38 



THE L0\\ ER PRIMATES 



fourth ventricle is bounded laterally by a fairly prominent clava but a much 

 less conspicuous cuneus. Both of these structures are most prominent in 

 their caudal portions. At their cephaHc extremities they become less conspic- 



FIG. 20. \ENTRAL SURFACE OF BRAIN STEM, LEMUR MOXGOZ. 

 [Actual Length, 35 mm.) 



Key to Diagram, cereb. peduncle. Cerebral Peduncle; optico.-ped.-space, Opticopeduncular Space; 

 PYR. dec. Pyramidal Decussation; trap, body, Trapezoid Body. 



uous as elevations. The decrease in surface rehei is more marked in the 

 cuneus than in the clava (Fig. 21). 



The relatively slight development of the cuneus may be interpreted as 

 due to a sensory representation of the forehmb which, as yet, has gained no 

 marked prominence. The cuneus itself consists of nerve fibers and cells 

 related to the transmission of sensory impulses arising in the upper trunk 

 and forelimb. The types of sensory impulses passing over this conduction 

 path pertain to the discriminative quality of sensibility, i.e., tactile, muscle. 



