Radius

Radius is one of the two bones (the ulna is the other) of the antebrachium (forearm). 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, 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 the horse, radius and ulna are welded together and cannot move independently of each other. 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.