, should
      never be bruised or squeezed until the time of opening. Pressure
      tends to break down the wall of white corpuscles and to spread the
      infection. Pus from a sore contains germs and should not, on this
      account, come in contact with any part of the skin. (See treatment
      of skin wounds, Chapter XVI.)

   11 Coagulation is not confined to the blood. The white of an egg
      coagulates when heated and when acted upon by certain chemicals, and
      the clabbering of milk also is a coagulation.

   12 If the blood be stirred or "whipped" while it is coagulating, the
      clot may be broken up and the fibrin separated as fast as it forms.
      The blood which then remains consists of serum and corpuscles and
      will not coagulate. It is known as "defibrinated" blood.

   13 Certain substances, called _opsonins_, have recently been shown to
      exist in the plasma, that aid the white corpuscles in their work of
      destroying germs. The opsonins appear to act in such a manner as to
      weaken the germs and make them more susceptible to the attacks of
      the white corpuscles.

   14 Some of the changes in the blood are very closely related to our
      everyday habits and inclinations. For example, a lack of nourishment
      in the blood causes hunger and this leads to the taking of food. If
      the fluids of the body become too dense, a feeling of thirst is
      aroused which prompts one to drink water.

   15 Metchnikoff, _The New Hygiene_.

   16 A physiological salt solution is prepared by dissolving .6 of a gram
      of common salt in 100 cc. of distilled water or pure cistern water.
      This solution, having the same density as the plasma of the blood,
      does not act injuriously upon the corpuscles.

   17 The term "circulation" literally means moving in a circle. While the
      blood does not move through the body in a circle, the term is
      justified by the fact that the blood flows out continually from a
      single point, the heart, and to this point is continually returning.

   18 The heart at first glance seems to bear little resemblance to the
      pumps in common use. When it is remembered, however, that any
      contrivance which moves a fluid by varying the size of a cavity is a
      pump, it is seen that not only the heart, but the chest in breathing
      and also the mouth in sucking a liquid through a tube, are pumps in
      principle. The ordinary syringe bulb illustrates the class of pumps
      to which the heart belongs. (See Practical Work.)

   19 The contraction of the heart is known as the _systole_ and its
      relaxation as the _diastole_. The systole plus the diastole forms
      the so-called "cardiac cycle" (Fig. 18). This consists of (1) the
      contraction of the auricles, (2) the contraction of the ventricles,
      and (3) the period of rest. The heart systole includes the
      contraction of both the auricles and the ventricles.

   20 Martin, _The Human Body_.

   21 The pressure maintained by the left ventricle has been estimated to
      be nearly three and one half pounds to the square inch--a pressure
      sufficient to sustain a column of water eight feet high. The
      pressure maintained by the right ventricle is about one third as
      great. In maintaining this pressure the heart does a work equal to
      about one two-hundredth of a horse power.

   22 The location of the heart in the thoracic cavity causes movements of
      the chest walls to draw blood into the right auricle for the same
      reason that they "draw" air into the lungs.

   23 Active exercise through short intervals, followed by periods of
      rest, such as the exercise furnished by climbing stairs, or by short
      runs, is considered the best means of strengthening the heart.

   24 Nosebleed in connection with any kind of severe sickness should
      receive prompt attention, since a considerable loss of blood when
      the body is already weak may seriously delay recovery.

   25 Newton, _Practical Hygiene_.

   26 On account of its position in the body, the lymph is not easily
      collected for examination. Still, nearly every one will recall some
      experience that has enabled him to see lymph. The liquid in a water
      blister is lymph, and so also is the liquid which oozes from the
      skin when it is scraped or slightly scratched. Swelling in any part
      of the body is due to the accumulation of lymph at that place.

   27 In certain small animals of the lowest types a single liquid,
      serving as a medium of exchange between the cells and the body
      surface, supplies all the needs of the organism. In larger animals,
      however, where materials have to be moved from one part of the cell
      group to another, a portion of the nutrient fluid is used for
      purposes of transportation. This is confined in channels where it is
      set in motion by suitable organs. The portion which remains outside
      of the channels then transfers material between the cells, on the
      one hand, and the moving liquid, on the other.

   28 Surgeons in opening veins near the thoracic cavity have to be on
      their guard to prevent air from being sucked into them, thereby
      causing death.

