THE ACOUSTIC COMPLEX OF THE OPOSSUM 413 



The inferior colliculus of the opossum brain is a large and con- 

 spicuous mass of ganghon cells and fibers forming the wall and 

 roof of the posterior portion of the Sylvian aqueduct. It is of 

 a somewhat uncertainly defined egg shape, the long axis slanting 

 downward and forward at an angle of about 45°. It is divisible 

 into two parts, a medial and a lateral, the medial being the more 

 distinctly nuclear mass and the lateral the more fibrous portion. 

 The medial, nuclear division is roughly bean-shaped, tapering to 

 an indefinite wedge-like lower extremity that becomes indistin- 

 guishable from the central grey substance. The lateral division 

 contains the main bundles of fibers of the lateral lemniscus as they 

 pass to their termination in the nucleus colliculi inferioris and 

 to the decussation of the inferior colliculi. It also includes the 

 fibers of the brachium colliculi inferioris and those fibers of the 

 lateral lemniscus whose doubtful destination has already been 

 mentioned. This lateral portion of the colliculus may be regarded 

 anatomically as a capsule-like investment of the medial portion, 

 roughly triangular in shape, with its apex at the middle genic- 

 ulate body. The brachium colliculi inferioris, uncertainly defined 

 within the colliculus itself, becomes a well-marked group of 

 fiber bundles as it approaches the middle geniculate. The fibers 

 decussating between the two inferior colliculi are gathered appar- 

 ently from both nucleus and capsule. The decussation is in the 

 roof, well forward in the colliculus. 



The corpus geniculatum mediale so far as reconstructed, is 

 an oval mass perhaps one-fourth or one-fifth as large as the nu- 

 cleus of the inferior colliculus, which forms a distinct protuberance 

 on the external surface of the brain stem, somewhat below the 

 median horizontal plane. Like the nucleus colliculi inferioris 

 it has a partial capsular investment of fibers along its upper and 

 posterior borders, composed largely of strands from the brachium 

 colliculi inferioris, which enters it on its posterior medial surface. 

 Anteriorly and above are other investing strands, whose course and 

 distribution could not be determined. 



This completes the description of the general morphology of 

 the auditory conduction path as reconstructed from the opossum 

 brain. Certain points better brought out by the typical sections 



