Lymph from the right side of the head and neck, the right upper limb, and the right side of the thorax is carried by lymphatic vessels that connect with veins on the right side of the neck.
Lymph from all other regions of the body is carried by lymphatic vessels that drain into veins on the left side of the neck.
Specific information about the organization of the lymphatic system in each region of the body is discussed in the appropriate chapter.
The nervous system can be separated into parts based on structure and on function: structurally, it can be divided into the central nervous system (CNS) and the peripheral nervous system (PNS) (Fig. 1.32); functionally, it can be divided into somatic and visceral parts.
The CNS is composed of the brain and spinal cord, both of which develop from the neural tube in the embryo.
The PNS is composed of all nervous structures outside the CNS that connect the CNS to the body. Elements of this system develop from neural crest cells and as outgrowths of the CNS. The PNS consists of the spinal and cranial nerves, visceral nerves and plexuses, and the enteric system. The detailed anatomy of a typical spinal nerve is described in Chapter 2, as is the way spinal nerves are numbered. Cranial nerves are described in Chapter 8.
The details of nerve plexuses are described in chapters dealing with the specific regions in which the plexuses are located.
The parts of the brain are the cerebral hemispheres, the cerebellum, and the brainstem. The cerebral hemispheres consist of an outer portion, or the gray matter, containing cell bodies; an inner portion, or the white matter, made up of axons forming tracts or pathways; and the ventricles, which are spaces filled with CSF.
The cerebellum has two lateral lobes and a midline portion. The components of the brainstem are classically defined as the diencephalon, midbrain, pons, and medulla. However, in common usage today, the term “brainstem” usually refers to the midbrain, pons, and medulla.
A further discussion of the brain can be found in
Chapter 8.
The spinal cord is the part of the CNS in the superior two thirds of the vertebral canal. It is roughly cylindrical in shape, and is circular to oval in cross section with a central canal. A further discussion of the spinal cord can be found in Chapter 2.
The meninges (Fig. 1.33) are three connective tissue coverings that surround, protect, and suspend the brain and spinal cord within the cranial cavity and vertebral canal, respectively:
The dura mater is the thickest and most external of the coverings.
The arachnoid mater is against the internal surface of the dura mater.
The pia mater is adherent to the brain and spinal cord.
Between the arachnoid and pia mater is the subarachnoid space, which contains CSF.
A further discussion of the cranial meninges can be found in Chapter 8 and of the spinal meninges in Chapter 2.
Functional subdivisions of the CNS
Functionally, the nervous system can be divided into somatic and visceral parts.
The somatic part (soma, from the Greek for “body”) innervates structures (skin and most skeletal muscle) derived from somites in the embryo, and is mainly involved with receiving and responding to information from the external environment.
The visceral part (viscera, from the Greek for “guts”) innervates organ systems in the body and other visceral elements, such as smooth muscle and glands, in peripheral regions of the body. It is concerned mainly with detecting and responding to information from the internal environment.
Somatic part of the nervous system
The somatic part of the nervous system consists of: nerves that carry conscious sensations from peripheral regions back to the CNS, and nerves that innervate voluntary muscles.