The arm bone is composed of three bones: the humerus, ULNA, and RADIUS. The humerus is the longest bone in the arm and is located between the shoulder and elbow joints. The ULNA is located on the medial side of the forearm and is the larger of the two forearm bones. The RADIUS is located on the lateral side of the forearm and is the smaller of the two forearm bones. Together, these three bones form the arm bone and provide support and movement to the arm.