352 



COLLEGE ZOOLOGY 



and spinal cord are filled with a fluid ap- 

 propriately called the cerebrospinal fluid. 

 Although this fluid looks somewhat like 

 lymph, the two are not identical in com- 

 position. 



The peripheral nervous system (Fig. 233) 

 includes 11 pairs of cranial nerves in the 

 frog and a number of pairs of spinal nerves. 

 Older textbooks give only 10 pairs of cranial 

 nerves, but the discovery of the terminal 

 nerve (nervus terminalis) makes it neces- 

 sary to revise the former figure. The terminal 

 nerve is found in many vertebrates, repre- 

 senting every class except the Agnatha and 



Gray matter 

 White matter 

 Dorsal root 

 Ventral root 

 Ganglion 



birds. Little is known about it, but it appears 

 to be sensory in function. The origin, dis- 

 tribution, and function of the cranial nerves 

 are indicated in the accompanying table. 



The spinal cord is a thick tube connected 

 directly with the brain; it passes through 

 the neural arches of the vertebral column. 

 The spinal nerves arise from the spinal cord 

 in pairs, one on either side in each body 

 segment, and pass out between the verte- 

 brae. Each nerve has two roots (Fig. 235), 

 a dorsal root (sensory), and a ventral loot 

 (motor). The dorsal root possesses a gan- 

 glion containing nerve cells. Its fibers carry 



Spina 

 nerve 



Central cana 

 Synapse 



Sensory nerve fiber ending 

 in skin (receptor) 



Motor nerve ending 

 in muscle (effector) 



Figure 235. Paths of sensory and motor nerve fibers. Arrows indicate direction of nerve im- 

 pulses. A reflex arc from sensory nerve ending by way of the spinal cord to muscles is shown at 

 lower right; also connections to and from the brain. 



impulses toward the spinal cord from var- 

 ious parts of the body and are therefore 

 sensory. The fibers of the ventral root carr\' 

 impulses from the spinal cord to the tissues 

 and are therefore motor. The structure of 

 the nerve cells (neurons, Fig. 236) is similar 

 to that of the earthworm. The direction of 

 the nervous impulses is indicated by arrows 

 in Fig. 235. 



On each side of the spinal cord is a chain 

 of ganglia which is connected at various 

 places with the central nervous system. This 



is known as the autonomic nervous system, 



once called the sympathetic nervous system, 

 but now that term is reser\'ed for one sub- 

 division of the autonomic system. These 

 ganglia send nerves chiefly to the digestive 

 tract, circulatory system, and glandular or 

 gans. 



Visceral and other activities of which we 

 are not ordinarily conscious are regulated 

 and controlled by the autonomic system; 

 these include movements of the stomach 

 and intestine, glandular activities, and the 



