The abductor pollicis longus muscle is one of three muscles in the forearm that facilitate the movements of the thumb. The others are the extensor pollicis brevis and extensor pollicis longus. These three muscles, along with the extensor indicis, make up the group of muscles called the deep extensors. On the proximal end, the abductor pollicis longus muscle attaches to two arm bones. These are called the ulna and radius. It also attaches to the interosseous membrane. On the distal end, it attaches to the base of the first metacarpal bone. The abductor pollicis longus muscle lies directly underneath the supinator muscle. In some people, these two muscles merge together. The posterior interosseous nerve and a continuation of the deep branch of the radial nerve serve the abductor pollicis longus muscle. Blood is supplied by the posterior interosseous artery. The function of the abductor pollicis longus muscle is to abduct and extend the thumb at the carpometacarpal joint. It also assists in flexing the wrist.