Overview
The ulna is the caudolateral bone of the forearm. In cattle it is complete, from the elbow to the carpus, unlike in the horse, where its distal part disappears. Only the proximal end is thick, however: the shaft is thin and joined to the radius along almost its whole length, and in the adult it fuses with it. Only two openings remain between the bones, the proximal and distal interosseous spaces. The ulna bears little weight; its main role is to act as a lever for the extensors of the elbow.
Proximal end
- Olecranon: large, projecting caudally behind the elbow. Its free end, the olecranon tuber (tuber olecrani), forms the point of the elbow and receives the triceps brachii.
- Anconeal process (processus anconeus): the proximal edge of the trochlear notch; it fits into the olecranon fossa of the humerus when the elbow extends.
- Trochlear notch (incisura trochlearis): concave articular surface that grips the humeral condyle together with the head of the radius.
Shaft and distal end
- Shaft: slender and triangular in section, applied to the caudolateral surface of the radius; with it, it bounds the interosseous spaces.
- Lateral styloid process (processus styloideus lateralis): the distal end of the ulna, fused to the radius, which forms the lateral prominence of the distal forearm and articulates with the ulnar carpal bone.
Clinical relevance
- Olecranon fractures: the triceps pulls the proximal fragment away and the animal cannot extend the elbow; it stands with a "dropped" elbow, a sign also seen in radial nerve paralysis.
- Forearm fractures: because the ulna is joined to the radius, fractures of its shaft usually accompany radial fractures.
- Examination: the olecranon tuber is palpable and is the landmark for locating the elbow.