The humerus, or upper arm bone, is fairly consistent across the various species. It varies mostly in its length and thickness relative to the other limb bones. It is short in the horse and ox, longer in the carnivores, and long and slender in primates. The upper end, the head, articulates with the scapula at the shoulder joint. The point on the outside front corner of the upper end of the humerus, although covered by thin muscle, may be seen on the surface. Called the point of the shoulder, it is an important landmark.
There are two bones in the forearm—the radius and the ulna. The radius is the weight-supporting bone in four-legged animals. In the horse and the ox, it is a strong bone that passes straight down the forearm to expand at the wrist (without the presence of the ulna). The expanded upper end of the radius lies in front of the much narrower ulna. In dogs, the radius passes downward from the outside of the elbow to the inner side of the wrist, crossing diagonally in front of the ulna
(which in dogs does reach the wrist). In cats and primates, the radius also begins at the outside of the elbow, but it changes position at the wrist as it rotates around the lower end of the ulna as the forearm is pronated or supinated (rotated so the palm side faces forward or back). Only the radius articulates with the carpal bones of the wrist.
The upper end of the ulna forms the point of the elbow, or olecranon, which becomes more prominent on the surface as the elbow is bent. The highly developed triceps muscle of the horse will actually overhang and conceal the olecranon in the standing position when the muscle is fully relaxed. In the horse, only the upper portion of the ulna is present; its lower end tapers to a point. In the ox, the lower portion of the ulna is tapered, but it does reach the wrist. In both animals, the ulna is fused to the back of the radius. In the dog and cat, the ulna extends down to the wrist, and is not fused to the radius.
In the dog, the radius and ulna are for the most part locked in place in the pronated position, with the "palm of the paw" permanently facing backward. The cat, however, can supinate and pronate its forearm, like primates, allowing its palm to face forward or backward. In this action, the upper end of the radius rotates in place at the outside of the elbow, while the wrist end pivots around the lower end of the ulna, moving from side to side. The forearm bones are oriented basically vertically in the standing position. In some of the deer, and in goats and sheep, they tend to be directed downward and inward, bringing the anatomical wrists (carpals) toward each other and giving the animal a knock-kneed appearance.
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