A member of the dorsal group;The extensor pollicis longus muscle arises from the dorsal surface of the ulna and the interosseous membrane, just distal to the abductor pollicis longus muscle. After passing through the extensor retinaculum, it crosses over the extensor carpi radialis longus and brevis muscles to insert on the base of the distal phalynx of the 1st digit. It's main action is to extend the distal phalynx of the 1st digit; but it can also abduct the hand.