Kirkes' handbook of physiology
(\hTMf\hS!\ I w w^ Vr •VVW ' '"Wr '»<'»V\Ar *VW| VW;;;Vvvv*vi f yl) VVv -^n. \ i =/00 1 >~> 1 > y r » » 1  TT Fig. 182. — Arterial Blood Pressure of the Dog, Showing the Effect on the Heart Rate of Cutting both Vagus Nerves as marked. The scale to the left shows the pressure in millimeters of mercury. Time in seconds. The momentary inhibition just before the nerves were cut is probably due to mechanical stimulation of the nerves. (New figure by Hill and Chilton.) The inhibitory center is influenced by afferent nerve impulses which may reach it from the heart itself by the depressor nerve, or from other parts of the body. These reflex stimulations of the heart through the vagus center are constantly occurring during our daily life and are the most potent factors in co-ordinations going on between the heart and the rest of the body. Rhythmical alterations of the heart rate occur in association with the effects of the mechanical variations of pressure of the thorax on the heart and blood vessels. Apparently the cardiac-inhibitory center is stimulated during the rise of blood pressure because of the better aeration of the sur- rounding blood and lymph. The activity of the center produces a slower rate of the heart during expiration, shown in figure 243. This variation in heart rate disappears when the vagi are cut off from the center. The Accelerator Nerves. — The influence of the accelerator nerves reaching the heart through the sympathetic, is the reverse of that of the vagus. Stimulation of the sympathetic, even of one side, produces acceler- ation of the rate of the heart-beats, and, according to certain observers, section of the nerve produces slowing. The acceleration produced by stimulation of the sympathetic fibers is accompanied by increased force, and so the action 204 THE CIRCULATION OF THE BLOOD of the nerve is more properly termed angmentor. The sympathetic differs from the vagus in several particulars other than the augmentation which it produces; first, the stimulus required to produce any effect must be more povirerful than in the case with the vagus stimulation; second, a longer time elapses before the effect is manifest; and third, the augmentation is followed by exhaustion, the beats being after a time feeble and less frequent. The stimulation of the vago-sympathetic in the frog, which usually produces Fig. 183. — Diagrammatic Representation of the Origin and Course of the Cardiac Nerves in the Dog, showing the Constituent Neurones, i* 1-5, First to fifth dorsal spinal nerves. Inhibitory fibers in blue, accelerators in red. (Modified from Moret.) inhibition, will occasionally produce acceleration, especially if the heart is beating feebly at the time of the stimulation. The fibers of the sympathetic system, which influence the heart-beat in the frog, leave the spinal cord by the anterior root of the third spinal nerve. They pass by the ramus communicans to the third sympathetic ganglion, OTHER INFLUENCES WHICH AFFECT THE HEART 205 thence to the second ganglion, the annulus of Vieussens (around the sub- clavian artery), through the first ganglion, and along the main trunk of the sympathetic to near the exit of the vagus from the cranium. There the two nerves join and run down to the heart within a common sheath, forming the vago-sympathetic trunk. In the dog the augmentor fibers leave the cord by the anterior roots of the second and third dorsal nerves, and possibly also by the first, fourth, and fifth dorsal nerves. They pass by the rami communicantes to the gang- lion stellatum, or first thoracic ganglion, and around the annulus of Vieussens to the inferior cervical ganglion of the sympathetic. Fibers from the annulus or from the inferior cervical ganglion proceed to the heart, figure 183. The course of the augmentor fibers in the spinal cord is not so well known except that they originate in an augmentor center in the medulla. The circulation of venous blood appears to stimulate the augmentor center, and of highly oxygenated blood the inhibitory center. The accelerator center, like the inhibitory, is in constant tonic activity; and the cardiac acceleration on cutting the vagi, shown in figure 182, is in part to be ascribed to this tone. When both nerves are stimulated together, the resulting rate is the algebraic sum of the opposed influences, according to Hunt. The accelerator center is influenced by afferent impulses arising throughout the body, and these reflexes contribute to the general co-ordina- tion of the chest with the activities of the body. In addition to direct and reflex stimulation, impulses passing down from the cerebrum may have a similar effect. Other Influences which Affect the Heart. — A great variety of special conditions influence the heart's action in the normal body, conditions that are not discussed directly under any of the categories treated above. Of these may be mentioned the coronary circulation, temperature, mechanical tension, age. The Coronary Circulation. — The contractions of the heart cannot long be maintained without a due supply of blood or other nutrient fluid. The nutrient fluid for the heart of man and the mammals is supplied from the coronary arteries and the vessels of Thebesius. The coronary arteries arise from the base of the aorta, where they