CHAP. VI.] UPPER EXTREMITY. 147 



Upper Extremity. The disposition and structure of the bones of 

 the upper extremity afford a marked contrast to those of the lower. 

 The latter are organs of support, and therefore are solid, firm, 

 strong, and, withal, elastic. The former are destined to perform 

 extended motions, as well as minute and nicely adjusted ones; 

 and, therefore, while they possess all the requisite strength, they 

 are light, present little expanse of surface, and are articulated by 

 numerous very movable articulations. 



The scapula and clavicle are the media through which the bones 

 of the arm are united to the trunk. The former bone is remark- 

 ably thin and light, and seems little more than a surface of attach- 

 ment for various muscles, on whose actions the extensive movements 

 of the arm depend. By the clavicle it is connected to the ster- 

 num, through the sterno-clavicular articulation, the movements of 

 which, although occurring only in two planes, intersecting each 

 other at right angles, are such as to favour a wide range of motion in 

 the shoulder. So necessary is this joint to the general movements of 

 the shoulder, that any injury or disease of it, or of the bone itself, 

 shews itself speedily in the impediment offered to those move- 

 ments. And the law of the development of this bone in the 

 lower animals is clearly connected with a necessity for a wide 

 range of motion in the anterior extremity. In those animals that 

 employ the anterior extremity only as an instrument of progressive 

 motion, there is no clavicle ; hence this bone is absent from the 

 skeletons of Pachydermata, Kuminantia, Solipeda, and the motions 

 of the shoulder are only such as may be required for the flexion and 

 extension of the limb. In the Carnivora, where there is a slight 

 increase in the range of motion of the anterior extremities, a rudi- 

 mentary clavicle exists; and in this class we observe that the size of 

 the bone bears a direct relation to the extent of motion enjoyed by 

 the limb. Thus it is smallest in the dogs, and largest in the cats : 

 in these animals it has no attachment to either the sternum or the 

 scapula, but is enclosed in the flesh, and does not occupy much 

 more than half the space between the two bones last named. "But 

 however imperfect," says Sir C. Bell, " it marks a correspondence 

 in the bones of the shoulder to those of the arm and paw, and the 

 extent of the motion enjoyed. When the bear stands up, we per- 

 ceive, by his ungainly attitude, and the motion of his paws, that 

 there must be a wide difference in the bones of his upper extremity 

 from those of the ruminant or soliped : he can take the keeper's 

 hnt from his head, and hold it; he can hug an animal to 

 death. The ant-bear, especially, as he is deficient in teeth, pos- 



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