44 THE LOWER PRIMATES 



the underbrush supplements the sense of sight and thus facihtates passage 

 amid obstacles in the environment. For this reason the head and face are 

 amply provided with a sensory equipment. 



The entire dorsal field of the section is fundamentally sensory in its 

 significance. Thus a hne ch-awn transversely through the central canal 

 bounds a territory which represents the animal's capacity in discriminative 

 sensibility for the whole body. This territory may be spoken of as the dorsal 

 field of sensory discriminuUon. It is the area ot the central nervous system 

 which affords the best index of an animal's capacity in discriminative sensi- 

 bility. Beginning at the more lateral portion of this sensor^' field are the 

 descending tract of the fifth nerve (Trd) and the substantia gelatinosa 

 (NR). These structures represent the head and face. The sensory 

 representation of the neck and arm lies contiguous to the substantia gelati- 

 nosa (NR), while occupying a mesial position, adjacent to the dorso- 

 mesial septum, is the area concerned in sensory transmission from the tail 

 and the leg ( CG ). Analysis of this sensory field in lemur indicates that 

 innervation is more generously provided for the leg, tail, face and head 

 than for the forelimb with its slightly ditlerentiated hand. The type of 

 sensibility which this area serves is preeminently discriminative. It is 

 particularly involved in finer adjustments of body posture and contact 

 relations with the outside world. Hence it not only plays an important 

 part in afl'ording discrimination in the analysis of elements encountered 

 in the environment, but must also be regarded as the sensory substratum 

 upon which the complexity of the skilled reactions depends. 



Other structures in this level need mention for topographical identifica- 

 tion. Among these are the ventral gray column (Ven) which, because 

 of the decussating bundles of the pyramid ( Pyx), has been detached 

 from the central gray matter (Cen), the latter now lying in contact 

 with the median line and surrounding the small central canal. Dorsolateral 