   29 Oxygen forms about 21 per cent of the atmosphere, nitrogen about 78
      per cent, carbon dioxide about .03 per cent, and the recently
      discovered element argon about 1 per cent. The oxygen is in a
      _free_, or uncombined, condition--the form in which it can be used in
      the body.

   30 The peculiar work devolving upon the organs of respiration
      necessitates a special plan of construction--one adapted to the
      properties of the atmosphere. Being concerned in the movement of
      air, a gaseous substance, they will naturally have a structure
      different from the organs of circulation which move a liquid (the
      blood). All the organs of the body are adapted by their structure to
      the work which they perform.

   31 In ordinary inspirations the force that causes the air to move
      through the passages is scarcely an ounce to the square inch, while
      in forced inspirations it does not exceed half a pound. On this
      account the closing of any of the air passages by pressure, or by
      the presence of foreign substances, would keep the air from reaching
      some part of the lungs.

   32 Coughing, which is a forceful expulsion of air, has for its purpose
      the ejection of foreign substances from the throat and lungs.
      Sneezing, on the other hand, has for its purpose the cleansing of
      the nostrils. In coughing, the air is expelled through the mouth,
      while in sneezing it is expelled through the nostrils.

   33 The amount of dust suspended in what we ordinarily think of as pure
      air is shown when a beam of direct sunlight enters an otherwise
      darkened room.

   34 Some children find it difficult to breathe through the nostrils on
      account of growths (called adenoids) in the upper pharynx. Such
      children should have medical attention. The removal of these growths
      not only improves the method of breathing, but in many instances
      causes a marked improvement in the general health and personal
      appearance.

   35 The weakest portions of the lungs appear to be the tiny lobes at the
      top. As they occupy the part of the thorax most difficult to expand,
      air penetrates them much less freely than it does the lobes below.
      In most cases of consumption (some authorities give as high as
      eighty per cent), the upper lobes are the first to be affected. Flat
      chests and round shoulders, by increasing this natural difficulty in
      breathing, have long been recognized as causes which predispose to
      consumption.

   36 The following exercise, from Dudley A. Sargent's _Health, Strength,
      and Power_, will be found most beneficial: "Stand with the feet
      together, face downward, arms extended downward, and backs of the
      hands touching. Raise the hands, arms, and elbows, keeping the backs
      of the hands together until they pass the chest and face. Then
      continue the movement upward, until the hands separate above the
      head with the face turned upward, when they should be brought
      downward and outward in a large circle to the starting point. Begin
      to inhale as the arms are raised and take in as much air as possible
      by the time the hands are above the head, then allow the breath to
      go out slowly as the arms descend."

   37 Colds may frequently be broken up at their beginning by taking a
      prolonged _hot_ bath and going to bed. After getting a start,
      however, they run a course of a few days, a week, or longer,
      depending upon the natural vigor of the individual and the care
      which he gives his body during the time. In throwing off a cold, the
      following suggestions will be found helpful:

      1. Dress warmly (without overdoing it) and avoid getting chilled. 2.
      Diminish the usual amount of work and increase the period for sleep.
      If very weak, stay in bed. Save the energy for throwing off the
      cold. 3. If able to be about, spend considerable time in light
      exercise out of doors, but avoid getting chilled. 4. Keep the bowels
      active, taking a cathartic if necessary. 5. To relieve pain in the
      chest, apply a mustard plaster or a flannel cloth moistened with
      some irritating substance, such as turpentine or a mixture of equal
      parts of kerosene and lard. Keep up a mild irritation until the pain
      is relieved, but avoid blistering.

   38 Not only do the lungs remove oxygen from the air and add carbon
      dioxide to it, but they separate from the body considerable moisture
      and, according to some authorities, a small amount of an impurity
      referred to as "animal matter." Odors also arise from the skin,
      teeth, and clothing which, if not dangerous to the health, are
      offensive to the nostrils. If on going into a room such odors are
      detected, the ventilation is not sufficient. This is said to be a
      reliable test.

   39 E.A. Schaffer, "Artificial Respiration in its Physiologic Aspects,"
      _The Journal of the American Medical Association_, September, 1908.

   40 Testing the prone-posture method by suitable apparatus, Professor
      Schaffer has found it capable of introducing more air per minute
      into the lungs than any of the other methods of artificial
      respiration, and more even than is introduced by ordinary breathing.

