border forms a guide in tying the brachial artery, as this artery runs along its inner side.
The brachialis anticus rises from the lower half of the outer and inner surfaces of the humerus and is inserted into the coronoid process of the ulna, thus covering and projecting the elbow-joint anteriorly. It is a flexor of the forearm.
Another smaller muscle on the anterior arm, which also aids in flexion, is the coraco-brachialis, which extends from the coracoid process of the scapula to the middle of the inner surface of the humerus.
Extending the entire length of the posterior surface of the humerus is the triceps, similar to the quadriceps extensor in the thigh and direct antagonist to the biceps and brachialis anticus muscles. It rises by a long head from below the glenoid fossa, by the external head from the upper third of the posterior surface of the humerus, and by the internal head from the middle and lower thirds of the posterior surface. It is inserted in the olecranon process of the ulna and serves to extend the forearm and arm.
The humerus is more often fractured by muscular action than any other bone. Usually the fracture occurs in the lower half of the bone and sometimes the musculo-spiral nerve is involved. There is a great tendency to non-union, probably due to interposition of soft parts. Sometimes the break is across and down between the condyles, T-fracture. Involvement of the elbow-joint is more serious than fracture of the humerus alone. Sarcoma of the humerus does occur and may require the removal of the clavicle and scapula as well as of the arm bone itself. In amputation of the humerus in children a long skin flap is left to allow for growth of the bone, as it is liable to grow again.
The Ulna.—In the forearm there are two bones, the ulna and the radius, of which the former is the longer. The ulna is on the inner side of the forearm and its upper end forms the greater part of the articulation with the humerus,
