I032 EVOLUTIONAL MODIFICATIONS 



whose further progress imparts such j^roniinenee to the e\-ohition;uy process 

 in the' primate brain. Thenceforth, throughout tlie order, this impulse toward 

 neo-olivary expansion becomes more j^ronouneed. \\ hen, however, the 

 demands of neokinesis called for greater functional activities in the sphere of 

 olivary action, no distinctly new element was added to the structure. 

 Phyletically ancient portions w ere so extended as to create, out of the old, 

 new and needed material for amplilied functional capacity. This is the 

 underlying ])rinciple which m the mam characterizes the modifications in 

 tile \ertebrate brain. The structural substratum ot t^xpansion usually exists 

 in rudimentary form awaiting only the demand and opjjortunity for further 

 dcM'lojjment. The inferior olivary body, illustrating this principle, expands 

 in rt^sponse to increasing demands lor lunctional control in its sphere ol 

 action. As e\ ideiice of an cvolutionarx unlolding, it is second to no other 

 element in the entire brain stem. The planimetric coellicients of the olivary 

 nucleus in jjrimates indicate that in passing from its lowest organization to 

 that representative ol man, there is a total gain ot almost 20 hundredths. 

 In the marmoset the planimetric cocflicient oi the mlerior olive is 3.8 per 

 cent, while In man it is 22.6 per cent. A MiluiiU'tric gain of such proportions 

 cannot possibly be without great biological sigmlicancc. More conspicuous 

 even than this increment in Nolunie is the increase in the olive's morpholog- 

 ical mdi\iduality and delinition. W itli this progressive oli\arv specializa- 

 tion, the connection of the nucleus with the lateral lobes of the cerebellum 

 becomes more extensive. This fact strongly suggests that the biological 

 impulse underlying this olivary diifercntiation is closely related to those 

 causes which activated the expansion of the lateral cerebellar lobes. It is 

 well established that the extensions in the lateral portions of the cerebellum 

 have been induced by the ])rogressi\e s]:)ecializations in the extremities, more 

 particularly in the upper extremity and hand. The inlerence seems lustilied, 

 therefore, that the oli\ e itself has also responded to this same incentive. 



