SHOULDER-JOINT, ELBOW-JOINT. 43 



is attached by muscles to the upper part of the back, and 

 the clavicle or collar-bone, which extends from the sternum 

 to the scapula, embracing the top of the chest. At the 

 angle formed by the superior border and the external edge 

 of the scapula an articular surface, called the glenoid cavity, 

 receives the upper extremity or head of the humerus, the 

 bone of the arm, which articulates at the elbow with the 

 ulna and the radius, the two bones of the fore-arm; these 

 form with the carpus the wrist-joint, which unites the fore- 

 arm to the hand. The deltoid, the great dorsal and great 

 pectoral, and other less powerful muscles, combine to form 

 the shoulder, and give motion to the humerus. The triceps 

 and biceps of the arm, &c., which surround the humerus, 

 flex or extend the fore-arm and turn it on its axis; and 

 numerous muscles cover the fore-arm and move the hand. 



The articulation of the humerus with the scapula or the 

 shoulder-joint is, of all joints, the one that permits the most 

 extended movement. The shallowness of the glenoid cavity 

 permits great freedom of motion to the rounded head of the 

 humerus. Thus the arm, which hangs parallel with the body 

 in a state of repose, can be raised vertically to the head, 

 laterally forward so as almost to enfold the chest, and also 

 backward, though in a more limited degree; it can be turned 

 on its axis in all these positions, and in the movement of 

 circumduction it describes a very flat cone, the base of 

 which, especially in front, approaches the apex. 



The elbow-joint is one of the most complicated in the 

 whole system. The lower extremity of the humerus, and the 

 upper extremities of the ulna and the radius, are adapted to, 

 and interlock with each other by a series of rounded surfaces 

 and pulley -grooves, which permit the fore-arm to flex itself 

 forwards upon the arm; while a protuberance of the ulna, the 

 olecranon, which forms the projecting part of the elbow, limits 

 the backward movement by resting in a cavity of the humerus. 

 It is into the olecranon that the tendon of the triceps of the 

 arm, the principal extensor of the fore-arm, is inserted, and 

 we shall see the analogy farther on between this process and 

 the knee-pan. 



The movements of the fore-arm singularly multiply by 



