The ulna is one of two bones that give structure to the forearm. The ulna is located on the opposite side of the forearm from the thumb. It joins with the humerus on its larger end to make the elbow joint, and joins with the carpal bones of the hand at its smaller end. Together with the radius, the ulna enables the wrist joint to rotate. The ulna is 50 percent larger in diameter than the radius at 4 to 5 months of age. During adult life, when remodeling and resorption are complete, the ulnar diameter becomes half that of the radius. The ulna is found, and has similar function, in both humans and four-footed animals, such as dogs and cats. If the ulna breaks, it will most commonly occur at either the point where the radius and ulna form a joint or where the ulna forms a joint with the hand’s carpal bones. Ulnar fractures cause severe pain, difficulty in moving the joint affected, and even deformity of the arm if the fracture is compound.