362 



THE INTERMEDIATE PRIMATES 



CephalacI to the eminences of the cuneus and clava, apparently in direct 

 continuity with them as they become attenuated and reach the level 

 of the ventricular floor, is a marked elevation. This is the vestibular area 



FIG. 169. VENTRAL SURFACE OF BRAIX STEM, MACACUS RHESUS. 



[Actual Length, 44 mm.| 



Key to Diagram, trap, body. Trapezoid Body; pyr. dec, Pyramidal Decussation; ventro med. sulcus, 



Ventromedian Sulcus. 



which contains the nuclear elements for the receipt of impulses from the 

 internal ear and particularly related to the bahmcing mechanism. The 

 marked prominence of this elevation in the relief of the lloor of the ventricle 

 is indicative of the degree to which macacus depends upon its balancing 

 function. A boundary between the cephaHc and caudal triangles of the 

 fourth ventricle is produced by the crossing striae acusticae, many of which 

 pass obliquely downward, while a few ascend, passing over the outer surface 

 of the vestil)ular eminences. The apex of the cephalic angle of the ventricle 

 marks the point of transition into the caudal orifice of the Sylvian aqueduct. 