   41 Osier, _The Principles and Practice of Medicine_.

   42 Huber, _Consumption and Civilization_.

   43 To prepare limewater some small lumps of _fresh_ lime (either
      slacked or unslacked) are added to a large bottle of water and
      thoroughly shaken. This is put aside until the lime all settles to
      the bottom and the water above is perfectly clear. This is now ready
      for use and may be poured off as needed. When the supply is
      exhausted add more water and shake again.

   44 An _element_ is a single kind of matter. Those substances are
      classed as elements which cannot be separated into different kinds
      of matter. Two or more elements combined in definite proportions by
      weight form a _compound_. The elements are few in number, only about
      eighty being known. Compounds, on the other hand, are exceedingly
      numerous.

   45 The term _energy_, as used here, has the same general meaning as the
      word _power_. See Chapter XII.

   46 The oxygen pressure of the atmosphere is that portion of the total
      atmospheric pressure which is due to the weight of the oxygen. Since
      oxygen comprises about one fifth of the atmosphere, the pressure
      which it exerts is about one fifth of the total atmospheric
      pressure, or, at the sea level, about three pounds to the square
      inch (15 x 1/5 = 3). This is the oxygen pressure of the atmosphere.
      The low oxygen pressure in the tissues is due to its scarcity, and
      this scarcity is due to its entering into combination at the cells.

   47 See footnote on oxygen pressure, page 109.

   48 The impression prevails to some extent that carbon dioxide, on
      account of its weight, settles out of the atmosphere, collecting in
      old wells and at the floor in crowded rooms. Any such settling of
      the carbon dioxide is prevented by the rapid motion of its
      molecules. This motion not only prevents a separation of carbon
      dioxide and air after they are mixed, but causes them to mix rapidly
      when they are separated, if they still have surface contact. The
      carbon dioxide found in old wells is formed there by decaying
      vegetable or animal matter. In rooms it is no more abundant at the
      floor than in other parts.

   49 On account of the formation of carbon dioxide in places containing
      decaying material, the descent into an old well or other opening
      into the earth is often a hazardous undertaking. Before making such
      a descent the air should always be tested by lowering a lighted
      lantern or candle. Artificial respiration is the only means of
      restoring one who has been overcome by this gas (page 97).

   50 While awaiting oxidation at the cells, the carbohydrates and fats
      are stored up by the body, the carbohydrates as glycogen and the
      fats as some form of fat. In this sense they are sometimes looked
      upon as serving to build up certain of the tissues.

   51 The following table shows the main elements in the body and their
      relation to the different nutrients:

                                [Nutrient Table]

   52 The recently advanced theory that the molecules of the mineral
      salts, by dissolving in water, separate into smaller divisions, part
      of which are charged with positive electricity and part with
      negative electricity, has suggested several possible uses for sodium
      chloride and other mineral salts in the body. The sodium chloride in
      the tissues is in such concentration as to be practically all
      separated into its sodium and chlorine particles, or ions. It has
      recently been shown that the sodium ions are necessary for the
      contraction of the muscles, including the muscles of the heart.
      There is also reason for believing that the different ions may enter
      into temporary combination with food particles, and in this way
      assist in the processes of nutrition.

   53 Chittenden, _The Nutrition of Man_.

   54 Compiled from different sources, but mainly from Atwater's _Foods:
      Nutritive Value and Cost_, published by the U.S. Department of
      Agriculture.

   55 The calorie is the adopted heat unit. As used in this table it may
      be defined as the quantity of heat required to raise 1 kilogram (2.2
      pounds) of water, 1 degree centigrade. The calories also show the
      relative amount of energy supplied by the different foods.

   56 While alcohol cannot be classed as a food, it is believed by some
      authorities to contain _food value_ and, in the hands of the
      physician, to be a substance capable of rendering an actual service
      in the treatment of certain diseases. It might, for example, be used
      where one's power of digestion is greatly impaired, since alcohol
      requires no digestion. But upon this point there is a decided
      difference of opinion. Certain it is that no one should attempt to
      use alcohol as food or medicine except under the advice and
      direction of his physician.

   57 A layer of connective tissue between the mucous membrane and the
      muscular coat is usually referred to as the _submucous_ coat. This
      contains numerous blood vessels and nerves and binds the muscular
      coat to the mucous membrane.

   58 The saliva may continue to act for a considerable time after the
      food enters the stomach. "Careful examination of the contents of the
      fundus (large end of the stomach) by Cannon and Day has shown that
      no inconsiderable amount of salivary digestion occurs in the
      stomach."--FISCHER, _The Physiology of Alimentation_.

