368 THE INTERMEDIATE PRIMATES 



Judgment based upon this eomparison would favor the pathway from the 

 upper extremity as being more capacious for the purpose of sensory con- 

 duction. Should this impression be borne out by other facts, the inference 

 that there has been an advance in the upper extremity, and especially the 

 hand as a sensory organ, seems justified. This fact, taken in conjunction with 

 the relatively small size of the substantia gelatinosa, as compared with the 

 similar structure in other species, implies that the facial region of the animal 

 has lost some of its original importance as a sensory directing organ. In 

 all probability much of this function has been delegated to more capable 

 sensory structures of the forclimb, particularly the hand. 



Occupying the most ventral position in the cross section is the pyramidal 

 system (Pyx), most of whose libers are actively engaged in the process of 

 decussation. On the ventrolateral aspect of the section, lying external to 

 the ventral gray matter, is the medullary substance constituting the circum- 

 ferential zone, and mesial to this is the intermediate zone. 



LEVEL OF THE CAUDAL LIMIT OF THE INFERIOR OLIVARY NUCLEUS (fIG. I 73) 



At this level certain striking changes have appeared. Most notable 

 among these is a dense mass of gray matter situated in the ventral portion 

 of the section, the caudal extremity of the inferior olivary nucleus (10). 

 In the dorsal sensory Held there appear the huge masses oi nuclear sub- 

 stance constituting respectively the nucleus of Goll (NG) and the nucleus 

 of Burdach (NB). A feature of the inferior olivary nucleus is the sharper 

 definition of its borders, giving it the appearance of a more discrete structure 

 than in any of the lower species. This nucleus, as is the case with other 

 structures, conveys the impression of an emergence from a more primitive 

 indefiniteness. The central gray matter (Cen) surrounding the central 

 canal occupies a position somewhat more dorsal than in the previous level, 

 indicatmg a general tendency of this portion ot the gray substance to move 



