THE NERVOUS SYSTEM 123 



most of the inuscles of the face. It consists of both motor and 

 sensory parts. 



The eighth or auditor tj nerve goes to the ear, and has to do with 

 the special sense of hearing. 



The ninth or glossopharyngeal nerve is mixed in character, and 

 conveys both motor and sensory impulses to the tongue and 

 pharynx. 



The tenth, vagus or pneumogastric nerve, is a very large nerve- 

 trunk with both motor, and sensory fibers that supply the stomach, 

 heart, lungs, trachea, pharynx, and related organs. 



The eleventh or spinal accessory nerve has motor fibers only. 



The twelfth or hypoglossal nerve is purely motor and innervates 

 the muscles of the tongue. 



The spinal cord is situated in the vertebral canal, and extends 

 from the medulla, which it continues, to about the middle of the 

 sacrum. Forty-two pairs of spinal nerves connect with the sides 

 of the spinal cord. The cord is enlarged where the nerves to the 

 limbs are connected. On cross-section a dorsal median septum and 

 a ventral median fissure are seen, also the central canal of the cord 

 (Fig. 41). White matter surrounds a core of gray matter. The 

 latter is arranged in the form of a capital H. Dorsal and ventral 

 gray horns, which give rise to nerve-roots with the same names, are 

 plainly visible. 



The sympathetic nervous system is composed of two chains of 

 gangha which are located one on each side of the vertebral column, 

 external to the spinal canal. At intervals communicating branches 

 join this system with the central nervous system. There are par- 

 ticular ganglia and plexuses for the main groups of visceral organs. 

 The most important is the solar plexus, which is situated on the 

 dorsal wall of the abdominal cavity in contact with the aorta. 



THE PHYSIOLOGY OF THE NERVES 



The function of the nerve-cell is to act as a center for the 

 nutrition of the nerve-fibers proceeding from it. It is acted upon 

 by stimuli from without. The stimuli are conducted to it by the 

 nerve-fibers. In the absence of such stimulation it has the power 

 of independent or automatic action. 



The function of the nerve-fibers, according to Starling, is 

 simply that of conducting impulses from the sense organs at the 



