TROGLODYTES NIGER, THE CHIMPANZEE 5-5 



organ. 1 he roots ot the tn^H-iiiinal iH-r\r come into relation w itli tlir pons 

 Varolii at tlu' point ol transition bttwrcn it and t lu' micklk- ci-rchfllar |)(,-(kin- 

 clc. Tlicsf libers pciU'tratr the |)ontiU' strata and make their wax to the teg- 

 mentum ol tin- axis. I hv ix'phalie extremity of the pons is marked by a well 

 deiiiu'd ponto-pediineular siikus separating the pons Varolii from the eerebral 

 pedimele. In tlu' midline this point of si'paration is I'urther emphasized b\ 

 till' extt'iision bi'iieath the |)ons ol the blind ending ol the- optieo-peduneular 

 spiice, the loramen eaeeiim antieum. 



The MiDiiKAiN. I lu' midbrain presents upon its dorsal surface the 

 usual specialization ol the c|uadrigi'minal plate. This consists of two sets of 

 colliculi corresponding to tlu' structures already obserx cd in all primates in 

 relation to the sense ol heai'ing and the si'iise of sight. The inferior colliculi 

 are less prominent than tlu' superior colliculi, and both ha\f uiukagone an 

 a|:)preciable decrease m their elevation abo\f the roof of the midbrain. 

 Longitudinall\ , the intercollicular sulcus at its cephalic extrcmit\ becomes 

 expanded to lorm tlu' pineal lossa, and it is particularl\ m this region that the 

 superior colliculi show their greatest attenuation. Judgt'd l)\ tlu'se marks of 

 identilication it seems clear that the chimpan/.t'c is less will i'C|uipped in tlu' 

 mechanism by means ol whuh it cxecuti's thost' immediate rellex react ions in 

 response to the stimuli ol hearing, and t hat the process of telenci'phalization 

 has adxa need much lurtlu'r than in tlu' inti'rmcdiate primates. 



The lateral surlaci- ol tlu' midbrain presents the ele\ation of the mesial 

 geniculate bod\ which ma\ bi' seen connected with tlu' inferior colliculus bv 

 means ol a iairly w ill-deline<l brachium i'on|iincti\Lim jjosticum. I lie 

 xcntral surlace ol the midljrain is characteri/.ed bx the ajipearance of the 

 optico-peduiHular spai'c, upon I'lthcr side ol which tlu' cerebral peduncle is 

 dixergmg to enter the hemisphcix'. I he ci'phalic boundarx' of this space is 

 lormed [)\ t lu' dixerging optic tracts and tlu- oj)t ic chiasm. The space contains, 

 as in all other forms, the point of emergence of the oeulomutor nerxe, the 