   59 Perhaps the simplest method of inducing vomiting is that of
      thrusting a finger down the throat. To make this method effective
      the finger should be held in the throat until the vomiting begins.
      An emetic, such as a glass of lukewarm salt water containing a
      teaspoonful of mustard, should also be taken, and, in the case of
      having swallowed poison, the vomiting should be repeated several
      times. It may even be advantageous to drink water and then vomit it
      up in order to wash out the stomach.

   60 Hammerstein, _Text-book of Physiological Chemistry._

   61 Amylopsin is absent from the pancreatic juice of infants, a
      condition which shows that milk and not starch is their natural
      food.

   62 The fact that butter is more easily digested than other fatty
      substances is probably due to its consisting largely of a kind of
      fat which, on splitting, forms a fatty acid (butyric) which is
      soluble in water.

   63 Fischer, _Physiology of Alimentation._

   64 Beginning the meal with a little soup, as is frequently done, may be
      of slight advantage in stimulating the digestive glands. To serve
      this purpose, however, and not interfere with the meal proper, it
      should contain little greasy or starchy material and should be taken
      in small amount.

   65 Dr. William Beaumont, an American surgeon of the last century, made
      a series of observations upon a human stomach (that of Alexis St.
      Martin) having an artificial opening, the result of a gunshot wound.
      Much of our knowledge of the digestion of different foods was
      obtained through these observations. In spite of the protests of his
      physician, St. Martin would occasionally indulge in strong drink and
      always with the same result--the lining of the stomach became much
      inflamed and very sensitive, and the natural processes of digestion
      were temporarily suspended.

   66 The lacteals (from the Latin _lacteus_, milky) are so called on
      account of their appearance, which is white, or milk-like, due to
      the fat droplets.

   67 Peptones and proteoses, when injected directly into the blood, are
      found to act as poisons.

   68 The soluble double sugars (maltose, milk sugar, and cane sugar) are
      reduced to the simple sugars (dextrose and levulose). Furthermore
      the action on the proteids does not stop with the production of
      peptones and proteoses, but these in turn are still further reduced.

   69 Energy, which is defined as _the ability to do work_, or _to cause
      motion_, exists in two general types, or forms, known as kinetic
      energy and as potential energy. _Kinetic_ energy is energy at work,
      or energy in the act of producing motion; while _potential_ energy
      is reserve, or stored, energy. All moving bodies have kinetic
      energy, and all stationary bodies which have within them the
      _capability_ of causing motion possess potential energy. A bent bow,
      a piece of stretched rubber, a suspended weight, the water above a
      mill dam, all have the capability of causing motion and all have
      potential energy. Examples of kinetic energy are found in the
      movements of machinery, in steam and electricity, in winds, and in
      currents of water. Kinetic is the active, and potential the
      inactive, form of energy.

   70 As the atoms of hydrogen and oxygen that make up the molecules of
      water separate, they unite with atoms of their own kind--the hydrogen
      with hydrogen and the oxygen with oxygen atoms. Since these
      combinations are weaker than those of the water molecules, energy is
      required to bring about the change. But when hydrogen burns in the
      oxygen, the change is from a weaker to a stronger combination. The
      stored-up energy is then given up or becomes active.

   71 In the evaporation of water, the energy of the sun is stored with
      reference to the force of gravity. In evaporating, water rises as a
      gas, or vapor, above the earth's surface, but on condensing into a
      liquid, it falls as rain. It then finds its way through streams back
      to the ocean. All water above the sea level is in such a position
      that gravity can act on it to cause motion, and it possesses, on
      this account, potential or stored-up energy. It is because of this
      energy that rapids and waterfalls are such important sources of
      power.

   72 Energy, like matter, can neither be created nor destroyed. It can,
      however, be transferred from one body to another and transformed
      from one form to another form. Whenever work is done, energy is
      transferred from the body doing the work, to the body upon which the
      work is done. During this process there may, or may not, be a
      transformation of energy. In turning a grindstone, kinetic energy is
      passed to the stone and used without transformation, but in winding
      a clock, the kinetic energy from the hand is transformed into
      potential energy in the clock spring. Then as the clock runs down
      this is retransformed into kinetic energy, causing the movements of
      the wheels.

