THE BRAIN OI- MODERN MAN 783 



timud tiansverscly hut a short distance, usually on the average of about \2 

 nmi. The line connecting this portion of the fissure and ihv prc-occipital 

 notch establishes an arbitrarx boundary between the occipital lobe bt'hind 

 and the temporal and |)arii'tal IoIjcs in front. In all of the anthrojjoids this 

 external portion of the parieto-occipital lissure is concealed in the sulcus 

 siniiaruni which interxx-nes between tlu' parietal and occipital lobes. It is 

 doubtful wluther any lissure, e\en as a vestige, in the human brain may be 

 homologized with the simian sulcus. Some authorities, however, maintain 

 that it is ])resent in the majoritN of human brains as a cur\ed sulcus, called 

 the sulcus occipitalis lunatus, situated on the lateral aspect ol the occipital 



lobe. 



LoBATioN OK THE Bkain. Lobatiou of the brain is not so clearly dcfiiu'd 

 as in the anthropoids. Tlu' boundary line established l)\ the Rolandic fissure 

 separates the [xirietal from the frontal lobe, while the Sylvian lissure sepa- 

 rates the frontal and parietal lobes respectively from the temporal lobe, and 

 tlu' arbitrar\ line from the parietal portion of the parieto-occipital sulcus 

 to the ])re-occii)ital notch in <ieneral is presumed to indicate the anterior 

 limit of the occipital lobe. In all four lolx-s discernible upon the lateral con- 

 vexit\- of the human brain, the most impressi\e feature is the richness ol 

 coinolution and of lissural pattern apparent in the frontal lobe. As compared 

 with this corresponding region in all of the great anthropoids and all otiier 

 primates, the comph'\it\ in coinolutional design is so striking as to con- 

 stitute perhaps the chiei" structural dillerencc between man and the upcs. 

 The greater degree of frontal de\x'lopnu'nt appart'iitls has mo\ed thesujx'- 

 rior e\trt-mit\ of the- cent ral sulcus further toward the occipital pole. In man 

 a little less than one-hall' the lateral aspect of the hemisphere is occupied by 

 the frontal conxolutions. The highest anthropoid fraction is one-third. By 

 actual planimetric measurement the frontal area in man covers 4" ])i.r cent 

 of thi' intire lateral surface, in chimpanzee 33 per cent, in gorilla 32 per 



