HUMERUS, the largest and longest bone of the upper extremity. It is the one bone of the arm proper, that is, that portion between the shoulder and elbow. It is divided anatomically into a shaft and two extremities. The upper extremity is rather the largest, and has a semi-globular bead which is received (partially, because the cavity is shallow) into the glenoid cavity of the scapula or shoulder blade, forming what is called 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 glen. oid, the relations being somewhat similar to what obtain in the hip joint (q.v.). The shaft of the humerus is nearly cylindrical in its tipper 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 fore-arm, the elbow joint (see ARM and 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 these depressions, on the posterior surface of the bone, is a d3ep triangular depression, called the olecranou fossa, for the reception of the important process of the ulna, called the olecranon process. 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, and is one of the most beautiful pieces of mechanism which can be conceived, especially in man, where it exhibits indications of design having reference to man's intellectual functions.