Chapter X\X 



THE INTERNAL STRUCTURE OF THE BRAIN STEM OF THE 

 PRIMATES. ITS EVOLUTIONAL MODIMCATION IN RELATION 

 TO THE DEVELOPMENT OF BEHAVIOR 



Essential Similurilits in Inttinul LUmcnls' 



"^HE external appearance of tiie priniate brain nia\- be sullieiently 

 conclusive ot the cN'olutional process in tins order of inaniniais. Sur- 

 face ap[)earances, lio\\e\er, are olten clecepti\c'. The intimate 

 internal organization of the brani ma\ exen chsqualilV tht' conchisions based 

 upon superficial sur\e\. II, on the other hand, tlu' e\ ideiice of the tnternal 

 structure corroborates that ollered by the outer surlace, the testinionx 

 beconies doublx con\ incmg. 



Concernnig the sniulant ies ol organization m the primates, cerebral 

 architectonics \ield an embarrassment ol riches. The homologous correspond- 

 ence of each |)articular is pronounced beyond cjuestion of doubt. Onl\ in the 

 early transitional stages Irom loux'r lorms ol mammals to llu- primate kind, 

 might there be room for dispute. A review of the brain stem in the lemur, or 

 even in the marmoset, ma\- seem to justify the objection that the intt-rnal 

 structure is not sufliciently specific to distinguish it from some relati\elv 

 lower mammal. E\en under these circumstances, more extended ins])ection 

 of the chief diagnostic features in the brain could not fail to reveal ellectual 

 ditTercntial characters. No such objection may be raised in connection with 

 the intermediate primates or with those constituting the group of higher 

 anthropoids. In fact, real dillicultii's might be experienced in distinguishing 

 between the identifying characters of the human brain and the homologous 

 structures in the brains of gorilla, of chimpanzee or of orang. A close scrutinv, 

 however, discloses definite structural \ariations u hich differentiate between 

 man and the great apes. 



993 



