The Radius, the bone on the outer side of the forearm, is a long bone with a shaft and two extremities; it differs from the ulna in having a small rounded head and a shaft which gradually widens out so that the lower extremity is much larger and triangular in shape. The head is circular, with a cup-shaped depression on its superior surface; it has a narrow articular surface all round for articulation with the radial notch of the ulna. Immediately below the head it is somewhat constricted to form a neck, and then widens out again into the shaft. The shaft is triangular in section, having three surfaces-anterior, outer, and posterior-and three borders, the interosseous one being the only well-defined one, as the surface is rounded and confluent with the other two. At the upper and inner side of the anterior surface is the bicipital tuberosity for the insertion of the biceps, and from the lower edge of that the oblique line passes across the anterior surface of the bone to the middle of the outer border. The lower end of the shaft is distinctly triangular; the edge of the narrow interosseous surface articulates with the lower end of the ulna; the anterior surface is smooth and concave, the posterior having a series of grooves for the extensor tendons to work in. On the radial border is a styloid process similar to that of the ulna.

The radius articulates above by means of the upper surface of its head with the capitellum of the humerus, and the lower end articulates with the scaphoid and semilunar bones of the carpus; on its interosseous surface it articulates at both ends with the ulna.

Ossification

Similar to that of the ulna.