TROGLODYTES NIGER, THE CHIMPANZEE 



60 1 



delegation to hi<i;hcr and more cHicieiit aix'as of s\ lUhcsIs in tlic cerebral 

 cortex. 



LE\EL Ol- THE SVPEKIOK COLLICULUS (EIG. 268) 



At this lc-\c'l two niodilicat lolls Ikinc occurred to dist in<j;uisli tlu' section; 

 first, tlie separation ol [\\r basal region of the axis to lonii the two cerebral 

 pechincles (CP) with the proiiuction of an interxal betwrt'ii them the 

 optico-peduiu'ular space; second, in a dorsal position, the a])pearance of the 

 two cephalic eliAatioiis ol the ciuatln^cniinal plate constituting tin- supt'rior 

 colliculi (SC). These striicturi's ari' associated with tln' \isual pathway 

 and are the \estigial remnants of lormcrlx ]jroniincnt structures lorniina; the 

 optic IoIk's. I heir luiK'tional si^nilicance, as in all tlu' i^rimates, arises 

 Ironi xhv lact ol tluir decline m structural promiiu'iicc and the attendant 

 diniiiuition in lunctional miportanct' with rererence to \ision. The same 

 process in\ol\cd in the diilerentiation of the inferior colliculus has been in 

 progress in this portion ol the midbrain. A [)rogressi\e delegation of \isual 

 lunction to in'w and more expansible art-as of the cerebral cortex has pro- 

 ceeded, until nearl\ all ol the \ isual acti\ities ha\-e been acKanccd to the 

 occipital lobes. The su|jerior colliculus, howi'\fr, still bears some e\idence 

 oi its tornur dominance m \ ision and retains a few of the original fourteen 

 layers olthis portion of llu' tectal cortex. At best the diilerentiation of these 

 layers is indelinite and indicati\-e ol an area in j^rocess of decline. 



The central gra\ matter (Cen ) cntirel\ surrounds the S\l\ian ac|ueduct 

 and in its central region is elongated lor the spt'ciali/.ation of the large 

 oculomotor nucleus ( Noc ) whit'h gi\-es rise to the libers of tlu- oculomotor 

 nerve (N3). Fibers arising in this nui'leiis make- their wa\ forward to 

 emerge Irom the brain stem 111 the optico-|jeduncular sjxice from the oculo- 

 motor sulcus. 



