STRUCTLR.\L CULMINATION 975 



lateral lobes represent coordinative control of the upjjer and lower extrem- 

 ities, the reason for this expansive srrow-th in the primate order becomes 

 clear. In prop>ort!on as the hands an^. cct are more capable of precise 



and complex ^ is essential to their op>eration that they be 



corresp>ondingIy regulated by accessions in coordinative control over the 

 muscles. 



EVOLVTIONAL SIGNIFICANCE OF St.H £5 IN THE BraIN StEM OF 



Primates. In many details, the surface of the brainstem reveals aprogressive 

 differentiation. The features up>on its ventral surface all pertain to the acqui- 

 sition of that new motor system which makes its first app>earance in mammals. 

 This system is important from the fact that it introduces the controlling 

 influences of the nc a. It brings into play a vast series of associational 



combinations ^" : ovide motor actr^itA' with a more extensive range of 



selective op; ry. The evidence of this progressr\"e expansion is seen in 



the presence of three prominent structures on the ventral surface of the 

 brain stem, the pyramid, the p>ons a; .erebral peduncle. At the lower 



extremity- of the series, all three of these structures are feebly marked. 

 Employed as a key to the combinations of the animal's adaptabilit\', they 

 indicate to what extent the lemur, for example, is limited in its behavioral 

 capacrtA". I: respects the cerebral p>eduncle, the f>ons \ arolii and the 



pyramid of ~ :t slight advance over the still lower mammals. 



Proceeding in the < : ection toward the upf>er extremhy of the primate 



group, it is evident almost upon casual insp>ection that these elements become 

 increasingly more prominent. The marmoset, jjerhaps, adds but little to this 

 developmen-. cfinite advance is clear in all of the Cebidae. In the 



old-worl <eys, such as baboon, macac: gibbon, a still further 



advance is demonstrable. The first actually decisr^e change in the prominence 

 of these structures app>ears in the higher anthrof>oids. In them the pyramid 

 and pyramidal decussation begin to stand out more conspicuously". The 



