THE ACOUSTIC COMPLEX OF THE OPOSSUM 411 



opossum brain, lying completely exposed posterior to the pons, 

 owing to the comparatively low development of the latter. It is 

 separated from the pons on the lateral aspect of the medulla by 

 an interval occupied by the entering fibers of the fifth nerve. 

 The fibers of the corpus trapezoideum follow the curve of the 

 surface of the medulla, slanting somewhat forward, fibers coursing 

 parallel and the whole tract bulging somewhat immediately below 

 the entrance of the eighth nerve as a result of lateral displacement 

 by the spinal root of the fifth. The main mass of the fibers is 

 anterior to the point of entrance of the cochlear nerve, and even in 

 the gross can be seen to be principally associated with the ventral 

 cochlear nucleus. The seventh and the N. vestibularis pass directly 

 through this thickest portion of the corpus trapezoideum. On the 

 ventral surface of the medulla, the origin of the lateral lemniscus 

 appears as a distinct rounded off-shoot from the anterior margin 

 of the corpus trapezoideum. It is easily identified in the unsec- 

 tioned brain. 



The internal surface of the corpus trapezoideum is shown by 

 reconstruction to present a median rounded elevation extending 

 across it from the posterior to the anterior border, where it be- 

 comes indistinguishable from the rounded internal surface of 

 the lower portion of the nucleus lemnisci lateralis. This eleva- 

 tion corresponds to the nucleus corporis trapezoidei and especially 

 in the more prominent posterior portion, to the nucleus olivaris 

 superior. Into the summit of this mass are received the fibers 

 of the olivo-cochlear tract. Attention should be called to the 

 close relation between the nucleus of the superior olive and the 

 nucleus of the seventh nerve, which lies immediately posterior 

 to it, and in actual contact with it. Anteriorly, the bifurcation 

 of the superior olivary nucleus with the formation of median and 

 lateral lobes is better shown in the sections (fig. 7). From the 

 more anterior portion of the superior olive springs also the oli- 

 vary peduncle already mentioned. 



The continuation of the cochlear conduction path into the mesen- 

 cephalon by way of the lateral lemniscus appears in the opossum 

 as a dense mass of fibers, springing from the anterior border of 

 the corpus trapezoideum. This tract, which at its origin is about 



