After passing through the supinator muscle between its two layers the nerve appears in the extensor compartment of the forearm and passes downwards over the abductor pollicis longus origin. It now dips deeply to reach the interosseous membrane on which it passes between the muscles as far as the wrist joint. Here it ends in a small nodule from which branches supply the wrist joint and intercarpal ligaments. The nerve supplies the muscles which arise from the common extensor origin and the deep muscles of the extensor compartment. It is sensory to the interosseous membrane and periosteum of the radius and ulna, and to the dorsal periosteum and ligaments of the carpal bones. It has no cutaneous branch.