204 The Vertebrate Organ Systems 



side the cord. By other processes, this ganglion makes connections with 

 the dorsal portion of the gray matter of the cord. These two roots 

 join into a single spinal nerve which emerges from the spinal column 

 through the intervertebral foramen. Clearly, all spinal nerves are 

 mixed nerves. 



Shortly after emerging from the spinal column, each of these nerves 

 divides into three main branches : the dorsal, ventral, and visceral 

 branches. The dorsal branch innervates the muscles and skin of the 

 dorsal region ; the ventral branch supplies the ventral body wall ; and 

 the visceral branch connects with the autonomic ganglia and eventu- 

 ally goes to the visceral organs. 



In man, there are thirty-one pairs of spinal nerves. The first 

 eight of these are grouped as the cervical nerves: one arises in front 

 of the atlas and one behind each of the cervical vertebrae. The next 

 twelve pairs are the thoracic nerves, one pair arising from behind each 

 of the thoracic vertebrae. Similarly, there are five lumbar nerves, 

 five sacral, and one coccygeal. More effective innervation of certain 

 areas is achieved when portions of nerves or usually entire nerves com- 

 bine to form plexuses. By so doing, an individual muscle may be 

 stimulated by several nerves or a single nerve may in turn affect sev- 

 eral different muscles. In man, there are three principal plexuses : the 

 cervical, composed of the first four cervical nerves for innervation of 

 the neck region ; the brachial, composed of the last four cervical nerves 

 and the first thoracic, all supplying the arm ; and the lumbar-sacral, 

 formed of the lumbar and sacral nerves which supply the leg. There are 

 minor variations in this arrangement in the different mammals. 



The remainder of the peripheral nervous system is formed of the 

 twelve pairs of nerves which originate from the brain (Fig. 61). These 

 are collectively termed the cranial nerves. Many of these innervate the 

 sense organs and have functions somewhat more specialized than those 

 of the spinal nerves. Inasmuch as they are quite varied in their function- 

 ing and distribution, they are best summarized by means of a table. 



In general, the function of the peripheral nervous system may be 

 considered to be the relaying of impulses from the various sense or- 

 gans to the central nervous system. Here they are interpreted, and 

 appropriate impulses are initiated. These impulses are transmitted 

 by the peripheral nervous system and stimulate muscles or glands to 

 activity. 



