The radius is the shorter of the two long bones of the forearm. The other is the ulna. The radius is the bone on the thumb side of the arm. The shaft of the radius has a broad base that joins the lower end of the ulna and the upper bones of the wrist at a large process called the radial styloid. The disk-shaped head of the radius, which is smaller than the base, joins the lower end of the humerus (bone in the upper arm) to form the elbow joint.