      Not only is kinetic transformed into potential energy and _vice
      versa_, but the different forms of kinetic energy (heat, light,
      electricity, sound, and mechanical motion) are readily transformed
      the one into the other. With suitable devices, mechanical motion can
      be changed into heat, sound, or electricity; heat into motion and
      light; and electricity into all the other forms of energy. These
      transformations are readily explained by the fact that the different
      varieties of kinetic energy are but different forms of motion (Fig.
      84).

   73 The simplest arrangement of the parts of a gland is that where they
      are spread over a plain surface. This arrangement is found in serous
      membranes, such as the pleura and peritoneum. These membranes,
      however, are not called glands, but secreting surfaces.

   74 In the oxidations that occur in the body it is not supposed that the
      nutrients are immediately converted to carbon dioxide, water, and
      urea. On the other hand, it is held that their reduction takes place
      gradually, as the reduction of sugar by fermentation, and that the
      wastes leaving the body are but the "end products" and show only the
      final results.

   75 Alcohol, if used in considerable quantity, leads to cirrhosis of the
      liver and Bright's disease of the kidneys, both very dangerous
      diseases. Dr. William Osler in his treatise, _The Practice of
      Medicine_, states that alcohol is the chief cause of cirrhosis of
      the liver. Dr. T.N. Bogart, specialist in kidney diseases, asserts
      that one third of all the cases of Bright's disease coming under his
      observation are caused by alcohol.

   76 Hall, _The Purin Bodies_.

   77 Review "Main Physiological Problems," page 21.

   78 In the production of motion in the body, as well as in the
      production of any kind of _purposeful_ motion outside of the body,
      three conditions must be fulfilled. There is required, in the first
      place, a mechanical device or machine which is so constructed as to
      produce a certain kind of motion. In the second place, energy is
      needed to operate this device. And, finally, there must be some
      controlling force, by means of which the motion is made to
      accomplish definite results. The driving of a horse hitched to a
      wagon will illustrate these conditions. The wagon is the mechanical
      device, the horse furnishes the energy, and the driver supplies the
      controlling force. In this, as in most cases, the machinery, the
      source of energy, and the controlling force are disconnected except
      when at work; but in the body all three occur together in the same
      structure.

   79 The dependence of the outer layers of bone cells upon the periosteum
      for nourishment causes a destruction of this membrane to affect
      seriously the bone beneath, producing in many instances a decay of
      the bone substance.

   80 It has been claimed that the introduction of vertical writing has
      reduced the number of cases of spinal curvature originating in the
      schoolroom, and statistics appear to prove the claim. It is shown,
      on the other hand, that unnatural positions also are unnecessary in
      the slanting system of writing, and that in either system the pupil
      who is permitted to do so is liable to assume an improper position.

   81 Lovett, _Lateral Curvature of the Spine and Round Shoulders_.

   82 See "Hygiene of Muscles," Chapter XV.

   83 On account of the striations of these cells the muscles which they
      form are called striated muscles.

   84 The striated muscle cells, having many nuclei, are said to be
      multi-nucleated.

   85 Every movement in the body has its opposing movement. This is
      necessary both on account of the work to be accomplished and for
      preserving the natural form of the body.

   86 The distance from the fulcrum to the power is called the _power-arm_
      and the distance from the fulcrum to the weight is called the
      _weight-arm_ (Fig. 115).

   87 The foot in lifting the body on tiptoe appears at first thought to
      be a lever of the second class, the body being the weight and the
      toe serving as the fulcrum. However, if the distance which the body
      is raised is compared with the distance which the muscle shortens,
      it is found that the _supposed_ weight has moved _farther_ than the
      power (Fig. 118). It will also be noted that the muscle which
      furnishes the power is attached at its upper end to the "weight."
      These facts show clearly that we are not here dealing with a lever
      of the second class. The foot in this instance acts as a lever of
      the first class with the fulcrum at the ankle joint and the toe
      pressing against the earth, which is the _actual_ weight. Since the
      earth is immovable, the body is lifted or pushed upward, somewhat as
      a fulcrum support is made to move when it is too weak to hold up the
      weight that is being lifted. In other words, we have the same lever
      action in the foot in lifting the body as we have when one lies face
      downward, and, bending the knee, lifts some object on the toes.

_   88 Walking_ is considered one of the very best forms of counter-active
      exercise for the brain worker (page 328).

   89 The epidermis does not afford complete protection against chemicals,
      many of them being able to destroy it quickly. The rule of washing
      the skin immediately after contact with strong chemical agents
      should always be followed.

