THE ACOUSTIC COMPLEX OF THE OPOSSUM 421 



below it, corresponds to that noted in the general description. 

 Externally it is of course entirely concealed from view by the 

 middle and posterior cerebellar peduncles and the overhanging 

 mass of the great flocculus-like lateral extension of the opossum 

 cerebellum itself. Internally the point of origin of the commis- 

 sure from the superior vestibular nucleus, in association with 

 fibers from the sensory nucleus of the fifth, is suggested in fig. 4. 

 The slender anterior projection of the medial nucleus, which is 

 also suggested, and its connection will be referred to in discussing 

 the sections. 



The lateral nucleus of the vestibular nerve as contrasted with 

 the superior, is easily recognized and outlined, owing chiefly to 

 the highly characteristic large ganglion cells of which it is com- 

 posed. It lies opposite and above the point of entry of the 

 nerve and corresponds in its apparent division into two parts, to 

 the outline description already given. The continuity of the two 

 can be readily made out, however, in the horizontal sections 

 especially. The upper part of Deiter's therefore appears as a 

 bulge in the side of the nucleo-cerebellar tract while the central 

 part lies within the ascending limb of the spinal tract itself, and 

 the lower forms a tongue or wedge-like piece inserted between 

 the olivo-cochlear tract and the descending vestibular root. 



DISCUSSION OF SECTIONS 

 (figs. 5-14) 



The closing part of this paper is devoted to a discussion of 

 the sections selected as illustrative of the topography of the two 

 divisions of the central acoustic complex in the opossum, as 

 previously discussed. While the sections have also a general 

 interest from the standpoint of other groups of structures in the 

 medulla, the intention in the present case is to confine the discus- 

 sion to salient points connected with the eighth and incidentally 

 the seventh nerves, and to direct attention to their bearing on the 

 models and to particulars in which they fill out deficiencies in the 

 plastic work. 



