148 LOCOMOTION. [CHAP. vi. 



sesses extraordinary power of hugging with his great paws ; and, 

 although harmless in disposition, he can squeeze his enemy, the 

 jaguar, to death. These actions, and the powers of climbing, re- 

 sult from the structure of the shoulder, or from possessing a collar- 

 bone, however imperfect." * 



In those Mammalia that dig and burrow in the ground, or whose 

 anterior extremities are so modified as to aid them in flight, or 

 which are skilful in seizing upon and holding objects with their 

 paws, the clavicle is fully developed, and extends the whole way 

 from the scapula to the sternum. Thus in the Rodentia this bone 

 is very perfect, as, for example, the squirrel, the beaver, the rabbit, 

 the rat, &c. The bat affords an example of a very strong and long 

 clavicle, as also do the mole and the hedge-hog among the Insec- 

 tivora. 



Among the Edentata those tribes possess a clavicle whose habits 

 are fossorial, as the ant-eater, the armadillo, and even the gigan- 

 tic extinct megatherium. In the Quadrumana the clavicles are 

 strong and curved, as in the human subject. 



The clavicle possesses considerable elasticity by reason of its 

 curves ; a property obviously of the greatest importance to it, be- 

 cause, as the bond of connexion of the shoulder to the trunk, it is 

 liable to participate in the many concussions to which the upper 

 extremity is exposed. This point has been put to the test of direct 

 experiment by Mr. Ward. He employed the clavicle of a well- 

 developed male subject, of the middle age : this was placed upon a 

 smooth surface, with its shaft perpendicular to the plane of a wall, 

 against which its inner extremity rested ; a smart blow was then 

 struck with a hammer on the outer extremity of the bone, in the 

 direction of its long axis; the hammer rebounded from the end 

 of the bone, which sprang to a distance of nearly two feet from the 

 wall. 



The humerus is the principal lever of the upper extremity : in 

 man it is light, and its articular extremities are constructed to con- 

 tribute to the formation of very moveable joints. The bones of the 

 fore-arm are also remarkable for their lightness and elasticity ; and 

 they move freely, not only on the humerus, but on each other. 

 The movements of pronatioii and supination, which are necessary to 

 the free and full use of the hand, are performed by the rotation of 

 the radius round an axis passing through its head and neck ; the 

 slight curve in the shaft of the radius causes its carpal extremity to 



* Bridgewater Treatise, p. 49. 



