THE ACOUSTIC COMPLEX OF THE OPOSSUM 423 



nucleus, and the medial nucleus is apparent, and the surface 

 markings in the floor of the ventricle outlining their position can 

 be inferred. Internal arcuate fibers are abundant in all sections 

 involving this part of the vestibular complex, strands of them 

 emerging from between the fasciculi of the descending root and 

 passing toward the median line, or more directly downward among 

 the bundles of the formatio reticularis. An interesting group of 

 fibers is indicated by the letter A. This group appears in a 

 series of eight or ten sections of which the most anterior ones 

 begin to show the fibers of the corpus trapezoideum, from which 

 however, it appears to be entirely distinct. The apparent origin 

 is among the fasciculi of the descending vestibular root, and its 

 termination in the nucleus of the seventh nerve, at whose lateral 

 border the fibers scatter. Such scattering of course may be due 

 to the interposition of an obstacle, and may not indicate an inter- 

 ruption in the path. I have not been able to find in von Kolliker's 

 or Cajal's descriptions or figures any reference to such a bundle 

 among the connections of the seventh nerve. It is of course 

 possible that this represents a collateral of the vestibular system 

 passing by way of the medial lemniscus to other levels in the mes- 

 encephalon or the cord. This section also shows the position and 

 bilobed form of the seventh nerve nucleus, and the characteristic 

 arrangement of the fine strands passing upward toward the genu. 

 Throughout this and other sections more or less characteristic 

 differences in size of ganglion cells may be made out without 

 special staining. Those of the medial and spinal vestibular nuclei 

 for example are quite small, those of Deiter's nucleus or the 

 nucleus of the seventh nerve very large. The two latter are so 

 conspicuous as to make the nuclei in question recognizable at 

 first glance. 



Fig. 6. Series A, slide 93, row 2, section 1 . This and the follow- 

 ing section are through the most complicated part of the opossum 

 medulla, and present especially interesting pictures. This sec- 

 tion is taken through the middle of the radix N. vestibularis and 

 the genu of the facial nerve. The anterior part of the ventral 

 cochlear nucleus is still prominent, but the tuberculum is repre- 

 sented only by a bit of grey matter from the anterior end. The 



