The radius is the shorter of the two long bones of the forearm, the other being the ulna. It extends from the elbow to the wrist, and is the bone on the thumb side of the arm. It rotates around the ulna and enables the hand to rotate and be flexible.
The radius is a long, prism-shaped bone, which is slightly curved longitudinally. It articulates with the capitulum of the humerus, the radial notch and the head of the ulna. The shaft of the radius has a broad base that articulates with the lower end of the ulna and with the upper bones of the wrist. Its two extremities are referred to as the proximal and distal ends. The distal end is a block of bone that can be felt at the distal end of the forearm's front, back and lateral sides. According to InnerBody, the radius is narrowest at the elbow and widens as it extends to the wrist. At its proximal end, the rounded head forms its pivoting part of the elbow joint that permits rotation of the lower arm and hand. It is considerably wider at its distal end than the ulna and forms the bulk of the wrist joint.