Now we’ll move on to look at the four short muscles of the thumb. One of them lies by itself, and three lie close together. The one that’s by itself is adductor pollicis, the other key pinch muscle.
Adductor pollicis has two heads, a transverse head and an oblique head. The transverse head arises from the third metacarpal. The oblique head arises from the ligaments in the base of the carpal tunnel. Adductor pollicis inserts on the ulnar sesamoid bone, and on the base of the proximal phalanx of the thumb. Adductor pollicis produces adduction at the carpometacarpal joint.
The other three thumb muscles make up this bulge, that’s called the thenar eminence. Collectively these three are called the thenar muscles. On the outside are flexor pollicis brevis, and abductor pollicis brevis. Deep to them both is opponens pollicis.
Abductor brevis arises from the trapezium, and from the flexor retinaculum. Flexor brevis arises from the flexor retinaculum and from the trapezoid. These two muscles insert here, on the base of the proximal phalanx of the thumb, on the radial side.
Opponens pollicis, here it is by itself, arises from the trapezium, and from the flexor retinaculum, and inserts along the radial side of the first metacarpal.
The three thenar muscles overlap, and their actions overlap too. Between them they produce abduction and flexion at the carpometacarpal joint, bringing the thumb away from the second metacarpal and across the palm, and thereby also rotating it medially. As we’ve seen, these movements add up to opposition of the thumb.
Lastly, let’s look at the three short muscles of the little finger. They make up this smaller bulge, the hypothenar eminence, and collectively they’re called the hypothenar muscles.
They’re arranged in much the same way as the thenar muscles, and their names are similar. On the outside are abductor digiti minimi, and flexor digiti minimi. Deep to them lies opponens digiti minimi.
The abductor arises from the pisiform bone, and inserts just like an interosseous muscle, partly into the base of the proximal phalanx, and partly into the extensor mechanism.
The flexor arises from the hamate bone, and the flexor retinaculum, and inserts near the abductor on the proximal phalanx. The opponens arises from the hook of the hamate, and inserts along the ulnar side of the fifth metacarpal. The ulnar nerve and artery pass underneath the flexor and opponens as they enter the hand.
The abductor has the same actions as an interosseous muscle. The flexor helps to flex the MP joint, and the opponens produces flexion of the fifth metacarpal at the carpometacarpal joint. These three muscles help to make the little finger specially mobile.