PITHECUS RHESUS, iMACACUS RHESUS 375 



in macacus, together with the tenclcnev toward clehnite convolution ol its 

 structure, which appears m this species in such conspicuous development 

 for the lirst time. This tenclcnev to convolution is one which will be seen 

 to become more marked as the higher apes are approached. It reaches its 

 greatest degree ot complexity m the brain stem ol man. 



The pyramid ( Py ) occupies its typical ventromesial position and affords 

 an opportunity for estimating its size in relation to the rest of the cross sec- 

 tion. Dorsal to It are the bundles constituting the mesial lillet (i\H ). Ventral 

 to Deiters' nucleus, laintly outlined at this level, is the substantia gelatinosa 

 trigemini (N R), upon the lateral border ol which are the collected fibers of 

 the descending trigeminal tract (Trd). The reticLilar formation (Ref) is 

 quite extensive and is penetrated by many arcuate fibers apparently arising 

 in the nucleus of Deiters, to form the Deiterso-spinal tracts. If one feature may 

 be signalized at this level, it is the great prominence of the vestibular com- 

 plex with the implication which it bears concerning the balancing function 

 of the macacus. 



LE\EL OF THE CEREBELLAR NUCLEI (FIG. 1 76) 



At the level of the cerebellar nuclei the organization of the cerebellar 

 connection is indicated. The two groups of cerebellar nuclei, namely, the 

 mesial and the lateral groups, have all of that indefinite development which 

 characterizes the lower primates. The lateral group (Ndt), whose largest 

 constituent part is the nucleus dentatus, is a more or less diffuse mass 

 of nuclear substance surrounded by medullary tissue. It has none of the 

 convoluted appearance or discrete outline prominent in the higher anthro- 

 poids. In this respect it allies itself closely with all the lower forms pre- 

 viously described. 



Situated in the roof of the fourth ventricle is the mesial nuclear group 

 ( N f g ), consisting of the nucleus fastigius and the nucleus globosus. Between 