receive the benefit of the highest arterial pressure. The coronary arteries are terminal arteries; that is, they do not permit the establishment of a collateral circulation when one of their branches is blocked. If the block be complete, that portion of the heart wall supplied by the branch dies. The immediate efl'ect of the closure of a large coronary branch, in the dog, may be occasional and transient irregularity or arrest of the ventricular contractions preceded by irregularities in the force of the contractions and a diminution in the amount of work performed. The force, rather than the rate, of the ventricular contractions is closely dependent upon the blood supply to the coronary arteries. Porter and others have 206 THE CIRCULATION OF THE BLOOD shown that the pressure in the coronary vessels follows closely the pressure in the aorta and that there is not, as formerly claimed, a closure of these vessels by the pressure of the systole of the ventricle. The vessels of Thebesius, which have been demonstrated to open both into the auricular and ventricular cavities, must now be looked upon, ac- cording to the investigations of Pratt, as an important source of cardiac nutrition. Blood may pass through them by way of connecting branches to the coronary arteries and veins. Pratt succeeded in maintaining cardiac contractions for several hours when the only source of nutrition was from these vessels. This source of nutrition may account for the survival of hearts for years where pronounced arterio- sclerosis of the coronary arteries exists. Alteration of Temperature. — The effect of cold is to slow the rate of the heart-beat. If the heart of a frog be cooled down to o° C. it wiU stop beating, but when the temperature of the surrounding lymph or blood is again raised, it will renew its spontaneous beats. The eilect of heat is to quicken and shorten the heart-beats, but at a moderate temperature, 20° C, the contractions are increased in force, iigure 177. The isolated mammalian heart is influenced by temperature variations in much the same way as that of the frog. It will contract slowly in a low temperature and rapidly in a temperature higher than that normal to the body. The very rapid heart in some high fevers is in part due to the increase in temperatures which affects the heart directly. Mechanical Tension. — The mechanical factors produced by the heart- beat are so prominent that it would be surprising indeed if there were no reaction of these mechanical conditions on the heart itself. The isolated cardiac muscle responds very quickly to variations in tension. Beginning with a low tension the activity of heart muscle is increased up to a certain optimum tension, after which further increase is unfavorable to the develop- ment of automatic rhythm. A quite strong stretching will paralyze the muscle. Tension on the whole heart influences its activity, not only through the effects on the muscle, but indirectly through the nervous mechanism. High tension, such as contracting against a high aortic pressure, stimulates sensory nerves of the heart which, acting through the depressor nerve on the inhibi- tory center, produce reflex slowing of the heart. It also produces reflex vaso-dilatation. Both reflexes relieve the liigh tension on the heart. This nerve reaction takes place with a tension which still mechanically stimulates the cardiac-muscle substance, and the inhibitory effects must therefore over- come the direct stimulating effect of the tension on the muscle fibers. Age, Sex, etc. — The average heart rate for the normal adult man is 72 times a minute, but this rate will vary much in different individuals accord- ing to the age, sex, size, and personal equation. The frequency of the heart's OTHER INFLUENCES WHICH AFFECT THE HEART 207 action gradually diminishes from the commencement to near the end of life, but is said to increase again somewhat in extreme old age, thus: Before birth the average number of pulsations per minute is 150 Just after birth 130 to 140 During the first year 11510130 During the second year 100 to 115 During the third year 90 to 100 About the seventh year 85 to 90 About the fourteenth year 80 to 85 In adult age 70 to 80 In old age 60 to 70 In decrepitude 6 5 to 75 The heart rate is greater in woman than in man. It is also greater in small than in large individuals. The rate varies from the type in certain individuals where no cause can be assigned other than personal equation. Poisons and Other Chemical Substances. — A large number of chemical substances have a distinct effect upon the cardiac contractions. Of these the most important are atropine, muscarine, digitalis, barium, nicotine, caffeine, etc. Atropine produces considerable augmentation of the heart-rate, and when acting upon the heart prevents inhibition by vagus stimulation. Its effects are produced by poisoning the nerve endings of the vagus within the heart. With these endings poisoned stimuli arising in the inhibitory center of the medulla (tonic activity), or artificially applied to the vagus, cannot reach the heart muscle, and inhibition is impossible. Muscarine, which is obtained from various species of poisonous fungi, produces marked slowing of the heart-beats, and, in larger doses, stoppage of the heart. It produces an effect similar to that