TROGLODYTES NIGER, THE CHIMPANZEE 605 



geniculate bodies arc shown (Mgb, Lgl)), the hitter reeei\iii<j; libers making 

 their way inward through tlie optic tract on their way to the visual area of 

 the cerebral cortex. The gi-nicuhitc bodies, both mesial and lateral, apparently 

 have develojjcd in the intiTcst of that delegation of auditory and visual func- 

 tion to the cerebral cortex which is characteristic of the inaninialian brain 

 and have become progressivelx more conspicuous in the primate series. The 

 superior extremity of the colliculi is also seen and lateral to it another 

 relaj^ station connected with \ Isual biiiction, the pulvinar of the optic 

 thalamus (Pul). The section is particularl\ important because of the 

 clear definition of the red nucleus (NRu), thus offering another criterion 

 l)\ w hich to judge the degree of coordinati\ c specialization in the chimpanzee. 



LEVEL OF THE OPTIC CIHASM (pIG. 2-o) 



At this level the contour has been changed b\ tin- appearance of the 

 vertical third ventricle and the extensive lateral masses of tlu' thalami bound- 

 ing it upon either sich'. The continuit\ of the third \fntricle is interrupted 

 by the appearance at this level of the large commissura mollis. Tlie optic 

 thalami here are separated laterall\ from the globus pallidus of the corj)us 

 striatum (GIp) by the interposition of a dense mass of m\ el inizecl fibers 

 constituting tlu' internal capsLilc (Cin). This latter structure is the con- 

 tinuation ccphalad of tlu' cerebral peduncle and serves to establish the 

 linal connection with the cerebral cortex. In the internal capsule at this 

 le^•el arc libers of the |:)allio-p()nt()-cerebcllar system, and also ot the 

 pyramidal system. The most M'litral structure of the section is the optic 

 chiasm (Opx), along whose dorsal aspect passes a set of less densely 

 myelinized libers constituting the supra-optic decussation of Meynert. 

 The optic chiasm serves as the point of partial crossing in the visual 

 jxithway, receiving at its cephalic angles the two optic ner\es and being 

 continuous at its caudal border with the optic tracts. These latter together 



