PAPIO O'NOCEPHALUS 307 



The influx of sensory impressions from this region depends upon the degree 

 to which the head is used in directing the animal's locomotion and as an 

 instrument for exploration of its environment. As a rule, animals provided 

 with hair-covered facial areas and long vibrissae, such as the rodents and the 

 cat family, are equipped with a large receiving center for sensory stimuli 

 arising in this region of the body. In animals whose facial cutaneous areas 

 are more or less completely denude of hair, in which also the hand and 

 fingers have more or less taken the place of the face for the purposes 

 of directing locomotion, there is a tendency for the substantia gelati- 

 nosa trigemini to decrease in size. The substantia gclatinosa in the 

 baboon is somewhat less conspicuous than it is in the several lower 

 primates; and certainly much less prominent than in the carnivores or 

 rodents. It is, however, more massive than in such forms as have come 

 to depend almost exclusively upon the hand for the direction of their 

 movements. 



The entire dorsal field in this section, as in the case of the other species 

 already considered, is significant as representing the complete discriminative 

 sensory territory of the body. The portion of this field adjacent to the dorso- 

 median septum conveys impressions from the tail, lower extremity and 

 lower portions of the trunk; the next succeeding portion serves the upper 

 trunk, arm and hand, while the most lateral portion represents the trigem- 

 inal areas of the face and head. It seems apparent in contrasting these 

 three areas which serve as indices to the sensory influx from the main dis- 

 criminative areas of the body, that the intermediate zone, or column of 

 Burdach, is probably the largest of the three in baboon, thus strongly 

 suggesting that the hand, forearm and arm have taken precedence from the 

 standpoint of sensory importance over the other areas of the body. 



Immediately lateral to the substantia gelatinosa, and lying upon the 

 circumference of the axis, is a dense bundle of fibers representing the descend- 