of prolonged vagus stimu- lation. The effect can be removed by the action of atropine, hence is supposed to stimulate the nerve endings of the vagus. Digitalis slows the heart by stimulating the vagi at their origin in the inhibitory center in the medulla. The heart muscle itself is also rendered more excitable. Veratrine and aconitine have a somewhat similar effect. Nicotine and caffeine are both very powerful cardiac stimulants. The great injurious effects of nicotine on the heart are .due to two causes, first, to paralysis of the nervous mechanism and relative loss of control, second, to the great direct stimulation of the cardiac muscle. The constant over- use of tobacco, therefore, very sharply weakens the efficiency of the heart. Caffeine does not lead to so great disturbance of the heart's nutrition as does nicotine. 2o8 THE CIRCULATION OF THE BLOOD THE CIRCULATION THROUGH THE BLOOD VESSELS. Blood-Pressure. — The subject of blood-pressure has been already incidentally mentioned more than once in the preceding pages; the time has now arrived for it to receive more detailed consideration. That the blood exercises pressure upon the walls of the vessels containing it is due to the following facts: The heart at each contraction forcibly injects a considerable amount of blood, 80 to 100 c.c, suddenly and quickly into the arteries. The arteries are highly distensible and stretch to accommodate the extra amount of blood forced into them. The arteries are already full of blood at the commencement of the ventricular systole, since there is not sufficient time between the heart-beats for the blood to pass into the veins. There is a distinct resistance interposed to the passage of the blood from the arteries into the veins by the enormous number of minute vessels, small arteries (arterioles) and capillaries, into which the main artery has been ultimately broken up. The sectional area of the capillaries is several hun- dred times that of the aorta, and the friction generated by the passage of the blood through these minute channels opposes a considerable hindrance or resistance in its course. The resistance thus set up is called peripheral resistance. The friction is greater in the arterioles, where the current is comparatively rapid, than in the capillaries, where it is slow. The interaction of these factors — heart-heat, elastic vessels, and periph- eral resistance — is sufficient to maintain a continuous flow of blood through the entire circulatory system. It is the interrelation of these factors which maintains an even and steady flow through the capillaries and past the tissues, where it is desirable that the conditions of blood flow should be most con- stant. In fact, we shall find that it is through the interaction of this same group of factors, together with the possibility of variations through the regu- lation of their nerve-motor mechanisms, that we have the great variations and adjustments of blood-pressure, speed of flow, volume of flow, and the regulation of volume in particular parts of the body. Arterial Blood-Pressure. — That the blood exerts considerable pres- sure upon the arterial walls in keeping them in a stretched or distended condition may be readily shown by puncturing any artery; the blood is instantly projected with great force through the opening, and the jet rises to a considerable height, the exact level of which varies with the size of the artery experimented upon. If a large artery be punctured the blood may be pro- jected upward for several feet, whereas if it is a small artery the jet does not rise so high. Another characteristic of the jet of blood from a cut artery, particularly well marked if the vessel be a large one and near the heart, is the intermittent character of the outflow. If the artery be cut across, the jet issues with force, chiefly from the central end. If there is considerable anastomosis of vessels in the neighborhood the jet from the peripheral end METHODS OF MEASURING ARTERIAL BLOOD-PRESSURE 2O9 may be as forcible and as intermittent as that from the central end. The intermittent flow in the arteries which is due to the action of the heart, and which represents the systolic and diastolic alterations of blood-pressure, may be felt if the finger be placed upon a sufficiently superficial artery. The finger is apparently raised and lowered by the intermittent distention of the vessel occurring at each heart-beat. This intermittent distention of the artery is what is known as the pulse, to the further consideration of which we shall presently return, but we may say here that in the normal condition the pulse is a characteristic of the arterial, and is absent from the venous, flow. At the same time it must be recollected that in the veins also the blood exercises a pressure on its containing vessel, though it is small when compared with the arterial pressure. As might be expected, therefore, the blood is not expelled with so much force if a vein be punctured or cut. The flow from the cut vein is continuous and not intermittent, and the greater amount of blood comes from the peripheral and not from the central end, as is the case when an artery is severed. Methods of Measuring Arterial Blood-Pressure. — The pressure in an artery may be measured by cutting the vessel and introducing into it a