THE ACOUSTIC COMPLEX OF THE OPOSSUM 407 



its close association with the choroid plexus of the fourth ventricle. 

 It is mentioned but not described in the text. It has been 

 repeatedly figured in atlases of the brain, but apparently without 

 immediate connection with the tuberculum acousticum. It is 

 easily seen in the gross in the brain of the pig embryo, appearing 

 as a welt-like strip passing from beneath the ventral lip of the 

 lateral recess, downward and forward around the lateral aspect 

 of the corpus restiforme between the roots of the seventh and 

 eighth nerves, forward to the caudal border of the pons at the 

 level of entrance of the fifth nerve. It is continuous dorsally with 

 the tissue from which the ganglion masses of the eighth nerve, and 

 especially the tuberculum acousticum, originate. In the opos- 

 sum, evidences of connection with the tuberculum acousticum are 

 even more decided (figs. 2, 3 and 5). In the gross it appears as a 

 prominent band beginning at the caudo-lateral border of the 

 tuberculum acousticum and extending over the restiform body 

 downward and forward over its lateral aspect, then almost hori- 

 zontally forward, innmediately above the roots of both the seventh 

 and eighth nerves, to the pons, where it merges with a capsule of 

 grey matter high up on its posterior border. Mention of its 

 appearance in the sections will be made below. The homology 

 between this structure and the one described by Essick may not 

 at first sight appear entirely undoubted, owing to a discrepancy 

 between the positions of the mass in relation to the seventh and 

 eighth nerves in the opossum and in higher mammalian brains. 

 The writer is of the opinion however, that such a difference is 

 readily explained by displacements due to the greater develop- 

 ment of the pons in the higher types, and by the difference in 

 position of the cochlear nuclei in the latter as compared with the 

 former. 



A third distinctive feature of the cochlear apparatus in the 

 opossum relates to the course of centrally directed fibers from the 

 cochlear nuclei. The close relation of the corpus trapezoideum 

 to the ventral cochlear nucleus is easily made out in the sections 

 and shows quite plainly in the models. This constitutes the ven- 

 tral conduction path, which however, is not confined exclusively 

 to axones from the ventral cochlear nucleus. The dorsal con- 



