258 THE LO\\ER PRIMATES 



than passing significance. The skillfull manner in which mycetcs employs its 

 tail supplies the reason for calling this organ a hfth hand, and hence the need 

 of sensory specialization in this caudal appendage is evident. The tail at its 

 tip is free of hair and presents upon its ventral surface parallel rugae not 

 unlike the rugous markings upon the balls of the fingers and thumb. The ani- 

 mal utilizes its tail not merelj- for purposes of suspension during locomotion, 

 but in many selective acts in which sensory discrimination is necessary. A 

 tail manifesting such a degree of deftness at once implies the accjuisition of a 

 large series of motor patterns. It must therefore be the case that the kines- 

 thetic development in connection with the movements of the tail is largely 

 expanded in mycetes as compared with lemur, tarsier and marmoset. 



This expansion, however, is not confined to the nucleus of Goll. The 

 planimetric coeflicicnts of the nucleus of Burdach show a similar increase in 

 the size. In lemur this nucleus has a planimetric coeflicient of 4.9 per cent, 

 in tarsius, of 2.9 per cent, in marmoset, of 4.3 per cent. In mycetes, the 

 coeflicient is 1 1.3 per cent. Since the nucleus of Burdach, in the main, repre- 

 sents the discriminative sensory influx from the upper extremitjs the hand 

 would prove a dynamic factor most likely to exert an influence favoring expan- 

 sion. In lemur, manual specialization is not far advanced. The thumb is short 

 and lacks much of the opposability characteristic of the higher species. Both 

 the forearm and arm are short and do not approximate the ideal proportions of 

 human conditions. In fact, the forelimb is still so largely implicated in locomo- 

 tion that it possesses many characters .inherent in a locomotor organ. Even 

 more submanual is the forelimb in tarsier and marmoset. In both of these 

 species, differentiation of the hand has made ineflectual attempts in the direc- 

 tion of human conditions. In the case of mycetes, however, the forelimb has 

 advanced decisively toward ultimate manual difTerentiation. Its locomotor 

 oflices have been supplemented by the use of the prehensile tail which, per- 

 mitting suspension of the body, aflords opportunities for the freer use ot the 



