6oo THE HIGHER ANTHROPOIDS 



eating that its original and jjrc-clomlnant I'linction with rt-lcTcnce to the sense 

 of hearing has cleert'ased eoiisiderably in importance. Sueh recession is due to 

 the fact that \hv aiichtorx activities ha\e been taken o\er !:>>• a higher [portion 

 of tlie ner\()us system, namely, the temporal area of tlie cerebral hemisphere. 

 .Mucii emphasis lias been laid u[)on the significance of this portion of the 

 brain stem in its bearing upon the evolutional process w hieh has operated in 

 tin' interest of giving to the sense of hearing a more expansible and more 

 cooperative cerebral region for the elaboration of the auditory sense. The roof 

 oi' the midbrain was so placed as to be delinitel\ embarrassed by its topog- 

 rajjhy, since any expansion in the endbrain must necessarily be attended by 

 corresponding dexelopment of the cerebellum. The mesencephalic roofplate, 

 from which the inferior collicLdi arise, under such conditions would be caught 

 between two slowly expanding structures, namelx, the endl^rain in front and 

 the cerebellum behind. In any case, a delegation of midbrain function to the 

 hemisj)heres has taken place and the dimensions of the inferior colliculus 

 have correspondingly decreased. 



Considered in terms of functional signilicance, it appears that auditory 

 sense has become less important as represented in tlu' midbrain ot the chim- 

 panzee than it is in the lowi'r primatt-s. The inferior colliculi are connected 

 across tlu' midline l)\ means of medullated commissural libt'rs. The tegmen- 

 tum in this region of the midbrain is separated Iroin the basis by the inter- 

 jjosition ol' the mesial lillet (Mf ) and t lu' central portion ol the field is 

 occLipied b\- the decussation of the superior cerebellar peduncle (Spx) 

 ])re|)arator\ to its entrance into the red nucleus at a somewhat higher le\el. 

 In the basis of the section are man\ ol' the iharacteristic pontile leatures, 

 while on the \entral surface a det'pening of the median sulcus is about to 

 determine the sej)aration of the two haKt's of this region of the stem to torm 

 the cerebral pitluncles. This section is important because of its bearing upon 

 the function of hearing, particularly as thai function appears to be undergoing 



<■' i- ^- •(. V'-. 



■> A A AT: !.! ;.... 



, ■ /."■'*. 