cannula and connecting the cannula with a tall vertical glass tube. When the blood in the vessel is released to the cannula, a column of blood will rise in the tube at once to the height that can be supported by the pressure in that par- ticular vessel. If the vessel be an artery, the blood will rise several feet, according to the distance of the vessel from the heart, and when the pressure has reached its highest point it will be seen to oscillate with the heart-beats. This experiment shows that the pressure which the blood exerts upon the walls of the containing artery equals the pressure of a column of blood of a certain height. In the case of the rabbit's carotid it is equal to 90 to 120 cm. of blood, or rather more than the same height of water. In the case of the vein, if a similar experiment be performed, blood will rise in the tube only for 8 or 10 cm. or less. The usual method of estimating the amount of blood-pressure differs somewhat from the foregoing simple experiment. Instead of a simple straight tube or glass manometer for measuring the pressure, a U-shaped tube containing mercury, the mercury manometer, is employed. The artery is connected with the manometer by means of the cannula inserted into the vessel as before, an arrangement being made whereby the cannula, tubes, etc., are first filled with a half-saturated solution of magnesium sulphate or other saline to prevent the clotting of blood when it is allowed to pass from the artery into the apparatus. The loss of blood is prevented during the preparation of the details of the experiment by a clamp or bull-dog for- ceps. The free end of the U-tube of njercury contains a very fine glass or metal rod with a bulb which floats upon the surface of the mercury and oscillates with the oscillations of the mercury. As soon as there is 14 2IO THE CIRCULATION OF THE BLOOD free communication between the artery and the tube of mercury, the blood rushes out and pushes before it the column of mercury. The mercury will therefore rise in the free limb of the tube, and will continue to do so until a point is reached which corresponds to the mean pressure of the blood-vessel used. The blood-pressure is thus communicated to the near limb of the column of mercury; and the depth to which the latter sinks, added to the height to which it rises in the other limb, the weight of the saline solution being substracted, will give the height of the column of mercury which the blood-pressure balances. For the estimation of the amount of blood-pressure one can make direct readings at any given moment and no further apparatus Fig. 184. Fig. 185. Fig. 184. — Arterial Cannula. T-form for convenience in washing out clots. Fig. 185. — Ludwig's Mercury Manometer. The mercury which partially fills the • tube supports a float in the form of a piston, nearly filhng the tube; a wire is fixed to the float, and the writing style or pen is guided by passing through the brass cap of the manometer tube; the pressure is communicated to the mercury by means of a flexible metal tube filled with fluid. han this is necessary. But in the more accurate study of the variations of pressure in the arterial system, as well as its absolute amount, the instru- ment is usually combined with a recording apparatus, called a kymograph. Numerous forms of recording kymographs are to be had in the market. These instruments, while all constructed on the same principle, vary chiefly in the accuracy of their construction and convenience of their adjustments. The essential part of a recording kymograph consists of a uniformly revolving cylinder accurately centered and carrying a paper on which a record is made of the physiological change which is being studied. This cylinder or drum may be driven by a weight, clock spring, electric motor, or other mechanical device that insures uniformity of speed and which is capable of speed regulation. The cylinder is covered with glazed paper, blackened in METHODS OF MEASURING ARTERIAL BLOOD-PRESSURE 211 the flame of a lamp, and the mercury manometer is so supported that its float, provided with a style, writes on the cylinder as it revolves. In some of the instruments, especially Ludwigs' continuous paper kymograph, a long paper band is made to pass over the recording surface and the record itself is written by various devices carrying ink. There are also many ways in which the mercury manometer may ■ be varied; in figure 185 is seen a form which is known as Ludwig's. In order to obviate the necessity of a large quantity of blood entering the tube of the apparatus and being lost to the animal, it is usual to have some arrangement by means of which the mercury may, previous to the experiment, be forced Fig. 186. — Tracing of Normal Arterial Pressure in the Dog, Obtained with the Mercurial Manometer. The smaller undulations correspond with the heart-beats; the larger curves with the respiratory movements. Pressure is in millimeters of mercury as shown by the scale to the left. Time in seconds. (New figure by March and Nugent.) up in the tube of the manometer to the pressure level corresponding to approximately the mean pressure of the artery experimented with, so that the writing style simply records the variations of the blood-pressure above and below the mean pressure. This is done by causing the