406 JOHN H. STOKES 



Before proceeding to the detailed description of the models, 

 certain respects in which they represent conditions peculiar to the 

 opossum may first be pointed out. 



The typical situation of the cochlear nuclei in such brains as 

 those of the cat, rabbit and man, is largely external to the cor- 

 pus restiforme, the tuberculum being lateral to and somewhat 

 above the ventral nucleus, and largely overlying it. In the 

 opossum this is not the case. The tuberculum acousticum lies 

 entirely medial to the corpus restiforme, and all but the more 

 anterior part of the ventral nucleus is similarly placed. The 

 anterior portion of the ventral nucleus escapes from under the cor- 

 pus restiforme as it ascends into the cerebellum, and extends 

 downward and backward between the entering strands of the coch- 

 lear nerve. This arrangement of the nuclei of reception makes it 

 inevitable that the posterior half of the fibers of the N. cochlearis 

 should be obliged to pierce the lower part of the restiform body 

 on their way to the cochlear nuclei. If the opossum brain be 

 regarded as the more primitive type, it is not difficult to conceive 

 that with the crowding of the primary structures in the medulla 

 by the relative increase in size of the cerebellar peduncles, the 

 inferior olive and the pons in higher types of brain, the nuclei of 

 reception of the cochlear apparatus have been forced downward 

 and outward along the root of the cochlear nerve and over the top 

 of the corpus restiforme to their present superficial position. 



Discussion of the relations of the dorsal cochlear nucleus 

 calls attention to a second structure in the opossum brain that 

 requires special mention in the description of the model. Essick 

 in 1907 described for the human brain a grey mass on the lateral 

 surface of the medulla in the region of the seventh and eighth 

 nerves, of which he had been unable to find any account in the 

 literature. This he designated the corpus ponto-bulbare, and 

 called attention to its connection with the pons. In his histo- 

 logical description mention is made of certain fibers from it which 

 join the striae acousticae. No other connection with the coch- 

 lear apparatus is noted. Streeter ('03) in the figures accom- 

 panying his description of the floor of the fourth ventricle labels 

 this mass the nucleus tela chorioidea inferior, presumably from 



