The ulna, (French : ulnlna) is placed at the inner side of the forearm ; it is a long and rather irregular bone, larger at the upper than at the lower extremity, a conformation the reverse of that which obtains in the radius.

Its body or shaft, is marked off into three surfaces by three prominent lines. These are all broader above than below, in consequence of the increasing size of the bone. The anterior surface, slightly depressed, is grooved longitudinally for the origin of the deep bender of the fingers (latin: flexor digitorum profundus), and marked at its upper third by a foramen,  directed obliquely upwards for the medullary vessels. The internal surface, smooth and somewhat excavated superiorly, is covered in the greater part of its extent by e deep bender of the fingers muscle; it becomes round inferiorly, where it is subcutaneous. The external surface, rough and irregular, is divided into two parts, of which one, of small extent, corresponds with the superior extremity of the bone, and forms an elongated depression for the anconeus muscle ; the other, reaching to the lower extremity, gives origin to the extensors of the thumb and the supinator muscle (lat. : supinator radii brevis). Of the three margins, the internal and posterior are round, and for the most part, smooth; the external is sharp, and gives attachment to the interosseous ligament.

At the superior extremity of the bone are situated two eminences, so placed as to bind the cavity by which it articulates with the humerus.

Of these, one called olecranon is nearly on a line with the shaft of the bone; the other, the coronoid process, projects from its anterior surface. The olecranon terminates in a rough tuberosity and an obtuse point; the former giving insertion to the triceps extensor; the latter being lodged, when the arm is extended, in the posterior cavity at the end of the humerus. Anteriorly, this process is smooth and hollowed out, to form part of the surface of articulation with the humerus; posteriorly, it presents a flat triangular surface, which is subcutaneous. The coronoid process, by a gentle ascent, rises upwards and forwards from the anterior surface of the bone, and terminates in a sharp ridge, which overhangs the articular surface, and is received during flexion into the anterior depression in the humerus ; its anterior surface, rough and triangular, gives insertion to the brachialis muscle (lat : brachialis anticus); its upper aspect is smooth and excavated : its inner border gives attachment to the internal lateral ligament; the external is hollowed into a smooth depression, the smaller sigmoid cavity, which articulates with the head of the radius. The great sigmoid cavity, formed by the junction of the smooth surfaces of these two processes, has been so named from some supposed resemblance to the form of the Greek letter Σ, as it was originally written. Covered by cartilage in its entire extent, it is divided into two parts (but unequally, the inner one being the larger,) by a smooth ridge running downwards from the peak or point of the olecranon to that of the coronoid process. This ridge and the concavities beside it correspond, the one with the groove, the others with the lateral parts of the trochlea. At the margins of the sigmoid cavity may be observed two notches which mark off the parts of the surface which belong to the olecranon and the coronoid process respectively.

At the inferior extremity of the bone, which is small and rounded, are situated two eminences, of which the external one, named the head of the ulna, round and covered with cartilage, presents two aspects, of which one, nearly circular in its form, looks towards the wrist-joint, and corresponds with the triangular fibro-cartilage of that articulation; whilst the external one, narrow and convex, is received into the semi-lunar cavity in the contiguous border of the radius. The internal eminence, named the styloid process, projects on a line with the posterior and inner surface of the bone; it is elongated in its form, and gives attachment to the internal lateral ligament of the joint. The head and the styloid process are separated posteriorly by a groove, which is traversed by the tendon of the extensor carpi ulnaris, and inferiorly by a depression at the base of the styloid process, into which the triangular fibro-cartilage is inserted.

Attachments of muscles

The anterior surface, superiorly, and in the middle, gives attachment to the the deep bender of the fingers ; inferiorly, to the pronator quadratus ; the posterior surface, to the anconeus, the extensor carpi ulnaris, supinator radii brevis, the extensores pollicis, and the extensor indicis; by the posterior longitudinal line, to an aponeurosis common to the flexor carpi ulnaris, flexor digitorum profundus, and extensor carpi ulnaris. The superior extremity, gives attachment by the summit of the olecranon to the triceps brachialis; the coronoid process, by its anterior rough surface, gives attachment to the brachialis amicus; and by its internal side, to the second origin of the pronator teres.

Articulations

The ulna articulates with the humerus and the radius; it has no point of contact with the carpal bones, but it is brought into relation with the cuneiform bone by means of the inter-articular fibro-cartilage.

Ossification

The ulna begins to ossify both in the shaft and the epiphyses a short time after the radius.

The ends are cartilaginous at birth. In the fourth year (sometimes later) a granule of bone forms in the lower end. It appears in the middle of the head, and afterwards extends to the styloid process. The upper epiphysis is very small, the greater part of the olecranon being formed from the- original center of ossification. Bone matter is perceptible in this part at the tenth year or soon after.

The epiphyses join the superior about the sixteenth year of age, and the inferior one about the twentieth year.