anti-coagulant solution, generally a saturated solution of sodium carbonate or of 10 per cent, magnesium sulphate, to fill the apparatus from a bottle suspended at a height about that of the pressure to be measured, and capable of being raised or lowered as required for the purpose. The cannula inserted and tied into the artery may be of several different kinds. A glass T-tube with the end drawn out and cut so that it is oblique, and provided with a slightly constricted neck to prevent its coming out of 212 THE CIRCULATION OF THE BLOOD the artery easily, is a very convenient form, figure 184. Of the two free ends of the T-cannuIa one is connected with the manometer, the other with the pressure bottle. The peripheral end of the cut artery is tied to obviate the escape of blood. By this means, the presssure communicated to the column of mercury is the forward, and not the lateral, pressure of blood, but there is very little if any difference. As soon as the experiment is begun, the writing float is seen to oscillate in a regular manner, and a curve of blood pressure is traced upon the smoked paper by the style (or, if a continuous roll of unsmoked paper be used, the trace is made by an inked pen) when a figure similar to figure 186 will be obtained. This indicates two main variations of the blood pressure. The smaller excursions of the lever correspond with the systole and diastole of the heart, and the larger curves correspond with the respirations, being called the respiratory undulations of blood-pressure, to which attention will be directed Fig. 187. — Tracing of Normal Arterial Pressure Taken from the Rabbit with a Hurthle Manometer. The horizontal lines show zero pressure. Time in seconds. (Dreyer.) in the next chapter. Of course, the undulations spoken of are seen only in records of arterial blood-pressure. They are more clearly marked in the ar- teries nearer the heart than in those more remote. The amount of the pressure in the smaller arteries as well as the indication of the systolic rise of pressure is, comparatively speaking, small. In order to record the details of the undulations of arterial pressure, it is better for some purpose to use the Hurthle membrane manometer than the mercurial manometer. Two views of this instrument are shown in figure 166. The instrument consists of a hollow tube and cup covered with rubber sheet against which a disc supported by a metal spring is adjusted. The appara- tus is filled with fluid, the interior of which is connected with the artery by means of a metal tube and cannula. The pressure transmitted to the appa- ratus tends to stretch the rubber and bend the spring, and the movement thus produced is communicated by means of a lever to a writing style and so to BLOOD-PRESSURE MEASUREMENTS IN MAN 213 a recording apparatus. This instrument obviates the errors which might be caused by the inertia of the mercury in the mercurial manometer; it also shows in more detail the variations of the blood pressure in the vessel during and after each individual beat of the heart. As regards the actual amount of blood-pressure, from observations which have been made by means of the mercurial manometer, it has been found that the pressure of blood in the carotid of a rabbit is capable of supporting a column of goto 120 mm. of mercury; in the dog 100 to 175 mm.; in the horse 152 to 200 mm.; and in man the pressure is estimated to be about the same as in the dog. To measure the absolute amount of this pressure in any artery multiply the area of its transverse section by the height of the column of mercury which is already known to be supported by the blood pressure in any part of the arterial system. The weight of a column of mercury thus found will represent the absolute pressure of the blood. Calculated in this way, the blood pressure in the human aorta is equal to i . 93 kilogrammeters; that in the aorta of the horse being 5 . 2 kilogrammeters; and that in the radial artery at the human wrist only 0.08 kilogrammeter. Supposing the muscu- lar power of the right ventricle to be one-fourth that of the left, the blood pressure in the pulmonary artery will be only o . 5 kilogrammeter. The amounts above stated represent the arterial tension at the time of the ventricular contraction. The arterial pressure is greatest at the beginning of the aorta, and de- creases toward the capillaries. It is greatest in the arteries at the period of th.e ventricular systole and least during the diastole. The blood-pressure gradually lessens as we proceed from the arteries near the heart to those more remote, and again from these to the capillaries, as it does also from the capillaries along the veins to the right auricle. Arterial Blood-Pressure Measurements in Man. — A number of instruments have been devised for estimating blood-pressure in man for clinical purposes. Some of these, though excellent in principle, are too com- plicated for general use. The first simple and approximately accurate form of apparatus was that devised by Riva-Rocci in 1896. This has been modi- fied and improved in minor points since, but the principles of the original instrument remain practically the same. In brief, the apparatus, figure 188, consists of an elastic tube ending in a rubber bag which can be adjusted about the arm or forearm, and a mercury