THE ACOUSTIC COMPLEX OF THE OPOSSUM 417 



the nucleus medialis N. vestibularis. From the median line to 

 the border of the spinal tract, the uniformity of the structure and 

 staining reaction are absolutely unbroken, there is not the slight- 

 est sign of a partition, the felt-work of fibers is characteristic 

 throughout, and even a significant dividing groove in the ventric- 

 ular floor is apparently lacking. If the nucleus intercalatus is 

 to be distinguished from the medial vestibular nucleus, it becomes 

 impossible to reconstruct the latter in the opossum from Weigert 

 preparations. The writer therefore followed what seems a legiti- 

 mate conclusion from this material, and included all the area 

 labelled as such in fig. 10, in the medial nucleus. 



A second feature of the opossum brain that has to be reckoned 

 with in the reconstruction of the anterior portion of the vestibular 

 complex, is the imperfect differentiation of the basal structures 

 of the cerebellum, especially the nuclei; and the confusing inti- 

 macy of the vestibular relations with these nuclei. The opossum 

 cerebellum has no dentate nucleus, a fact which probably has its 

 influence upon the primitive confusion of the base, since the func- 

 tions and connections thus differentiated fall presumably at least 

 in rudiment to the basal nuclei. The nucleus fastigii is a rather 

 indefinitely defined structure, the chief clue to whose identity is 

 its relation to the superior peduncle anteriorly. One other nuclear 

 mass, medial to the nucleus fastigii and below, can be recognized 

 in some of the sections. Figs. 6, 7 and 8 may serve to convey 

 some idea of the difficulties of reconstructing in this region. The 

 decussation in the roof seems to offer an exception to this rule, 

 its parts standing out with considerable clearness. 



A third point requiring brief mention in the same connection 

 relates to the intimacy of the association between the superior 

 nucleus of the vestibular nerve and the sensory nucleus of the 

 fifth nerve, below it. The commissural connection of Bechterew's 

 nucleus seems to' be associated with a decussation between the 

 two fifth nuclei, rather than with that of the superior peduncle as 

 described by Sabin ('01) for the human brain. In the cross sec- 

 tions, boundaries for Bechterew's nuclei cannot be assigned, but 

 the horizontal sections are somewhat more satisfactory in this 

 respect. 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 12, NO. 4 



