Photograph of the bones of the left forearm in anterior aspect. The ulna, the inner (medial) bone is at left; the shorter radius (right) partially revolves about the ulna, allowing the hand to be turned downwards (pronation). The head of the radius (top) articulates with the humerus, and the lower end with the scaphoid and lunate bones of the carpus (wrist). The ulna articulates with the humerus and radius above and with the radius and indirectly with the carpus below. The upper prominence on the ulna is the olecranon process, which projects behind the elbow joint.