854 ^lAN 



between the dorsal and ventral aecumulations of gray matter. At its junction 

 with the fourth ventricle the cochlear nucleus shows its greatest dimensions, 

 the dorsal nuclear subdivision bemg eontmued mesially under the lloor ol the 

 fourth ventriek', separating tlie subependymal gray matter of the fourth 

 ventricle ior a considerable distance from the dorsal surlaee of the vestil)ular 

 complex. 



The CoLLicuLi 



These masses of gray matter appear to be relatively insignificant in the 

 reconstruction oi the gray matter of the human brain stem. In particular 

 the inlerior colliculus is represented by a relatively small collection ol nuclear 

 material. It rests upon the cephalic continuation of the deep layer of the 

 reticular formation in the mesencephalon, this nuclear matrix separating the 

 colliculus Itself from the gray matter surrounding the aqueduct of Sy!\ ius. 

 The colliculus itself is made up of a number of layers of gray matter, the most 

 definite of which is the external layer which is represented in the reconstruc- 

 tion. The interior ol the colliculus is formed very largely of white fibers and as 

 these fibers approach each other and coalesce they form a fibrous core for the 

 colliculus. Arising from the lateral aspect of the colliculus is a bundle of libers, 

 the inferior brachium, which leaves the eollieulus and passes toward the 

 mesial geniculate body, producing a deep groove and in some places a tunnel 

 in the lateral surlaci- of the mesencephalic reticular formation. The inferior 

 colliculus is supported laterally by the lateral extension of the reticular for- 

 mation while mesi;ill\' it rests upon the reticular formation forming the dorsal 

 raphe ol the mesencephalon and a group of libers which emerge from the 

 colliculus and cross to the opposite side as the commissure of the inferior 

 colliculus. It is separated cephalicallx from the superior eollieulus 1)\ a dorsal 

 extension ol the reticular formation. I his dorsal prolongation arises liom the 

 deep layer of the mesenee|ihalic rt'tieiilar lorination extending dorsally and 



