PITHECUS RHESUS, MACACUS RHESUS 



359 



THE BRAIN STEM 



In general, the outline of all t)f the structures appearing upon the surface 

 of" the I^rain stem in macacus has a clearness of definition which is not the case 



HG. l()(). BASE OF BRAIN OF xMACACUS RHESUS. 



jActiial Length 56 mm.] 



in the lo\\er primates, nor even in more closely allied species, such as Papio 

 cynocephalus. 



The Oblongata. The oblongata, upon its ventral surface, presents a 

 well-defined ventromedian sulcus, flanked upon either side by two pro- 

 nounced elevations, the pyramids. These pyramids extend from their bases 

 which are situated above, in relation with the lower border ol the pons 

 Varolii, downward, and gradually attenuate to an apex, where many 

 of the fibers pass inward and backward to form the pyramidal decussa- 

 tion. Lateral to the pyramid, and separated from it by a well-defined 

 sulcus, is the eminence produced by the inferior olivary nucleus. 

 The sulcus separating this structure from the pyramid is the pre-olivary 

 sulcus. Dorsally the olivary eminence is bounded by the pOst-olivary sulcus 

 separating it from a rather faintly outlined elevation on the lateral surface, 



