THE BRAIN 01- PREHISTORIC MAN 877 



Three somatic tactors lia\(.' \)vvn \\v\d especially respmisihlc lor this Increase: 

 (irst, tlic cle\ elopnieiit ol tlie huiiian or luiiiianoicl loot; second, the assump- 

 tion ol the erect posture, and thiixl, the emancipation ol the hand Irom loco- 

 motor lunctions. 01 tliese lactors, [Iw third has perhaps the <i;reatest cogency. 

 The increase in the jjarietal t'liimence iinoKes a cortical area assigned to 

 sensory perceptions ol the u|jper extrt'inity. 



THE OCCIPITAL LOBIi Ol PITHECANTHROPI S EKECTl'S 



The occipital lobe is little more productive ol cerebral landmarks than 

 the parietal. The occipital pole- ol the brain extends o\-er the cerebt'llum, w hile 

 the divergence ol the two hemis|)heres is jjronounced in this region. A bilat- 

 erally' symmetrical groo\e corrt'sponding to the trans\-erse sinus separates 

 the cerebellum from the cerebrum and thus establislu-s the caudal boundary 

 ol tile occipital lobe. No other boundar\' ol this lobe is discernible m the cast 

 nor does any indication appear either of a sulcus simiarum (sulcus lunatiis — 

 Smith) or parieto-occipital incisure. The demarcation between occipital 

 and parietal lobes is, therelore, \vholl\- conjectural. This latter indeliniteness, 

 however, does not impose an embarrassment more serious than is the case 

 with Homo sapiens, m whom the parieto-occipital dividing line is an arbitrary 

 one. It is probable that in tlu- human brain there is an intermediate, transi- 

 tional area between the parietal and occipital lobes where the cortical types 

 ol these two regions manilest a histological mutualitx'. This area permits 

 of a physiological blending in the acti\ities of the two /ones. In the apes and 

 more particularl\- m the great man-like apes, a sharply delined boundary 

 created by the sulcus simiarum exists between these two regions. No actually 

 homologous fissure appears in the cerebrum of modern man, although 

 several authorities maintain that the sulcus lunatus closely resembles the 

 simian fissure and ma\' be identilied in some human brains. 



In the occijjital lobe on the right side a long crescent fissure begins at 

 the superior longitudinal fissures. It curves downward and forward in the 



