PITHECUS RHESUS, MACACUS RHESUS 389 



peduncles now contain the compact bundles of the pallio-pontile system which 

 connect the cerebral cortex with the lateral lobes of the cerebellum and the 

 compact bundles of the cortico-spinal tract for the transmission of voHtional 

 impulses to the somatic muscuhiture. Immediately dorsal to the cerebral 

 peduncle is a large mass of gray matter, the substantia nigra (Sbn) 

 whose relation to the control of certain automatic associated movements 

 has been mentioned. 



LEVEL OF THE OPTIC CHLA.SM (FIG. 1 82) 



At this level, one of the most cephalic sections of the brain stem in this 

 series is shown. A prominent feature is the optic thalamus which is sepa- 

 rated by the internal capsule (Cin) from the lenticular nucleus, whose 

 major portions are well illustrated, the globus pallidus (GIp) and 

 the putamen (Put). As this specialization in the diencephalic portion 

 of the brainstem is highly complex and deserves separate treatment, the 

 structures at this level are merely referred to as indicative of the marked 

 changes in transition in the level at which the brain stem forms its 

 final continuity with the endbrain. It is here that contact is ultimately estab- 

 lished with those great expansions of the brain, the cerebral hemispheres. 

 They, in later stages of evoIutitMi, have dominated all of the far-reaching 

 modifications in behavior characteristic of mammals, reaching their con- 

 summate expression in the achievements of man. 



