HU?MERUS (Lat., shoulder). The largest and longest bone of the upper extremity; the bone of the arm proper, extending from the shoulder to the elbow. It is divided anatomically into a shaft and two extremities. The upper extremity is rather the larger, and has a semi-globular head which is partially received into the shallow glenoid cavity of the scapula or shoulder-blade, forming a ball-and-socket joint. Two processes or projections of the shoulder-blade assist the glenoid cavity in completing the cavity or seat of the head of the humerus. There are three ligaments which hold the humerus to the scapula--the capsular, the coraco-humeral, and the glenoid, the relations being somewhat similar to those of the hip-joint (q.v.). The shaft of the humerus is nearly cylindrical in its upper part, but triangularly prismatic below, becoming flattened and broad at the lower extremity, where are placed the two condyles, with their articular surfaces, and the trochlea between them, which form, with the two bones of the forearm, the elbow-joint. (See Arm; Skeleton.) The broad, flat lower extremity has two depressions on the anterior aspect of the bone; one slight one on the outer side called the radial depression, which is for the reception of the anterior border of the head of the radius, when the arm is strongly flexed; the other, called the coronoid depression, for the reception of the coronoid process of the ulna during flexion of the arm. Opposite the latter depression, on the posterior surface of the bone, is a deep triangular depression, called the olecranon fossa, for the reception of the olecranon process of the ulna when the forearm is extended. The humerus forms with the scapula, as above mentioned, a ball-and-socket joint, the shoulder-joint (q.v.). The elbow-joint is a hinge joint, and, to a certain extent, in its relation to the head of the radius, a ball-and-socket joint.