The scapula is seldom _broken_ because it is quite movable and is covered with large muscles and because it lies on the chest, which serves as an elastic cushion. The acromion process is the part most frequently broken and occasionally the neck is fractured. _Tumors_ occur and may necessitate the amputation of the whole upper extremity. =The Humerus.=--The bone of the upper arm, the humerus, is the largest bone in the upper extremity and articulates with the scapula above and with the ulna and radius below. At its upper end are the _head_ and the _anatomical neck_, with the _greater tuberosity_ external to and the _lesser tuberosity_ in front of them. The constriction of the surgical neck is below the tuberosities, and extending from between them downward and inward along the upper third of the bone is the bicipital groove for the long head of the biceps. Though round above, below the _shaft_ becomes flattened from before backward and curves slightly forward, terminating in the internal and external _condyles_, from the former of which the flexors and the round pronator arise and from the latter the extensors and supinators. From the external condyle also there projects in front the _radial head_ or _capitellum_ for articulation with the radius. Internally to the capitellum in front and in a corresponding position on the back of the bone are the trochlear surfaces for articulation with the ulna, there being a depression in front called the _coronoid fossa_ for the reception of the _coronoid process_ of the ulna in flexion of the forearm, and another depression behind, the _olecranon fossa_, to receive the tip of the olecranon process during extension. On the lower half of the humerus at the back is the spiral groove for the _musculo-spiral nerve_ and the _superior profunda artery_, while the _ulnar nerve_ runs in a groove back of the internal condyle. The humerus is almost completely covered with muscles, the only part that is subcutaneous being a small portion of the external and internal condyles. The head can be felt under the muscles and the greater tuberosity forms the point of the shoulder. When the arm is at the side, the biceps appears at the front and inner side and the brachialis anticus on either side below, while on the back of the arm, with its largest swelling above, is the triceps. [Illustration: FIG. 70. FIG. 70.--Superficial muscles of shoulder and arm (from before): 1, Pectoralis major; 2, deltoid; 3, biceps brachii; 4, brachialis anticus; 5, triceps; 6, pronator radii teres; 7, flexor carpi radialis; 8, palmaris longus; 9, flexor carpi ulnaris; 10, supinator longus; 11, extensor ossis metacarpi pollicis; 12, extensor brevis pollicis; 13, flexor sublimis digitorum; 14, flexor longus pollicis; 15, flexor profundus digitorum; 16, palmaris brevis; 17, abductor pollicis. (Dorland’s Dictionary.)] [Illustration: FIG. 71.