36 Anatomy of the Nervous System 



forward.^ It is quite distinct from the latter nucleus in 

 sections owing to the arrangement of the nerve-fibres, and 

 the character of the cells in the two centres differs (Pis. III., 

 IX.-XI.). 



Some of the secondary fibres from the dorsal nucleus pass 

 ventrally along the medial aspect of the ventral nucleus, 

 where they join those arising in the latter. These fibres form 

 a large tract which passes ventrally and a little anteriorly 

 over the surface of the medulla until it gets near the median 

 line, where many of the fibres become deeper in position, 

 breaking up into smaller fascicles and leaving only a thin 

 layer superficial to the pyramids. They decussate and pass 

 towards the lateral part of the other side of the oblongata in 

 a somewhat more diffuse condition than in the earlier part 

 of their course. Up to this point the tract is known as the 

 trapezoid body {corpus trapezoides), but it now changes its 

 direction, running anteriorly, and is given a new name, the 

 lateral lemniscus {lemniscus lateralis, lateral fillet) (Pis. II., 

 IX. -XIV.). In the human brain the trapezoid body is com- 

 pletely covered superficially by the greatly enlarged pons. 



The other secondary fibres arising in the dorsal cochlear 

 nucleus pass medially {striae medullares acusticae), dipping 

 some distance below the floor of the fourth ventricle in the 

 form of small scattered groups of arcuate fibres. About two- 

 thirds of these decussate {decussation of v. Monakow) and, 

 running forward dorsal to the superior olive, join the lateral 

 lemniscus. 



^There may be distinguished in the ventral cochlear nucleus: dorso- 

 medial, ventro-medial, dorso-lateral, central cell groups, and in the dorsal 

 cochlear nucleus: superficial, middle, and deep layers may be observed. 



The difference in the level of the two nuclei is greater in man, where 

 they do not overlap, the dorsal nucleus having been pushed back by the 

 enlargement of the restiform body and the ventral nucleus having become 

 imbedded in the side of the brachium pontis. Also, the dorsal nucleus 

 is relatively smaller and the ventral nucleus is relatively larger in the 

 human brain. 



