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. The shaft is convex dorsad and concave ventrad. The distal end is somewhat pyramidal. From its medial or radial side a wedge-shaped process, the =styloid= process (_f_), extends distad. The distal surface of the end together with the lateral surface of the styloid process form a concave articular cavity (_g_) which fits against the scapholunar bone. [Illustration: FIG. 49.--RADIUS AND ULNA, DORSOLATERAL VIEW. FIG. 50.--RADIUS AND ULNA, VENTROMEDIAL VIEW. _I_, radius; _II_, ulna. _a_, head of radius; _b_, neck; _c_, bicipital tuberosity; _d_, articular circumference; _e_, facet for radial fossa of humerus; _f_, styloid process of radius; _g_, facet for articulation of scapholunar bone; _h_, semilunar (or great sigmoid) notch of ulna; _i_, coronoid process; _j_, olecranon; _k_, area for insertion of brachialis and clavobrachial muscles; _l_, rough area for attachment of interosseous membrane; _m_, styloid process of ulna.] The dorsal surface of the distal end is marked by longitudinal grooves for tendons, and its lateral or ulnar surface bears a concave facet for articulation with the ulna. =Ulna= (_II_, Figs. 49 and 50).--The ulna is a long slender bone, flattened mediolaterad. It is enlarged at its proximal end and becomes gradually smaller toward the distal end. The proximal end is marked ventrally by a deep excavation, the =semilunar notch=, or =great sigmoid cavity= (_h_). By the saddle-shaped articular surface of the semilunar notch it articulates with the trochlea. This articular surface is divided into two parts by a transverse non-articular area. The distal boundary of the semilunar notch is a blunt process, the =coronoid= process (_i_), which bears on its lateral surface a concave facet, the =radial notch=, for the head of the radius. The portion of the bone proximad of the semilunar notch is called the =olecranon= (_j_). It fits into the olecranon fossa of the humerus when the arm is straightened, and is rough at its end for the insertion of tendons.