manometer connected with this tube and also with some form of air pump used for inflating the tube about the arm and thus exerting pressure upon its blood-vessels. The elastic tube is covered by some inelastic tissue, usu- ally a leather cuff, in order that the inflation of the bag may cause the full in- crease of pressure to be exerted upon the encased arm. By inflating the bag until the pulse at the wrist just disappears, and reading the height of the column of mercury in the manometer, the maximum or systolic pressure is 214 THE CIRCULATION OF THE BLOOD obtained in millimeters of mercury. If now the pressure on the arm is re- duced until the widest oscillations of the mercury column are obtained, the lowest position of the mercury meniscus represents the diastolic pressure. The apparatus depends on the principle that an external pressure just equal to the ma.^mal pressure within an. artery wiU hold the vessel in the collapsed condition, a fact that has been proven for vessels that are exposed. An external pressure that wUl just equal the minimal or diastolic pressure will cause a complete collapse of a vessel during diastole and will allow a complete expansion of an artery to its maximal limits during the systolic period of pressure. In other words, the mercury of the manometer will oscillate to its ma.ximal. If the pressure is reduced to a still lower point, it Fig. i88. — Riva-Rocci Apparatus (schematic) for Determining Blood Pressure in Man. will not be sufficient to compress the artery completely, and the mercury oscillations will again become smaller. In applying the instrument to the brachial artery, one must, of course, deal with a vessel deeply buried in mus- cular and other tissues. These latter tissues probably consume a certain small percentage of the pressure, an error which may be ignored for all comparative purposes. Erlanger has perfected a form of sphygmomanometer which contains a very ingenious and compactly arranged recording device, figure 189. This instrument has a mercury manometer from which the pressures are read off directly. On a side limb of the manometer there is a rubber bag enclosed in a glass bell. The cavity of the beU outside of the rubber bag is connected VENOUS BLOOD -PRESSURE AND CAPILLARY PRESSURE 2IS with a recording tambour, the entire apparatus being fully supplied with the necessary valves and adjusting devices which make it mechanically very perfect. The instrument is mounted on a stand with a small clock and recording cylinder adapting it to convenient clinical use. The brachial arterial pressure of man when taken by this form of appara- !-i r — ; 1 '^ I 1 ^m l^^m \ :i '^^^^^V ' m^ m^M !^2KK- 'it^^^ ( ■ Fig. i8g. — Erianger's Sphygmomanometer, Shown \rith the Rubber Bag Attached to the Arm. The picture is taken at the end of an experiment after the pressure in the instru- ment is run up again to above the systohc pressure. The upper part of the cylinder shows a sphygmogram taken with the instrument. (Experiment and photo by Hill and Watkins.) tus has been found to vary greatly, but Erlanger gives no mm. of mercury as the average of observations on young adults in the determination of the systolic pressure; i.e., the maximal arterial pressure. He gives for the dias- tolic pressure 40 to 45 mm. of mercury below the systolic pressure. Other observers using the same method find a somewhat higher average pressure, see figure 190, which represents a fair type of observation. The Venous Blood-Pressure and Capillary Pressure. — The blood- pressure in the veins is nowhere very great, but is greatest in the small veins, 2l6 THE CIRCULATION OF THE BLOOD while in the large veins near the heart the pressure may become negative, or in other words, when a vein is put in connection with a mercurial manom- eter the mercury may fall in the arm farthest away from the vein and will rise in the arm nearest the vein, the action being that of suction rather than pressure. In the large veins of the neck the tendency to suck in air is es- pecially marked, and is the cause of death in some accidents or operations in that region. The amounfof pressure in the brachial vein is said to support 9 mm. of mercury, whereas the pressure in the veins of the neck may fall to a negative pressure of from — 3 to — 8 mm. The variations of venous pressure during systole and diastole of the heart are very slight, and a distinct pulse is never seen in veins except under Fig. 190. — Tracing taken with Erlanger's Sphygmomanometer. The figures indicate pressure in millimeters of mercury. Systolic pressure 160; diastolic pressure, 120. (New figure by Hill.) extraordinary circumstances. In certain forms of cardiac valvular insuffi- ciency there may be considerable regurgitation of the blood with a strong venous pulse. Careful observations upon the web of the frog's foot, the tongue and mesen- tery of the frog, the tails of newts and small fishes, and upon the skin of the finger behind the nail (von Kries) ; as well as estimations of the amount of pressure required to empty the vessels of blood under various conditions, all indicate that the capillary blood-pressure is subject to very great varia- tions. Apparently the variations follow the variations of pressure in the arteries, though the measurements of the capillary pressure of the skin