SIM! A SAT^'RIS, THE ORANG-OUTANG 507 



separated troin tiu' t'lniiu'iitia lri<:c'iiiiii[ hy llu' sulcus mttTiiU'dius. The 

 spinocerebellar eminence contains ascending fibers from the proprioceptive 

 system destined lor the cerebellum, and comprising the dorsal spinocerebellar 

 tract. This i-inlnence is a well-dehned IV'ature of the brain stem of the 

 primates, but becomes especially' demarcated as a surface feature m the 

 anthropoids and man. Its progressive distinction through the higher aiuliro- 

 l)oids, until It reaches its most clearly delined demarcation m man, gives fur- 

 ther evidence of tliat process which results in the sharpi'r delinilion ol all 

 brain stem structures in ascending tlu' primate series. 



Lateral to the external limit ol' the pyramid on the circumterence is a 

 small groove which indicates the caudal e\tremit\' oi the preoli\arv 

 sulcus. Further along the periphcr\- is the caudal e\tremit\ ol the |)ost- 

 olivary sulcus. The eminence which is thus demarcated, although ol slight 

 prominence, occu|)ies the position at which the mierior oli\ar\ nucleus 

 appears in higher levels, in conseciuence ol whose appearance the olivary 

 eminence is determined. 



At this level the more or less detached Acntral gra\- column is contained 

 within the limits indicated by the beginning ot the two olivary sulci. 

 On the outer margin of the ventral gray column, \entrolateral to the central 

 gray column, are the descending Deiterso-spinal tract ( DT), the rubrospinal 

 (Rst) and spinothalamic (Spt) tracts. Upon the actual perlphi-r\- of the 

 circumferi'iit iai /.one are the libers constituting the \entral sj)inocerebellar 

 tract ( Gow). 



This section indicates accessions to the animal's sensor\- sphere, espe- 

 cially in the held of discriminative sensibility. It also points to a lesser 

 degree of sensorx inlhix from tlu' lower extremity. The extent of Nolitional 

 control inlu'reiit in this animal is indicated b\ the \'olume ol the |)\ramidal 

 decussation (Pyx). The tendenc\ of the central gray matter to move 

 into a dorsal j^osition in the cross section is show n as a step preparatory to the 

 ventriculalioii which occurs m tlu' hi<rher KacIs ol tlu' oblongata. 



