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.
The body of the ulna becomes triangular distad. The distal end is slightly larger than the shaft just proximad of it, and bears on its radioventral side a hemispherical head for articulation with the radius. Distad of the head the bone continues as the flattened =styloid= process (_m_), which projects distad from its dorsolateral side and is smooth on the medial side of its apex, for articulation with the cuneiform bone of the wrist.