THE GRAY MATTER. 605 



the middle nerve, the vagus, begin and end spring from three sources, the 

 combined nucleus, the nucleus ambiguus, and the ascending root. 



531. The eighth or auditory nerve. This nerve differs from the other 

 nerves which we are now considering, in being a nerve of special sense ; its 

 arrangements are complicated. In a view of the base of the brain- (Fig. 

 131, (?) the nerve is seen to leave the surface of the brain from the ventral 

 surface of the fore part of the restifbrm body at the hind margin of the pons 

 as two strands or roots, one of which winds round the restiform body so as to 

 reach its dorsal surface, while the other appears to sink into the substance of 

 the bulb to the median side of the restiform body ; and in a transverse sec- 

 tion of the bulb (Fig. 133), just behind the pons, the two roots may be seen 

 embracing the restiform body, one passing on its dorsal and the other on its 

 ventral side. The former is called the dorsal root (Fig. 133), or sometimes 

 the lateral root, or, since it reaches back or lower down than the other, the 

 posterior or inferior root; the latter is called the ventral root (Fig. 135), or 

 sometimes the median root, or, since it reaches further forward or higher up 

 than the other, the anterior or superior root. When we come to study the 

 ear we shall find that one division of the auditory nerve is distributed to the 

 cochlea alone, and is called the nervus cochlearis, the rest of the nerve being 

 distributed to the utricle, saccule and semicircular canals as the nervus ves- 

 tibularis. As we shall see, there are reasons for thinking that the vestibular 

 nerve carries up to the brain from the semicircular canals impulses other 

 than those, or besides those, which give rise to sensations of sound, whereas, 

 the cochlear nerve appears to be exclusively concerned in hearing ; and in 

 some structural details these two divisions of the auditory nerve differ from 

 each other. Hence it is important to note that according to careful inves- 

 tigations the cochlear nerve is the continuation of the dorsal root and the 

 vestibular nerve the continuation of the ventral root. With these roots of 

 the auditory nerve proper also issues, a little in front of the ventral root, the 

 small nerve called the portio intermedia Wrisbergi, which goes to join the 

 facial nerve. 



The auditory nucleus, as a whole, is a broad mass, having in transverse 

 sections of the bulb a somewhat triangular form, lying in the lateral parts 

 of the floor of the fourth ventricle, reaching in front somewhat beyond the 

 level of the strise acusticse, and overlapping behind the front parts of the 

 nucleus ambiguus and the combined accessory-vago-glosso-pharyngeal nu- 

 cleus : it extends laterally some distance outside the former nucleus. 



The nucleus, however, consists of two distinct parts, a median or inner 

 nucleus (Fig. 138, VIII., m.), characterized by the presence of small cells, 

 and a lateral or outer nucleus (Fig. 138, VIII., I.), the cells of which are 

 much larger, some of them being very large. The lateral nucleus is placed 

 somewhat deeper than, ventral to, the median nucleus ; it also extends farther 

 forward (Figs. 133 and 134, VIIL, /?), so that the front end of the whole 

 nucleus is furnished by the lateral nucleus alone, which at its front end 

 occupies a more dorsal position than at its hind end. 



Moreover, this auditory nucleus thus placed in the floor of the fourth 

 ventricle is not the whole of the nucleus of the auditory nerve. At the 

 convergence of the dorsal and ventral roots on the ventral surface of the 

 restiform body is placed a group of cells, forming a swelling which, in its 

 general appearance and in the characters of its cells is not unlike a gan- 

 glion on the posterior root of a spinal nerve. This is called the accessory 

 nucleus. 



When we trace the fibres of the nerve centralward into the brain, we find 

 that a large number, at least, of the fibres of the dorsal root, cochlear nerve 

 (Fig. 133), end, according to most observers, in the cells of the accessory 



