SKELETON OF THE QUADRUMANA. 209 



structure of tlie unguiculate limb. The- clavicle, 58, con- 

 nects the acromion with the sternum, and affords a fulcrum 

 to the shoulder-joint. The humerus, articulating below 

 with a radius and ulna which can rotate on each other, 

 develops ridges above both inner and outer condyles for 

 the extended origin of the muscles of pronation and 

 supination. The brachial artery pierces the entocondyloid 

 ridge. The carpal bones, answering to the scaphoid and 

 lunar in the human wrist, are here confluent. The digits 

 are five in number, enjoy free, independent movements, 

 ard are each terminated by a sharp -curved claw. 



SKELETON OF THE QUADRUMANA. 



The sloth is an exclusively arboreal animal ; its diet is 

 foliage ; it has but to bring its mouth to the leafy food, 

 and the lips and tongue serve to strip it from the branches. 

 The extremities, as we have seen, serve mainly to climb 

 and cling to branches, and occasionally to hook down a 

 tempting twig within reach of the mouth. There is, how- 

 ever, another much more extensive and diversified order 

 of arboreal mammals destined to subsist on the fruits and 

 other more highly developed products of the vegetable 

 kingdom than mere leaves. In the monkeys, baboons, 

 and apes the extremities are endowed with prehensile 

 faculties of a more perfect and varied character than in 

 the sloths ; and this additional power is gained by a full 

 development of the digits in normal number, with free 

 and independent movements, which in one of them — the 

 first or innermost — are such as that it can be opposed to 

 the rest, so that objects of various size can be grasped. 

 This modification converts a foot into a hand ; and, as the 



18-^ 



