SUMMARY OF STRUCTURES 459 



Compared group for group, the intermediate primates show a greater 

 pianimetric coefiicient in the pyramidal system than the lower primates. In 

 one instance alone does this lower group approach the intermediate one in 

 this respect. Mycetes is about on a footing with gibbon. In general, the inter- 

 mediate primates show a striking advance in the organization of the pyram- 

 idal system, although this increase in pyramidal volume is not surprising 

 when the behavioral expansions of the group are brought into contrast with 

 those of such forms as the lemur the tarsius and the marmoset. The closer 

 approximation of mycetes to the figures of the intermediate species empha- 

 sizes again the high degree of speciahzation in these remarkable new-world 

 monkeys. Because of their manual differentiation, as well as the functional 

 capacity of their prehensile tails, they have developed a range of behavioral 

 performances comparing favorably with their presumably more capable 

 congeners of the old w^orld. 



II. The Olivary Nucleus in Its Relation to the Regulation 

 OF Skilled Learned Performances 



The olivary nucleus, by comparison among the intermediate primates, 

 clearly indicates that the gibbon has gained preeminence in this specialization 

 of the brain. In definition of outline, in size, in degree of convolution, the 

 olivary nucleus of the gibbon has every advantage over that structure in the 

 macacus or baboon. From the functional standpoint, the conclusion may be 

 drawn that in the simultaneous movements of the eye, head and hand, the 

 gibbon has made a notable advance over its intermediate associates. This 

 conclusion is in some respects difficult to reconcile with the known facts of 

 the animal's behavior. It does not accord with the conception that the fore- 

 limb, particularly the hand, of the gibbon is in many particulars inferior 

 to that of the other intermediate primates described. In no sense can it be 

 said that the skilled manual performances of the gibbon are comparable with 