in ma-n indicate that it is occasionally markedly influenced by the venous pressure variations (Hough). In the skin in man it is from 30 to 50 mm. mercury. The pulse in the arterioles, capillaries, and venules becomes more and more evident as the extravascular pressure is increased. The pressure in the web of the frog's foot has been found to be equal to about 14 to 20 mm. of mercury; in other capillary regions the pressure is found to be equal to from one-fifth to one-half of the ordinary arterial pressure. GENERAL VARIATIONS IN BLOOD PRESSURE 217 General Variations in Blood-Pressure. — The arterial blood-pressure may be made to vary by alterations in either of the chief factors upon which the pressure in the vessels depends, but primarily by the cardiac contrac- tions and the peripheral resistance. Thus, increase of blood-pressure may be brought about by either, i, a more frequent or more forcible action of the heart, or, 2, by an increase of the peripheral resistance. On the other hand, diminution of the blood-pressure may be produced, either by a, a diminished force or frequency of the contractions of the heart, or by b, a diminished peripheral resistance. These different factors, however, al- though varying constantly, are so combined that the general arterial pressure Fig. 191. — Schema Showing the Relation between Blood Pressure, Velocity of Flow, and Vascular Area, in the Arteries, Capillaries, and Veins. Ordinates represent height of pressure and speed of flow. The abscissa, b-c, represents zero pressure and speed. Space between lines a-b and d-e represents arterial system; between d-e and f-g, capillary system, and between f-g and h-i, the venous system. Line A-B equals pressure; line C-D, speed of flow; and line E-F, vascular area. (Modified from Gad.) remains fairly constant. For example, the heart may, by increased force' or frequency of its contractions, distinctly increase the blood pressure, but this increased action is almost certainly followed by diminished peripheral re- sistance, and thus the two altered conditions may balance, with the result of bringing back the blood-pressure to what it was before the heart began to beat more rapidly or more forcibly. It will be clearly seen that the circulation of the blood within the blood- vessels must depend upon the diminution of the pressure from the heart to the capillaries, and from the capillaries to the veins, the blood flowing in the direction of least resistance. We shall presently see further that the local flow also depends upon the relations between the heart's action and the peripheral resistance both general and local. 2l8 THE CIRCULATION OF THE BLOOD The Arterial Flow. — The character of the flow of blood through the arterial system depends to a very considerable extent upon the structure of the arterial walls, and particularly upon the elastic tissue which is so highly developed in them. The elastic tissue of the arteries, first of all, guards them from the sud- denly exerted pressure to which they are subjected at each contraction of the ventricles. In every such contraction, as is above seen, the contents of the Fig. 192. — Cross Section of the Aorta to Sho\y Elastic Tissue; e, elastic elements. (Bailey.) ventricles are forced into the arteries more quickly than they are discharged through the capillaries. The blood, therefore, being for an instant resisted in its onward course, a part of the force with which it is impelled is directed against the sides of the arteries; under this force their elastic walls dilate, stretching enough to receive the blood, and becoming more tense and more resisting as they stretch. Thus by yielding they break the shock of the force impelling the blood. On the subsidence of the pressure, should the ventricles cease contracting, the arteries are able by the same elasticity to resume their former caliber. The elastic tissue in the same way equalizes the current of blood by main- taining pressure on it in the arteries during the period at which the ventri- THE ARTERIAL FLOW IS RHYTHMIC 219 cles are at rest or are dilating. If the arteries were rigid tubes, the blood, instead of flowing as it does in a constant stream, would be propelled through the arterial system in a series of spurts corresponding in time to the ventric- ular contractions and with intervals of almost complete rest during the in- action of the ventricles. But in the actual condition of the vessels, the force of the successive contractions of the ventricles is expended partly in the direct propulsion of the blood and partly in the dilatation of the elastic ar- teries; and in the intervals between the contractions of the ventricles, the force of the recoil is employed in continuing the flow onward. Of course the pressure exercised is equally diffused in every direction, and the blood tends to move backward as well as onward. All movement backward, however, is prevented by the closure of the semilunar valves, which takes place at the very commencement of the recoil of the arterial walls. The Arterial Flow is Rhythmic. — By the exercise of the elasticity of the arteries, all the force of the ventricles is expended upon the circulation. That part of the force which is used up or rendered potential in dilating the arteries is restored or made active or kinetic when they