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 be-
