--0 THE HIGHER ANTHROPOIDS 



this lissurc with the occipito-paric-tal fissuif. In the human brain, this deep 

 anncctent gyre is sul:)mergecl anti thus [)erniits the (leeij^ito-parietai sulcus to 

 give the appearanee of continuity in which its parietal portion incises the 

 lateral surface. This apparent independence of the two di\isions of the parieto- 

 occipital sulcus in the chinipanzee atlords some ground to doubt the actual 

 homology with the parieto-occipital lissurc m man. It also secnis to oiler a 

 reason for questioning whether the sulcus smiiarum has a representative 

 vestige in the human brain. 



THE BASAL SURFACE OF THE HEMISPHERE 



Upon the basal surface of the hemisphere, certain characteristic markings 

 indicate that the brain of the chimpanzee is more primitK'e than that ol the 

 gorilla. Thus the orbital conca\'ity, into which the orbital portion of the fron- 

 tal boiu' extends, is a particularly marked feature. This concavity indicates 

 a faihire on the part of the frontal lobe to expand at the expense of the orbit. 

 The olfactor\ Inilb and tract are detachable as far back as the trigonum 

 olfactorium, and a well-delincd gyrus rectus is demarcated by the presence 

 of an olfactory sulcus. The angulation at the chiasm, Ijoth w ith the optic 

 nerves and the optic tracts, is characteristic of all other simian forms, tend- 

 ing to be obtuse rather than acute as it is in most of the lower mammals. 

 The uncus is present on the mesial surface of the temporal lobe, and, although 

 well delined, is less prominent than in gorilla. The ccrcbt'llar concavity 

 in the occi[)ital region is mort' prominent than it is in the gorilla, jjarticu- 

 larly that portion of it which constitutes the postspleiiial lossa, into which 

 projects the highly ele\aled cephalic |jortion of the supi'rior \ermis oi the 

 cerebellum. In all of its features on the basal surlace ol the lu'inisphere the 

 brain of the chimpanzee approaches more closely that ol the intermediate 

 primates than of man, and a|)pears to be less acKancecl in this dirt'ction than 

 the brain of gorilla as judged by these crucial points of basal identilication. 



