INTERNAL STRUCTURE OF THE BRAIN STFAI 1025 



locdiiiotion. Tliiis, till' priniatr brain, through its i;am,s 111 nrokiiu'tif expan- 

 sion, bt'i^an to sliow tliosi' real accessions \sliicli arc sought lor in \ain 

 throughoLit all other orckas ol nianinials. ^ et the priniati's weri' not without 

 tlu-ir own serious t'lnbarrassnicnts. The ([uadrunianal cle\ t'lopnient so charac- 

 teristic ol all simian tribes giNcs neither the hand nor the loot its lull or ample 

 opportunity lor ultimate dillerentiation. Arboreal hie im]>osed a severe 

 haiulicap in that it demanded m upper and lowi'r t'\tremit\ alike a high 

 degree of j^rehcnsilc s])eciali/ation lor locomotor lunction. On tlu'other hand, 

 tlu' progressi\'e acKance in tlu' neokinetic indices ol the brain, t-speciall\' in 

 the pyramid, tin- pons \ arolii and the cerebral |:)ed uncles, disclost's the manner 

 in which an excpiisite arboreal speciali/ation has been slowly superseded, 

 first by si-mi-arboreal adaptation, then In imperlectl\ organized terri'strial 

 life, with the gradual assumption of the erect posture and the almost exclu- 

 sive adoption of the surface of the earth for transportation. DLiring this 

 process tlu- hand became the su|3reme agent lor externalizing the lull t'xpres- 

 sion of these ad\ances. 



Othek SiKLCTiRES IN Till: Bhain Ste.m Indicaii.ng Necjkinetic 

 Development in iiil; Phi\iait:s. TIu' insistenci' of neokinesis to iind 

 adec|uate expression is again and etpiallx well illustrated b> structural extt'ii- 

 sion of certain t-lemcnts functionally connected with the maintenance ot 

 coordination. From the standpoint of \olumetric cx])ansion and progressive 

 adx'ancc in organic delinition, thtae is no structure in the brain w Inch appears 

 to be moresusct'ptibleto the inlluences of adaptation than thedentate nucleus. 

 Its plan imetriccocHicient, which in marmoset is "."percent and in man i ".6 per 

 cent, shows a progressive gain ol about 10 hundredths in \ohime. Even more 

 proiiouiux'd is its increasing discreteness as a lUuK'ar structure. \\ ith t lu- pos- 

 sil^Ie exct'])tion ol the inlenor oii\ai"\ nucleus, the nucleus (kaitatus is the most 

 striking example ol ]jrogressi\e delinition m the entire brain stem. Startingas it 

 does in a dillusi', irngular nuchar aggregation, it passes tlirough many 



