The central nervous system comprises the brain and spinal cord and is composed of myriads of neurones. These are connected to form a very complex chain and network through an innumerable number of synaptic relationships. The cell bodies are massed in the gray matter of the brain and spinal cord while the nerve fibers constitute columns of white matter.
The peripheral nervous system is composed of fibers whose cell bodies reside in the gray matter of the central nervous system or in certain peripheral ganglia lying outside of the central nervous system, that is, the afferent and efferent fibers which compose the nerve trunks previously described.
From the brain stem and medulla issue twelve pairs of cranial nerves, namely, the olfactory, optic, oculomotor, trochlear, trigeminus, abducens, facial, auditory, glossopharyngeal, vagus, spinal accessory, and hypoglossal nerves. From each side of the spinal cord there arise eight cervical (neck), twelve thoracic (trunk), five lumbar (middle of the back), five sacral, and one coccygeal nerve, thus, making a total of thirty-one pairs of spinal nerves.
Each nerve arises from the cord from two roots, the one ventral and the other dorsal. It has been conclusively demonstrated experimentally, by a number of observers, that those fibers contained in the dorsal root are afferent in function, that is, carry impulses only toward the central nervous system, while those in the ventral root are efferent or carry impulses from the central nervous system to the various tissues and organs. Situated on the dorsal root is an enlargement, the dorsal root ganglion, which contains the nerve cells of the afferent neurones.