TROGLODYTES GORILLA 653 



Tlir expansion ol tiic lateral lobes of the cerebellum shows itself even 

 more conspicuously upon the occipital surface of the organ where the bulging 

 due to increase in size has produced two elevations. These rise above the 

 inferior \ermal j;)()rtion ol" the cerebellum and cause it to sink deeply in a 

 mesial depression, the vallecula. Lying in this positifxi the itiferior vermis 

 appears relati\cly insignilieant w hen comj)ared with the two massive lateral 

 lobes upon either side of it. II the lateral lobes are separated, two para- 

 median sulci may be discerned Interruptnig the eontnuiity of the folial fissures 

 as they pass Irom the vermis in the direction of either lateral cerebellar 

 expansion. This sulMiiergence ot the inlerior \ermis in the \ allecular dej)res- 

 sion is a characteristic seen only in the higher anthrojjoids and man. It is one 

 of the most significant indications ol the expansion m the lateral lobes of the 

 cerebellum. Nothing demonstrates more clearly the potential expansibility 

 of the cerebellum than these lateral extensions. Prom this it appears that the 

 rapidly increasing functional demand lor augmented cerebellar control is 

 made, not upon the primordial and more rigidly fixed central portion of the 

 organ, but upon those plastic extensions of it which constitute the lateral 

 lobes. The expansion of these cerebellar jiortions, m conjunction with 

 simultaneous expansion of the cerebral hemispheres, indicates the funda- 

 mental nature of the Ijond existing between these two structures. 



The jK'troso-ventricular surface of the cerebellum shows no great varia- 

 tion in its adjustment to the petrosal j^ortion of the temporal bone and the 

 roof of the fourth ventricle. At the cerebello-pontile angli' there is a fairly 

 vvell-defincd but not conspicuous flocculus. The other markings correspond to 

 all of the lower forms. This surface, therefore, represents a phyletically more 

 fixed region than is the case with either the tentorial or occipital surface. 



THE BRAIN STE.M 



The Oblongata. The external appearance of the brain stem in gorilla 

 gives the impression of an increasing definition in the outlines of all impor- 



