THE NERVOUS SYSTEM 529 



VI. Abducens. The roots of the abducens nerve in man emerge from 

 the ventral side of the medulla near the posterior boundary of the pons. 

 In lower vertebrates it is a meta-otic nerve. Its somatic efferent fibers 

 come from a nucleus in the somatic motor column near the midventral 

 line of the medulla, and innervate the external rectus eye muscle. The 

 eye muscle nerves (III, IV, VI) are an especially conservative group, 

 which, like the muscles they innervate, differ little from fishes to man. 



VII. Facialis. The facial is a mixed nerve with two roots. Its major 

 root is purely motor, the fibers, which belong to the special visceral motor 

 group, innervating the superficial facial and scalp muscles, the posterior 

 belly of the digastric muscle, and the stylohyoid. Its motor nucleus lies 

 lateral and posterior to that of the abducens, the fibers of the facial looping 

 around those of the abducens to form the genu of the facial. 



The minor root of the facial forms the glossopalatine or intermediate 

 nerve of the facial. Most of it enters the chorda tympani nerve, which 

 consists of special visceral sensory fibers from the anterior taste buds of the 

 tongue. Some general visceral efferent fibers innervate the submaxillary 

 and sublingual salivary glands. The sensory fibers of this nerve come 

 from cells in the geniculate ganglion. With the disappearance of lateral 

 line organs in land amphibians, the seventh nerve loses its special somatic 

 sensory (lateralis) components. 



Vm. Acusticus. The auditory nerve is relatively short, with two 

 branches, cochlear and vestibular, which have their root or origin near 

 that of the facialis. The fibers which develop from the spiral and ves- 

 tibular ganglia belong to the special somatic sensory group. The cochlear 

 is the nerve of hearing; the vestibular, of equilibration. The dendrites 

 of both receive their nervous impulses from receptor hair-cells. Their 

 central connexions are with the cochlearis and vestibularis nuclei in the 

 oblongata. The centers of hearing, however, are located in the temporal 

 lobes. Ganglion cells of both nerves are primitive and unique among 

 amniote afferent neurones in being bipolar. 



IX. Glossopharjmgeus. The glossopharyngeal is a mixed nerve which 

 contains special and general visceral fibers, both sensory and motor. Its 

 roots are posterior to those of the acusticus and in line with those of the 

 vagus. The sensory fibers come from the petrosal ganglion and are 

 distributed to the posterior part of the tongue and to the pharynx. The 

 central nucleus is that of the tractus solitarius which lies between the 

 cochlear and vestibular nuclei. The secondary cortical connexions and 

 the centers of taste are unknown. In some fishes the glossopharyngeus 

 receives special somatic afferent fibers from the supratemporal line of 

 sense organs. 



The motor fibers of the glossopharyngeus come from the inferior 

 salivatory and ambiguus nuclei in the medulla. The general visceral 



