INTERNAL STRUCTURE OF THE BRAIN STEM 1009 



system. The evidence of its brain indicates to what extent the pyramidal 

 system has increased in volume oAer and above lower mammalian forms. 

 Thus begun in lemur, the process of expansion goes steadily forward from 



CO.MPARATIX E PLANI.METRIC GRAPHS 

 A. Oli\ E AND Pyramid 



c 



A 



T 



G 

 O 

 R 

 I 



L 

 L 

 A 



A 



N 



FIG. 493. GRAPHS CONSTRUCTED ON THE BASIS OF THE PLANI.METRIC INDICES. 



This representation clearly indicates the hesitating advances made by subprimate vertebrates in the expan- 

 sion of important structures connected vrith neoldnetic evolution. The effects of arborealism are plainlv 

 showTi in the marked advances of the Primate^ o\er other mammals. Such advances are attributed to the 

 .nous differentiation determin. real habitat. Broken line represents the inferior olr\-e, 



• the pj-ramid. 



