246 THE LOW ER PRIMATES 



While these figures may not in any sense be accepted as dclinitive, they 

 afford a close approximation to an actual estimate of the fact. From them it 

 seems permissible to conclude that the increment in pyramidal volume is 

 proportional to that increasing demand for control of motor performances 

 made possible through the progressive development ol the hand. 



MORPHOLOGICAL CONSIDERATIONS IN THE COMPARISON OF 

 UPPER EXTREMITIES IN MAN AND LOWER PRIMATES 



If the human upper extremity be accepted as the structural standard in 

 the process of manual differentiation, certain morphological conditions must 

 be taken into account when comparing these parts of the human body with 

 similar structures in the lower primates. Among these conditions are the 

 proportional length of the arm to the body, of the forearm to the arm, ot the 

 hand to the forearm, as well as the proportions of the metacarpals and 

 phalanges, particularly the proportion of the metacarpal bones and phalanges 

 of the thumb. There should likewise be included the degree of opposability of 

 the thumb and the differentiation of the fingers, especially the linger nails 

 and the cutaneous pads connected with the distal phalanges. Estimated in the 

 light of such criteria, the upper extremity in lemur and tarsius falls consider- 

 ably short of complete manual differentiation. In the case of marmoset, the 

 proportions in the major segments in the limb are at variance with the 

 accepted human standard. The differentiation of the fingers is particularly 

 primitive. The thumb is short, the cutaneous pads of the distal phalanges 

 correspond more with the conditions presented by animals possessed of 

 claws, and finally, the nails upon the lingers are much more claw-like than in 

 any other form of primate. 



In mycetes, however, the behavioral reactions made possible through 

 differentiation of the hands place the animal much closer to man than 

 lemur, tarsier or marmoset. The performances of the howling monkey are 



