FIG. 71.--Superficial muscles of shoulder and arm (from behind): 1, Trapezius; 2, deltoid; 3, rhomboideus major; 4, infraspinatus; 5, teres minor; 6, teres major; 7, latissimus dorsi; 8, triceps; 9, anconeus; 10, brachialis anticus; 11, supinator longus; 12, extensor carpi radialis longior; 13, extensor carpi radialis brevior; 14, extensor communis digitorum; 15, extensor carpi ulnaris; 16, flexor carpi ulnaris; 17, extensor ossis metacarpi pollicis; 18, extensor brevis pollicis; 19, tendon of extensor longus pollicis. (Dorland’s Dictionary.)] =Upper Arm Muscles.=--The _biceps_ is the most important arm muscle. It rises by a short head from the coracoid process of the scapula and by a long head from a tubercle on the upper margin of the glenoid cavity, the tendon arching over the head of the humerus and descending in the bicipital groove. It is inserted into the back of the tuberosity of the radius and by a broad aponeurosis into the fascia of the forearm. It flexes and supinates the forearm and renders the fascia tense. Its inner 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.