extensor pollicis brevis muscle

Its middle third is broad, slightly concave, and gives origin to the Abductor pollicis longus above, and the extensor pollicis brevis muscle below.

Its lower part is narrow, and covered by the tendons of the abductor pollicis longus muscle and extensor pollicis brevis muscle.

The lateral surface of this process is marked by a flat groove, for the tendons of the abductor pollicis longus muscle and extensor pollicis brevis muscle.