recoil. There is no loss of force, neither is there any gain; for the elastic walls of the artery can- not originate any force for the propulsion of the blood; they only restore that which they receive from the ventricles. Since the ventricular discharge is intermittent, there will be intermittent accessions of pressure, and therefore the flow of blood in the arteries will be periodically accelerated. The volume of blood discharged from a cut artery increases and decreases with the systole and diastole of the ventricles, or with the systolic and diastolic pressures of the arteries themselves, the maximal speed being at the moment of maximal systolic pressure, see page 215. The equalizing influence of the resistance of the successive arterial branches reacts so that at length the intermittent accelerations produced in the arterial flow by the discharge of the heart cease to be observable, and the jetting stream is converted into the continuous and even movement of the blood which characterizes the flow in the capillaries and veins. The resistance which is offered to the flow of the blood stream in these vessels is a necessary agent in the production of a continuous stream of blood in the smaller arteries and capillaries. Were there no greater ob- stacle to the escape of blood from the larger arteries than exists to its en- trance into them from the heart, the stream would be intermittent, notwithstanding the elasticity of the walls of the arteries. The muscular element of the middle coat co-operates with the elastic element in adapting the caliber of the vessels to the quantity of blood which they contain; for the amount of fluid in the blood-vessels varies quite con- siderably even from hour to hour, and can never be quite constant. Were the elastic tissue only present, the pressure exercised by the walls of the 220 THE CIRCULATION OF THE BLOOD containing vessels on the contained blood would be sometimes very small and sometimes inordinately great. The presence of a muscular element, how- ever, provides for a certain uniformity in the amount of pressure exercised: the muscles are in constant action or tone, and it is by this adaptive, uniform, gentle muscular contraction that the normal tone of the blood-vessels is maintained. Deficiency of this tone is the cause of the soft and yielding arterial pulse, and the sluggish blood flow through the arterioles. Incidentally it may be mentioned that the elastic and muscular contrac- tion of an artery may also be regarded as fulfilling a natural purpose when, the artery being cut, the sudden contraction at first limits, and then, in con- junction with the coagulating blood, completely arrests, the flow of blood. It is only in consequence of such contraction and coagulation that we are free from danger through even very slight wounds; for it is only when the artery is closed that the processes for the more permanent and secure pre- vention of bleeding are established. The Velocity of the Arterial Blood Flow. — The velocity of the blood current at any given point in the various divisions of the circulatory system is inversely proportional to the united sectional area at that point. If the united sectional area of all the branches of a vessel were always the same as the area of the vessel from which they arise, and if the aggregate sectional area of the capillary vessels were equal to that of the aorta, the mean rapidity of the blood's motion in the small arteries and in the capillaries would be the same as in the aorta. If a similar correspondence of capacity existed in the veins there would be an equal correspondence in the rapidity of the circula- tion in them. But the volume of the arterial and venous systems may be represented by two truncated cones with their apices directed toward the heart; the area of their united bases, the sectional area of the capillaries, being about eight hundred times as great as that of the truncated apex rep- resenting the aorta. Thus the velocity of blood in the smallest arterioles and the capUlaries is about one-eight-hundredth of that in the aorta. The velocity of the stream of blood is greatest in the neighborhood of the heart. The rate of movement is greatest during the ventricular systole and diminishes during the diastole. The rate of flow also decreases along the arterial system, becoming least in the parts of the system most distant from the heart. Chauveau has estimated the rapidity of the blood stream in the carotid of the horse at over 20 inches per second during the heart's systole, and nearly 6 inches during the diastole (520-150 mm.) see figure 191. The Capillary Flow. — It is in the capillaries that the chief resistance is offered to the progress of the blood; for in them the friction of the blood is greatly increased by the enormous multiphcation of the surface with which it is brought in contact. When the capillary circulation is examined in any transparent part of a full-grown Hving animal by means of the microscope, figures 193, 194, the THE CAPILLARY FLOW 221 blood is seen to flow with a constant equable motion; the red blood corpus- cles moving along, mostly in single file, and bending in various ways to ac- commodate themselves to the tortuous course of the capillary, but instantly recovering their normal outline on reaching a wider vessel. 