i66 THE FROG CHAP. 



lower jaw and to the muscles of the hyoid. It is a mixed 

 nerve. 



The eighth or auditory nerve (Figs. lo and 49, VIII) 

 arises from the medulla just behind the seventh, passes 

 through an aperture in the inner wall of the auditory cap- 

 sule, and is distributed to the auditory organ or membranous 

 labyrinth (see Chapter XI). It is the nerve of hearing, 

 and is purely sensory. 



The ninth ox glossopharyngeal (Figs, 49 and 53, IX) arises 

 behind the auditory nerve. It sends a branch to join the 

 facial and supplies the mucous membrane of the tongue and 

 pharynx as well as certain small muscles connected with the 

 hyoid. It is also a mixed nerve. 



The tenth or vagus (Figs. 49 and 53, Jt') is a large nerve 

 arising in common with the ninth, and dilating, shortly after 

 leaving the skull, into a vagus ganglion. It supplies the 

 larynx iXlar), the heart (Xcd), the lungs {X pul), and the 

 stomach {Xgas), and is therefore often known as thepneunio- 

 gastric. It has thus an extraordinarily wide distribution, being 

 in fact the only cerebral nerve which supplies parts beyond 

 the head. It is a mixed nerve, and contains many motor 

 fibres, but its branches — some of which have to do with the 

 regulation of the heart's contraction and with respiration — 

 are better described as efferent and afferent than as motor 

 and sensory : the meaning of these terms will be explained 

 later on. The ninth and tenth nerves leave the skull close 

 together through the aperture noticed in the exoccipital bone. 



The sympathetic nerve {Sy) e.Kteiid? forward.? from its junction with 

 the first spinal nerve, joins the vagus, and finally ends anteriurly in the 

 Gasserian ganglion. 



Microscopic Structure of Nervous Tissue. — Examination 

 of a piece of nerve under the microscope shows it to be 



