The Ulna

The ulna of the horse is a reduced long bone situated behind the radius, with which it is partially fused in the adult.

The shaft (Corpus ulnæ) is three-sided and tapers to a point below. The anterior surface (Facies dorsalis) is applied to the posterior surface of the radius, and below the interosseous space the two bones are fused in the adult. The surface which enters into the formation of the space is smooth and usually presents a small nutrient foramen, directed upward. Above the space it is rough and is attached to the radius by an interosseous ligament which is usually permanent. The internal surface (Facies medialis) is smooth and slightly concave. The external surface (Facies lateralis) is flattened. The internal and external borders are thin and sharp, except at the interosseous space. The posterior border is slightly concave in its length and is rounded. The lower end is pointed and is usually a little below the middle of the radius. It is commonly continued by a fibrous cord to the distal external tuberosity of the radius, but this band may be replaced in part or entirely by bone.

The proximal extremity is the major part of the bone. It projects upward and somewhat backward behind the lower end of the humerus, and forms a lever arm for the extensor muscles of the elbow. The internal surface is concave and smooth. The external surface is convex and is roughened above. The anterior border bears on its middle a pointed projection, the processus anconæus or “beak,” which overhangs the semilunar notch or sigmoid cavity (Incisura semilunaris). The latter is triangular in outline, concave from above downward, and articulates with the humerus; in the middle of its lower part is an extensive synovial fossa. Just below the notch are two convex facets which articulate with those on the posterior aspect of the proximal end of the radius. The posterior border is nearly straight, and is thick and rounded. The free end or summit is a rough tuberosity, the olecranon, which gives attachment to the triceps brachii and other muscles.

The primitive distal extremity has, as previously stated, fused with the radius.

Development.—The ulna ossifies from three centers, of which one is for the main part of the bone, one for the olecranon, and one for the distal end. The cartilaginous embryonic ulna extends the entire length of the forearm. The lower part of the shaft is usually reduced to a small fibrous band or may disappear entirely; in some cases a variable remnant of it ossifies. The distal extremity fuses early with the radius. The olecranon unites with the rest of the bone at three to three and a half years. A medullary canal appears to occur constantly in the adult—contrary to the statements of some authors.