PAPIO CYNOCEPHALUS 



299 



THE BRAIN STEM 



Numerous features upon the several surfaces of the brain stem in 

 baboon make this portion of the central nervous system stand out in 



FIG. 141. LEFT LATERAL SURFACE OF BRAIN, PAPIO CYNOCEPHALUS. 

 [Actual Length 89 mm.] 



Key to Diagram, ramus post.. Ramus Posterior of Sulcus Temporalis Superior; sulc. occip., Sulcus 

 Occipitalis (Inferior); sulc. prec. sup., Sulcus Precentralis Superior; sulcus retrocentralis inf.. 

 Sulcus Retrocentralis Inferior; sulc. temp, med., Sulcus Temporalis Medius. 



greater prominence than is the case with any of the lower primates. 



The Oblongata. The oblongata presents a well-marked intermediate 

 sulcus and two ventrolateral sulci upon its ventral surface. Two well-detined 

 pyramidal elevations lie upon either side of the ventromedial sulcus at its 

 cephalic extremity; while the pyramids themselves are separated irom two 

 well-defined inferior olivary eminences by the ventrolateral sulci. The pyra- 

 mids, as in other forms, taper caudally as they approach the lower extremity 

 of their decussation, and in this area a faint indication of interlacing bundles 

 interrupting the ventral sulcus may be seen, indicating the position of these 

 crossing fibers. The relative prominence of the pyramidal elevations as well 

 as the olivary eminences, as compared with the lo\\er primates, signifies 

 a progressive increase in the dimensions of these structures, and thus 

 probably bespeaks some extension in the functions with which these two 



