INTERXAl, STRlCTri^F OF THE BRAIN STEM 1033 



Like the lateral lobes ol the eeieln'Muni, it must he lunetioiially associated 

 with till' (le\ elopnients ol iieokinetie progress. The inferior oli\ar\ inieleus 

 thus corroborates the exicieiice supplied by the p\ ramid, the pontile nuclei, 

 thi' cerebral jicduneles, the dentate nucleus, the ri'd luicK'Us and the su])erior 

 cerebellar pi'duncle. All ol these j^arts of the brain, by reason of their expan- 

 sion, stand out as prominent signals ol e\'olutionar\ acKance. 



Certam constituentsw ith ec|nal I orce denote this process, not by e\])ansi()n 

 but by progressive (hmnuition m si/e. Among these structures the most 

 prominent are the superior and the mierior collieuli of the midbram. Their 

 importanci- ni relation to the special senses of hearing and of sight has been 

 emphasized, d he dellort'scence \n the inferiorcolliculus isparticularl\ striking 

 showing, as it does, a decrease in planimetric coefhcients In'twci'ii lemur and 

 man ot over 1 5 hundredths. W hile m the case of the superior colhculus the 

 decrease in structural i^romineiice is not so pronounced, it is iu'\erthelcss 

 clearly evident that a progressive decline proceeds through all of the jjrimate 

 species from lemur to man, with a total decrease of something mori' than 4 

 hundredths. 



That thi' mesencephalic colliculi are remnants ol prominent features in 

 lower vertebrates, and that the\ still retain in these classes a pri'dominance in 

 encephalic organization, has been freciucntly rt'iterated. This is the condition 

 in birds, ri-ptik's, amphibians and fish, but with the appearance of the 

 mammals, the introduction of the neopallium concentrated the full force of 

 development upon the hemispheres. In consequence, this part of the brain 

 soon gained ])re(.niinence which in large measure was due to the unremitting 

 attemjjts to capitalize still lurtlur the new endowments of neokinetie behav- 

 ior. Each of the special senses, w it h the exception of the olfactory sense, partici- 

 pated in these attempts. These senses sought an advantage in proximity to 

 each other w Inch representation in the cerebral cortex afforded them together 

 with more am])le ojiportunities for functional expansion. 1 he optic lobe of 



