Proximad of the trochlea the dorsal surface presents a deep fossa, the =olecranon fossa=, which receives the olecranon of the ulna when the arm is straightened. On the ventral surface (Fig. 47) are two shallower fossæ separated by a longitudinal ridge. The one over the trochlea receives the coronoid process of the ulna when the arm is bent, and is called thence the =coronoid fossa= (_l_). The one over the capitulum, the =radial fossa= (_m_), receives a triangular facet on the proximal end of the radius at the same time. Between the radial and coronoid fossæ on one side and the olecranon fossa on the other is only a thin plate of bone. On the medial surface of the distal end is a considerable roughened elevation, the =medial epicondyle= (_n_) (epitrochlea). It gives origin to flexor muscles and to the ulnar collateral ligaments of the elbow-joint. Opposite the medial epicondyle over the capitulum is the =lateral epicondyle= (_o_) for the origin of extensor muscles of the forearm and of the radial collateral ligaments of the elbow-joint. From the lateral epicondyle a ridge, the lateral =supracondyloid ridge= (_p_), continues proximad, curving onto the dorsal surface of the bone and ending about opposite the junction of the deltoid and pectoral ridges.
Proximad of the medial epicondyle the bone is pierced near its medial margin by an oblique oval foramen, the =supracondyloid foramen= (_q_).
=Radius= (_I_, Figs. 49 and 50).--In the usual position the radius lies with its proximal end on the lateral side of the arm, articulating with the capitulum of the humerus. The proximal end is thus laterad of the proximal end of the ulna. Its distal end, however, lies on the medial side of the distal end of the ulna, so that the radius in the natural position crosses ventrad of the ulna.
The radius is a curved bone slightly flattened dorsoventrally, with enlarged ends. It may be described as consisting of a shaft and of a proximal and a distal end. Its proximal end presents on the ventral surface a tuberosity, the =bicipital tuberosity= (_c_), for the insertion of the tendon of the biceps muscle. Proximad of this the bone is contracted to form a =neck= (_b_) which is surmounted by a =head= (_a_). The head has on its proximal surface a depressed oval facet by which it articulates with the capitulum, and on its ulnar border a long narrow facet, the =articular circumference= (_d_), for articulation with the radial notch of the ulna; also a triangular facet (_e_), which fits into the radial fossa of the humerus.