(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.
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.
[Fig. 49].—Radius and Ulna, Dorsolateral View.
![[Fig. 49]](#3418956513767812439_illo068alg.jpg.id-7632893323812678451.wrap-0.html.html){kind=link}
[Fig. 50].—Radius and Ulna, Ventromedial View.
![[Fig. 50]](#3418956513767812439_illo068blg.jpg.id-6259272620303467526.wrap-0.html.html){kind=link}
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.