THE BRAIN OF PREHISTORIC MAN 88i 



of which were reflected in the hrain. Standing u[)riti;ht in itself rec]uires a 

 complex sensorN' iiieehanisni to reeei\e and adjust the inipidses from the 

 proprioce])tors. How intricate and esst'iilial this mechanism is ma\ he seen 

 in those diseases characterized h\ patholouical changes m the sensor\ s\ stem. 

 Locomiitor ataxia is an oiitstandmii; examjjlc. Here the disease is confined to 

 the sensory elements hut exprcssi's itself pret'mmeiit l\ in t he motor adjust- 

 ments of standintj; and walking. 



To what extent sensory orientation ol the hody is essential to proper 

 x'oluntary mo\c'ment is sc'cn m such pat hological condit ions as acragnosis, 

 in which the patient loses his limh sense to the degree that iu' may not 

 appreciate the positions ol his extrt-mitics or recognize passixe move- 

 ments ol them. The adec|uate sensing of the dillerent parts of the hody in 

 rest and action is essential to kinesthetic sensihility and the mdispensahlc 

 phj'siological hasis of all xoluiitary movements. Tlu' transactions of this 

 highlyspeciaiized sense mv thelunction ol the j^arictal lohe. As they hecomc 

 more extensi\e and complex, the parietal area has expanded to meet the new 

 demands mack" upon it. Throughout the long history of primate adaptation 

 and progress, the parietal lohe lias manifested the most consistent expansion. 

 It has espcciafly seemed to keep pace w ith the progressive tendency to assume 

 the erect posture, to (k'\elop |)lantigrade locomotion, to acquire himanual 

 characters. If the [)arietal expansion is a reliahle index to the e\()hition of 

 kinesthetic seiisihilit \ , then this essential attrihute of \()luntary nioxcment 

 became more extensive in direct proportion as the u[)per extremity was 

 emancijjated to perform the duties of the human liand. A sim|)le exam])le 

 may suffice to elucidate this point. The indi\idual digits m the ])aw of a cat 

 or dog have not acquiri'd independent mo\ements similar to fingers. Their 

 sensory representation m the hram is consec|uently much less, and requires 

 less cortical area. The man\ indi\ idual mo\emeiits of the fingers lia\e nvcd 

 of much more cortical surlace lor their sensor\- orientation. It would seem to 



