Subdivisions of the Nervous System

The vertebrate nervous system consists of three subdivisions, all intimately associated with one another, namely: (1) the Central Nervous System, consisting of the brain and spinal cord; (2) the Peripheral Nervous System, including the cranial and spinal nerves; and (3) the Autonomic Nervous System which is a special portion of the peripheral nervous system innervating smooth muscles and glands. Sense Organs, to be considered in Chapter XX, are made up of specialized endings of cranial and spinal nerves together with various non-nervous elements.

The peripheral nervous system is an indispensable auxiliary of the central nervous system. It consists of nerves and ganglia. Nerves are groups of fibers which put sense organs and effectors into physiological connection with the brain and cord, while ganglia are groups of cell bodies. Corresponding parts within the central nervous system are tracts and centers, respectively. In general the cell bodies of all sensory neurons are in ganglia while those of all motor neurons, except certain autonomic ones, are within the central nervous system.

Medullated nerve. Part of a cross section of a human median nerve

“Nerves” are cables of neurites and dendrites in which the independence of the strands composing them is maintained. They extend through mesenchymatous tissues to all parts of the body, except into cartilage and the epidermal layers of the skin. Although the number of nerves is not large primarily, the neurons, or morphological elements that combine to make them (Fig. 604), may be, according to Donaldson, as many as three or four millions in man. The nerves associated with the central nervous system are in pairs, and are either spinal or cranial, according to whether they connect with the cord or brain.