Abductor pollicis brevis m

One of the 3 thenar muscles, the abductor pollicis brevis originates from the carpal bones (scaphoid and trapezium) and inserts into the proximal phalanx of the thumb.  The action of the muscles is to abduct the thumb, and it is innervated (as are all 3 of the thenar muscles) by the recurrent branch of the median n.