<P> The tendons of the extensor pollicis longus and extensor pollicis brevis form what is known as the anatomical snuff box (an indentation on the lateral aspect of the thumb at its base) The radial artery can be palpated anteriorly at the wrist (not in the snuffbox). </P> <P> The intrinsic muscles of the thumb can be divided into two groups; the thenar eminence and other muscles . The thenar eminence refers to the group of muscles on the palm at the base of the thumb . The three muscles composing the thenar eminence are the abductor pollicis brevis, flexor pollicis brevis and opponens pollicis . The other two muscles that influences movement of the thumb are the adductor pollicis and the first dorsal interosseous muscle . </P> <P> The most superficial muscle in the thenar group is the abductor pollicis brevis . The abductor brings the thumb away from the other four fingers . The flexor pollicis brevis, which lies next to the abductor, will flex the thumb, curling it up in the palm . The opponens pollicis lies deep to abductor pollicis brevis . As its name suggests it opposes the thumb, bringing it against the fingers . This is a very important movement, as most of human hand dexterity including grip comes from this action . </P> <P> The abductor pollicis brevis originates on the scaphoid tubercle and the flexor retinaculum . It inserts to the radial sesamoid bone and the proximal phalanx of the thumb . It is innervated by the median nerve (C8 and T1). </P>

When opposing the thumb which muscle is active