288 THE NERVOUS SYSTEM 



the cheek is flaccid; the lips are immobile and saliva may flow 

 from that corner of the mouth; the buccinator is paralyzed, and 

 there is often great difficulty in mastication because of the accu- 

 mulation of food between the cheek and the teeth; the unop- 

 posed action of the muscles of the opposite side greatly distort 

 the facial features, the affected side being quite expressionless. 

 Facial monoplegia is common; facial diplegia is very uncommon. 

 The Chorda Tympani. This branch of the seventh is con- 

 cerned especially in gustation. The fibers of which it is com- 

 posed undoubtedly come from nerve of Wrisberg. Section of 

 the seventh involving also the nerve of Wrisberg causes not only 

 facial palsy but also a loss of the sense of taste in the anterior 

 two-thirds of the tongue. The sense of taste will receive later 

 notice. 



Eighth Nerve (Auditory) . 



Origin. This is a nerve of special sense. Its apparent 

 origin is by two roots one from the groove between the olivary 

 and restiform bodies at the lower border of the pons, the other 

 coming around the upper end of the restiform body to join the 

 first in the groove. The deep origin of the two roots is different. 

 That of the median root is the dorsal auditory nucleus in the 

 floor of the fourth ventricle; that of the lateral root is mainly 

 from the ventral auditory nucleus in front of the restiform body 

 between the two roots. 



Course and Distribution. Crossing the posterior border of 

 the middle peduncle of the cerebellum, it enters the internal audi- 

 tory meatus in company with the facial nerve and the nerve of 

 Wrisberg. At the bottom of the meatus it receives fibers from 

 the seventh, and divides into branches which pass to the cochlea, 

 semicircular canals and vestibule. 



Function. This nerve receives and conveys to the brain im- 

 pressions produced by sound waves; it is the nerve of hearing 



