682 THE HIGHER ANTHROPOIDS 



primates. Because of the advanced diU'erentiation in the nucleus ruber, 

 the reticular formation ( Ref) seems to be a less ditluse and more defi- 

 nitely circumscribed structure. This tendency for the archaic constituents 

 of the brain to gain an increasingly more precise definition is a feature which 

 distinguishes gorilla in its evolutionary progress toward the standards 

 attained by the highest development of the nervous system. 



The dense reticular formation passes laterally without a sharp line 

 of demarcation into the protuberance forming the mesial geniculate body 

 (Mgb). It appears to become more sharply defined in a ventral direction 

 as a discrete nuclear mass extending from the midline to the extreme periph- 

 ery of the section. This is the substantia nigra (Sbn) whose function 

 in connection with c'crtain automatic associated movements has already been 

 suggested, although much doubt still exists with regard to its exact physio- 

 logical significance. Ventral to the substantia nigra is a very dense mass of 

 fibers constituting the cerebral peduncle (CP) which contains the col- 

 lected nerve fibers of the pyramidal system as well as all those axons 

 constituting the pallio-ponto-cerebcllar system. Thus the cerebral peduncle 

 offers an opjjortunitx to estimate at a glance the size and proportion of those 

 systems of libers w hich convey the two great concurrent streams necessary to 

 all skilled motor performances: first, tlu' palliospinal or pyramidal system 

 which serves for the transmission of impulses necessary for the voluntary 

 c(jntrol of the movement, and second, tlie pallio-|)onto-cerebellar system, 

 for the transmission of impulses whicli call into play the simultaneous 

 cooperation of the eoordinative regulation by the cerebellum. 



In the more lateral portion of the reticular formation is an irregular 

 collection of fibers constituting the mesial fillet ( M f ) which has here 

 departed somewhat from the jjositioii occupied b\ it throughout tlu' pons 

 Varolii. This deflection of its course is preparatoi\ to its entrance into the 

 optic thalamus. 



