GENERAL ANATOMY OF THE NERVOUS SYSTEM 73 



as they meet. The feature thus produced is like an English 

 X or the Greek letter Chi, whence it takes its name of the 

 optic chiasma. Close behind the chiasma a short stalk 

 depends from the brain and is connected with the pitu- 

 itary body, an unpaired organ which is lodged in a hollow 

 of the sphenoid bone. 



The cranial nerves are of very unequal size and func- 

 tional importance. A large one is the fifth (trigeminal) , 

 which enters the pons and thus serves as a convenient 

 reminder that four pairs of the nerves are anterior and 

 seven posterior to this structure. In this trunk are con- 

 veyed a great majority of the afferent fibers from the face, 

 the front of the eye, the linings of the nose and mouth, 

 and the teeth. It is responsible for a large share of 

 human suffering. But efferent fibers are present also, 

 and the chief muscles employed in mastication are gov- 

 erned through their agency. The seventh cranial nerve, 

 springing from the medulla just behind the pons, governs 

 the small muscles which are responsible for facial expres- 

 sion. The eighth nerve, entering the central axis very 

 near the seventh, is the auditory, interesting not only as 

 the pathway on which our hearing depends, but also for the 

 part which it plays in the maintenance of equilibrium. 



The tenth nerve is one to which reference must often 

 be made. It is called the vagus or pneumogastric. Its 

 striking peculiarity lies in the fact that it is not dis- 

 tributed so largely to structures belonging to the head 

 and neck as to the organs of the trunk both above and 

 below the diaphragm. Hence its fibers are, as a rule, 

 much longer than those of other cranial nerves. They 

 reach muscular and glandular elements in all parts of the 

 digestive system, while certain ones end in the heart and 

 are of particular interest because of the role which they 

 fulfil in the regulation of its beating. Still others come 

 into relation to the larynx. Many of the vagus fibers are 

 afferent and bring impulses from the tissue of the lungs, 

 from the heart, and from other localities. 



As to the less conspicuous nerves of the cranial series, 