   90 "Rough calculations have placed the number of sweat glands on the
      entire body at about 2,000,000." Rettger, _Studies in Advanced
      Physiology_.

   91 Heat also leaves the body by the lungs, partly by the respired air
      and partly through the evaporation of moisture from the lung
      surfaces. Respiration in some animals, as the dog, is the chief
      means of cooling the body.

   92 "The story is told of some woodsmen who were overtaken by a severe
      snowstorm and had to spend the night away from camp; they had a
      bottle of whisky, and, chilled to the bone, some imbibed freely
      while others refused to drink. Those who drank soon felt comfortable
      and went to sleep in their improvised shelter; those who did not
      drink felt very uncomfortable throughout the night and could get no
      sleep, but in the morning they were alive and able to struggle back
      to camp, while their companions who had used alcohol were frozen to
      death.... This, if true, was of course an extreme case; but it
      accords with the universal experience of arctic travelers and of
      lumbermen and hunters in the northern woods, that the use of alcohol
      during exposure to cold, although contributing greatly to one's
      comfort for the time being, is generally followed by undesirable or
      dangerous results."--HOUGH AND SEDGWICK: _The Elements of Hygiene and
      Sanitation_.

   93 Foods that are difficult to digest, or which cause disturbances of
      the digestive organs (a coated tongue being one indication), have a
      bad effect upon the skin. It is in this way that the use of tea and
      coffee by some people induces a sallow or "muddy" condition of the
      complexion.

   94 A most valuable antiseptic ointment is prepared by the druggist from
      the following formula:

          Lanolin, 25 grams.
          Ichthyol, 6 grams.
          Yellow vaseline, 20 grams.

      This is applied as a thin layer on the surface, except in the case
      of boils or abscesses. In treating these a heavy layer is spread
      over the affected part and then covered with absorbent cotton or a
      thin piece of clean cotton cloth.

   95 In a larger sense adjustment includes all those activities by means
      of which the body is brought into proper relations with its
      environment, including the changes which the body makes in its
      surroundings to _adapt them_ to its purposes.

   96 Almost to the present time, physiologists have described the nervous
      system as being made up of two kinds of structural elements which
      were called _nerve cells_ and _nerve fibers_. The nerve cells were
      supposed to form the ganglia and the fibers to form the nerves.
      Recent investigators, however, employing new methods of microscopic
      study, have established the fact that the so-called nerve cell and
      nerve fiber are but two divisions of the same thing and that the
      nervous system is made up of, not two, but one kind of structural
      element. The term "neuron" is used to denote this structural
      element, or _complete nerve cell_.

   97 Many of the axons in the brain and spinal cord have no primitive
      sheath. Axons without the medullary sheath are found in the
      sympathetic nerves. These are known as non-medullated axons and they
      have a gray instead of a white color.

   98 The difference in weight between the brain of man and that of woman
      is due mainly to the fact that man's body is, as a rule,
      considerably larger than that of woman's.

   99 The nervous tissues present, at different places, two colors--one
      white, and the other a light gray. Great significance was formerly
      attached to these colors, because it was supposed that they
      represented two essentially different kinds of nervous matter. It is
      now known that the protoplasm in all parts of the neuron
      proper--cell-body, axis cylinder, and dendrites--has a grayish color,
      while the coverings of most of the fibers are white. Hence gray
      matter in any part of the nervous system indicates the presence of
      cell-bodies, and white matter the presence of nerve fibers.

  100 In very early life the spinal cord entirely fills the spinal cavity,
      but as the body develops the cord grows less rapidly than the spinal
      column, and, as a consequence, separates at the lower end from the
      inclosing bony column.

  101 Fibers passing between the spinal cord and the cerebrum cross to
      opposite sides--most of them at the bulb, but many within the cord--so
      that the right side of the cerebrum is connected with the left side
      of the body, and _vice versa_. This accounts for the observed fact
      that disease or accidental injury of one side of the cerebrum causes
      loss of motion or of feeling in the opposite side of the body.

  102 In general, _afferent_ neurons or fibers are those that convey
      impulses _toward_ the central nervous system (brain and cord), while
      _efferent_ neurons or fibers are those that convey impulses _from_
      the central system.

