CHAP, ii.] THE BRAIN. 959 



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 striae acusticae, and 

 overlapping behind the front parts of the nucleus ambiguus and 

 the combined accessory- vago-glossopharyngeal nucleus ; it extends 

 laterally some distance outside the former nucleus. 



The nucleus however consists of two distinct parts, a median 

 or inner nucleus (Fig. 115, VIII. m.), characterized by the presence 

 of small cells, and a lateral or outer nucleus (Fig. 115, 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 forwards (Figs. 110 

 and 111, VIII. /3), 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 ganglion on the posterior 

 root of a spinal nerve. This is called the accessory nucleus. 



When we trace the fibres of the nerve centralwards into the 

 brain, we find that a large number at least of the fibres of the 

 dorsal root, cochlear nerve (Fig. 110), end, according to most 

 observers, in the cells of the accessory nucleus, or in nerve cells 

 lying dorsal to the accessory nucleus and especially in a group of 

 cells giving rise to the tuber culum acusticum, which, small in man, 

 is conspicuous in some animals. Hence the farther part of this 

 dorsal root as it winds round the lateral and dorsal surface of the 

 restiform body, consists largely, if not wholly, of fibres which are 

 derived not directly from the trunk of the nerve, but indirectly 

 through the relay of the accessory nucleus or of other cells. 

 Reaching the dorsal surface of the restiform body, these fibres 

 appear on the floor of the fourth ventricle as the striae acusticae 

 (Fig. 108, sir), and end partly in the median nucleus, partly in 

 other regions of the bulb. The exact determination, however, of 

 the endings of this root is a matter of considerable difficulty ; 

 some observers regard the accessory nucleus as homologous, not 

 with the Gasserian and ,with the spinal ganglia, but with the 

 other, true, cranial nuclei ; and in any case we must probably 

 consider the median division of the auditory nucleus, not as a 

 nucleus in the sense in which we are now using it, but rather as 

 a secondary connection within the bulb. 



F. 61 



