264 THE LOWER PRIMATES 



adjustment, operating upon a less stable base in mycetes ealls into play the 

 functional activity of additional balancing reflexes. While the problems of 

 balancing change from species to species according as the locomotor adapta- 

 tion is made to arboreal, terrestrial or intermediate modes of life, the 

 demands for equilibration remain essentially the same throughout the 

 primate order. This undoubtedly is due to the fact that although the equili- 

 bratory requirements of arboreal life may recede as the animal approaches 

 nearer to a strictly terrestrial habit of living, these are replaced by new 

 requisites for balancing induced by the gradual assumption of the erect 

 posture, by bipedal and ultimate plantigrade locomotion. 



That tarsius and mycetes should exceed other primates in the size of 

 their vestibular areas is due to the fact that they have added to their balanc- 

 ing equipment certain ancillary mechanisms essential to peculiarities in their 

 locomotor specializations. Such specializations are conditioned by the pre- 

 hensile tail or saltatory locomotion. 



In their general evolutional signilicance, the vestibular nuclei do not 

 contribute so striking an example of progressive unfolding as is the case with 

 many other structures. Nevertheless, their high specialization in mycetes is 

 worthy of note. They reveal to what extent the development of a plastic charac- 

 ter like the prehensile tail may influence so fundamental a function as that 

 regulating the equilibrium of the body. The absence of such a tail in lemur, 

 tarsier and marmoset seems to make this differential element all the more 

 significant in the evolutional sense. 



v. The Cerebellar Nuclei and the Nucleus Ruber in Relation 

 TO Coordination 



Several relatively prominent cell groups are recognized and distinguished 

 as cerebellar nuclei. They include the nucleus dentatus, nucleus emboliformis, 

 nucleus globosus and nucleus fastigii. The globosal and fastigial nuclei belong 