  103 At different times the nervous impulse has been regarded as a
      current of electricity; as a progressive chemical change, likened to
      that in a burning fuse; as a mechanical vibration, such as may be
      passed over a stretched rope; and as a molecular disturbance
      accompanied by an electrical discharge. The velocity of the nervous
      impulse, which is only about one hundred feet per second, proves
      that it is not a current of electricity. It takes place with little
      or no exhaustion of the cell protoplasm and consequently is not due
      to chemical action. And the loose, relaxed condition of the nerves
      prevents their transmission of physical vibrations, like those on a
      stretched rope. The view that the impulse is a progressive molecular
      disturbance, accompanied by an electrical discharge, has much
      evidence in its favor, but it has only recently been proposed and is
      likely to be modified upon fuller investigation.

  104 The surface of the body includes the linings of the air passages,
      food canal, and certain cavities, as well as the external covering
      or skin.

  105 Derived from the Latin _re_, back, and _flectere_, to turn or bend.

  106 A frog from which the brain has been removed is suspended with its
      feet downward and free to move. If a toe is pinched, the foot is
      drawn away, and if dilute acid, or a strong solution of salt, is
      placed on the tender skin, the feet are moved as if to take away the
      irritating substance. This of course shows that reflex action can
      take place independently of the brain.

      Now if the spinal cord is also destroyed, there is no response when
      the irritation of the skin is repeated. The animal remains perfectly
      quiet, because the destruction of the cord has interrupted the
      reflex action pathway. This shows that some part of the central
      nervous system is necessary to reflex action.

  107 Review description of the spinal nerves, page 295.

  108 Where a deep-seated cause for worry exists, there may be occasion
      for grave concern. Many people have become insane through continued
      worry about some _one_ thing. In cases of this kind the sufferer
      needs the aid of sympathetic friends, and sometimes of the
      physician, in getting the mind away from the exciting cause. A
      change of scene, a visit, or some new employment is frequently
      recommended, where the actual cause for the worry cannot be removed.

  109 Any part of the body which is overworked or which works at a
      disadvantage tends to disturb, more or less, the entire nervous
      system and to produce nervousness. Especially is this true of such
      delicate and highly sensitive structures as the eyes. If the eyes do
      not focus properly or if the muscles that move the eyeballs are out
      of their natural adjustment, extra work is thrown upon these
      delicate parts. One of the first and sometimes the only indication
      of eye strain is that of some disturbance of the nervous system. For
      this reason it is important to carefully test the eyes in
      determining the cause of nervousness (page 385).

  110 One form of neck exercise recommended for this purpose is easily
      taken on retiring at night. Lying flat on the back, without a
      pillow, lift the head slowly from the bed and let it as slowly
      settle back to the level of the body. Repeat several times, lying on
      the back, and then again on the face and again on each side.
      Practice these exercises every night during an interval of a month
      or until relief is secured.

  111 Insurance statistics show that habitual _moderate drinkers_ do not
      live so long as abstainers.

  112 Organs very frequently affected by tobacco are the heart and the
      eyes. It induces, as already stated (page 56), a dangerous nervous
      derangement called "tobacco heart," and it causes a serious disorder
      of the retina (retinitis) which leads in some instances to loss of
      vision. Tobacco smoke also acts as an irritant to the delicate
      lining of the eyes, especially when the tobacco is smoked indoors.

  113 Of 4117 boys in the Illinois State Reformatory, 4000 used tobacco,
      and over 3000 were cigarette smokers. Dr. Hutchison, of the Kansas
      State Reformatory, says: "Using cigarettes is the cause of the
      downfall of more of the inmates of this institution than all other
      vicious habits combined."

  114 The term "mind" is used in this and preceding chapters in its
      popular, not technical, sense.

  115 The problem of social adjustment is but a phase of the general
      problem of establishing proper relations between the body and its
      surroundings.

  116 A vibrating body is one having a to-and-fro movement, like that of a
      clock pendulum or the string of a violin on sounding. Bodies to give
      out sound waves must vibrate rapidly, making not less than sixteen
      vibrations per second. The upper limit of hearing being about 40,000
      vibrations per second, certain bodies may even vibrate too rapidly
      to be heard.

  117 Somewhat as the waves on a body of water impart motion to the sticks
      and weeds along the shore, sound waves are able to cause bodies that
      are small or that are delicately poised to vibrate.

