OSSEOUS SYSTEM OF MAMMALIA. 



77 



cavity of the scapula ; as will be best understood by referring to the an- 

 nexed figure (77). 



The bones which enter into the formation of the shoulder, consist of 

 the humerus, or single bone of the arm, the scapula, and the clavicle. 

 The humerus, at its lower extremity, is articulated with the radius and 

 ulna, the mechanism of the elbow-joint resulting from their peculiar union. 

 The upper extremity of this long cylindrical bone is moulded into a round 

 head, standing obliquely backward: this head is received into a very 

 shallow cavity (called the glenoid cavity) in the anterior angle of the 

 scapula, a (fig. 77) ; and is retained in its situ- 

 ation by ligaments, which allow to the arti- 

 culation the greatest liberty. 



Of all the joints in the human frame, that 

 of the shoulder is by far the most free ; and 

 this freedom is necessary, in order that Man 

 may use his hands without any impediment. 

 He can rotate the arm from the shoulder, 

 describing a large circle with the hand : he 

 can, in like manner, raise the arm or depress 

 it, thrust it forward or draw it backward, at 

 will ; but this extent of motion, which handed 

 animals alone enjoy, depends not only upon 

 the nature of the connexion of the humerus 

 with the scapula, but upon the character of 

 the scapula, and the mode of its union with the 

 body. The point or tip of the human shoulder 

 is produced by the extremity of a large process 

 of the scapula, termed the acromion, d (fig. 77), overhanging the joint. 

 From this process, d, to the top of the sternum, f, proceeds a long bone, 

 shaped something like the italic letter /, termed the collar-bone, or clavi- 

 cle (clavicula), c. Its presence is of great importance: it keeps the 

 shoulders at a proper distance, and apart from the sides of the chest, pre- 

 venting the muscular stress, which is perpetually exercised upon the arm, 

 as a lever, from drawing it inward. It may be taken as a rule, that all 

 animals which freely use the hand either for holding, digging, climbing, 

 or flying, and, consequently, have the fore-arm in a greater or less degree 

 capable of revolving, possess the clavicle more or less developed. In many 

 animals it is much stronger than in Man, as, for example, in the Echidna, 

 the Mole, the Chlamyphorus, the Armadillo, &c. In these creatures, 

 whose anterior limbs are expressly formed for digging, and are, besides, 

 endowed with extraordinary power, the clavicle is thick and rugged, 

 with bold processes, for the better attachment of voluminous muscles, which 

 act upon the arm ; and it is capable of enduring the utmost stress. In the 



Lateral view of the osseous framework 

 of the human chest and shoulder, shewing, 

 a, the scapula ; b, the humerus ; c, the 

 clavicle ; d, the acromion process of the 

 scapula ; e, the coracoid process of the 

 scapula; f, the sternum. 



