TROGLODYTES GORILLA 657 



gorilla stands next to that ol man in proniini'iUT. \ icwcd as an index of the 

 differentiation In t lie cerebral heinisphercs, it signiiies an cndbrain organi- 

 zation highly advanced ni its deveiopnuiit. 



Othhk Structures i\ the Midbrain. A deep Ixisilar groove extends 

 cephalad and grachiallx widens toward the cejjhahc margin of tlie pons 

 Varohi. At this point the divergence in the basis of tlie midbrain begins, the 

 cerebral peduncles become apparent, and a huge oi)tico-i)eduncular space is 

 disclosed. This space contains the mammilhuv bodies, the attacliment of the 

 mfuncbf)ular stalk and the tuber cmereum. It is bounded at its cephalic 

 extremity by the optic chiasm and the di\erging optic tracts. Upon the 

 dorsal surface of the midbrain appear the usual specializations of the 

 ciuadrigeminal j)lates, lorming the su[)enor and the inferior colliculi. Of 

 these, the superior colliculi are considerably larger but both sets of eleva- 

 tions ha\'c lost in the prominence ol their surface relief. The sulci between 

 them, especiallx the transverse intercollicular sulcus, are less defined than in 

 the intermediate piimati'S, while the cephalic extremity of the longitudinal 

 intercollicular sulcus shows a much wider separation to form the j)ineal 

 fossa. Upon the lateral surface of the midbrain is a fairly well-outlined mesial 

 geniculate body. 



Internal Structure of the Brain Stem in Gorilla 



As in all of the primati's, the le\els ujjon which these descriptions are 

 based show the critical changes in the brain stem. 



level of the P'iRAMIDAL DECUSSATION AND THE CAUDAL EXTREMITY OF THE 

 DORSAL NUCLEI (FIGS. 297 AND 298) 



At these liAcIs the sections have the characteristic aj^pearance incident to 

 the decussation in the motor jjathway with the introduction into the dorsal 

 field of nuclear masses for the relav of sensorv stinuih to the higher levels 