  118 Some idea of how the movements of the cartilages change the tension
      of the cords may be obtained by holding the fingers on the larynx,
      between the thyroid and cricoid cartilages, and making tones first
      of low and then of high pitch. For the high tones the cartilages are
      pulled together in front, and for the low tones they separate. As
      they pull together in front, they of course separate behind and
      above, where the cords are attached.

  119 It is only the central portion of the pinna that aids the entrance
      of sound into the auditory canal. If by accident the outer portion
      of the pinna is removed, there is no impairment of the hearing.

  120 The middle ear is also called the _ear drum_, and, by the same
      system of naming, the membrana tympani is referred to as the _drum
      membrane_.

  121 The inner projection of the temporal bone is known as the petrous
      process.

  122 A small opening in the bone at this place is called the _fenestra
      rotunda_.

  123 Consult some work on physics on the different kinds of lenses and
      their uses.

  124 With respect to its adjustments the eye does not differ in principle
      from various other optical instruments, such as the microscope,
      telescope, photographer's camera, etc., which, in their use, form
      images of objects. These all require some adjustment of their parts,
      called focusing, which adapts them to the distance. The eye's method
      of focusing, however, differs from that of most optical instruments,
      in that the adjustment is brought about through changes in the
      curvature of a lens.

  125 The converging power of convex lenses varies as the curvature--the
      greater the curvature, the greater the converging power.

  126 An oculist is a physician who specializes in diseases of the eye.

  127 Some of the more common symptoms of eye strain are nervousness,
      headache, insomnia, irritations of the eyelids, sensitiveness to
      bright light, and pain in the use of the eyes.

  128 Pyle, _Personal Hygiene_.

  129 "An infectious disease is one in which disease germs infect (that
      is, invade) the body from without. Among the infectious diseases are
      some that are quite directly and quickly conveyed from person to
      person and to these the term contagious is applied. Formerly a sharp
      line was drawn between infection and contagion, but to-day it is
      recognized that no such line exists."--HOUGH AND SEDGWICK, _The
      Elements of Hygiene and Sanitation._

  130 The arctic explorer, Nansen, states that during all the time that
      his party was exposed to the low temperature of the arctic region,
      no one was attacked by a cold, but on returning to a warmer climate
      they were subject to colds as usual. The difference he attributes to
      the absence of germs in the severe arctic climate. There seems to be
      no doubt but that most of our common colds are due to attacks of
      germs.

  131 An interesting biological fact is that the female _Anopheles_, and
      not the male, sucks the blood of animals and is the cause of the
      spreading of malaria.

  132 The habit of spitting upon the floors of public buildings and street
      cars, and also upon sidewalks, is now recognized as a most dangerous
      practice. Not only consumptives, but people with throat affections,
      may do no end of harm in the spreading of disease by carelessness in
      this respect.

  133 For further information on the care of consumptives, consult Huber's
      _Consumption and Civilization_.

  134 As typhoid fever is a disease of the small intestine, great care
      must be exercised in taking food and in the bodily movements. Solids
      greatly irritate the diseased lining of the intestine, and the
      weakened walls may actually be broken through by pressure resulting
      from moving about.

  135 Alcoholic beverages include all the various kinds of drinks that owe
      their stimulating properties to a substance, ethyl alcohol (C2H5OH),
      which is made from sugar by the process of fermentation. They
      include _malt liquors_, such as beer and ale, which contain from
      three to eight per cent of alcohol; _wines_, such as claret, hock,
      sherry, and champagne, which contain from five to twenty per cent of
      alcohol; and _distilled liquors_, such as brandy, whisky, rum, and
      gin, which contain from thirty to sixty-five per cent of alcohol.
      Alcoholic beverages all contain constituents other than alcohol,
      these varying with the materials from which they are made and with
      the processes of manufacture. The distilled liquors are so called
      from the fact that their alcohol has been separated from the
      fermenting substances by distillation.

  136 Duncan, _The Chemistry of Commerce_.

  137 Alcohol is "denatured" by adding substances to it such as wood
      alcohol, which render its use as a beverage impossible.

  138 The tobacco plant, _Nicotiana tobacum_, is a native of America, and
      the use of tobacco began with the American Indians. It was taken
      back to Europe by the early explorers, Sir Walter Raleigh being
      credited with introducing it to the nobility of England.

  139 Most headaches are the result either of eye strain or of digestive
      disturbances, such as indigestion and constipation, and are to be
      relieved through the work of the oculist or through attention to the
      hygiene of the digestive system.

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redpajama_set_name: RedPajamaBook