The Ulna, the inner bone of the forearm, is a long bone with a shaft and two extremities. The head is formed of two processes, the olecranon posteriorly and the coronoid anteriorly. The olecranon process forms a continuation of the shaft, and is hollowed out anteriorly for articulation with the trochlear surface of the humerus. The coronoid process juts out from the anterior surface of the shaft, and its upper surface is in continuation with the anterior surface of the olecranon process, the two between them forming the semilunar notch or sigmoid fossa.

Styloid Process of Radius

Fig. 4. - The Right Radius and Ulna (Anterior View).

On the outer side of the upper end of the shaft is another articular surface, the radial notch, for articulation with the head of the radius. The posterior surface of the olecranon is smooth and subcutaneous. Just below the coronoid process is a rough tuberosity for the insertion of brachialis anticus. The shaft is triangular for about two-thirds of its length, then tapers gradually, and becomes smooth and rounded. It has three surfaces-anterior, inner, and outer-and three borders-posterior, inner, and interosseous. The posterior border is subcutaneous throughout its length. The lower end of the bone is much smaller than the upper, and nearly circular. On its inner surface it has a projection, the styloid process, pointing downwards, and on its outer surface an articular facet for the lower end of the radius. The inferior surface is smooth for articulation with the triangular fibro-cartilage of the wrist-joint.

The ulna articulates above with the trochlear surface of the humerus by means of the semilunar notch, and below with the articular disc of the wrist-joint; on its outer surface it articulates with the radius at either extremity.

Ossification. - The centre for the shaft appears before birth, and secondary centres for the olecranon process and the lower end of the shaft appear later to form epiphyses which unite with the shaft in adult life.