At the elbow, it joins with the capitulum of the humerus, and in a separate region, with the ulna at the radial notch.

The long narrow medullary cavity is enclosed in a strong wall of compact bone.

It is thickest along the interosseous border and thinnest at the extremities, same over the cup-shaped articular surface (fovea) of the head.

The upper extremity of the radius consists of a somewhat cylindrical head articulating with the ulna and the humerus, a neck, and a radial tuberosity.

The distal end of the radius forms two palpable points, radially the styloid process and Lister's tubercle on the ulnar side.

The volar surface (facies volaris; anterior surface) is concave in its upper three-fourths, and gives origin to the flexor pollicis longus muscle; it is broad and flat in its lower fourth, and affords insertion to the Pronator quadratus.

A prominent ridge limits the insertion of the Pronator quadratus below, and between this and the inferior border is a triangular rough surface for the attachment of the volar radiocarpal ligament.

At the junction of the upper and middle thirds of the volar surface is the nutrient foramen, which is directed obliquely upward.

That for the body makes its appearance near the center of the bone, during the eighth week of fetal life.

An additional center sometimes found in the radial tuberosity, appears about the fourteenth or fifteenth year.

The lower quarter of the body attaches to the pronator quadratus muscle and the tendon of the supinator longus.

Its structure is similar in most terrestrial tetrapods, but it may be fused with the ulna in some mammals (such as horses) and reduced or modified in animals with flippers or vestigial forelimbs.

[6] This article incorporates text in the public domain from page 219 of the 20th edition of Gray's Anatomy (1918)