Most of the hand muscles are located on the anterior or palmar side of the hand. Keep in mind the midline of the hand is digit number 3. The thumb or thenar muscles can be identified by moving from superficial to deep, and by observing the distal attachment of these muscles. These muscles form the uniquely human thenar prominence. They arise from a point near the midline of the proximal palmar area - the flexor retinaculum. They then pass in a distal and lateral direction.

The abductor pollicis brevis is the most superficial and lateral of this group, while the medial superficial muscle is the flexor pollicis brevis . Both of these muscles insert on the lateral side of the proximal phalanx of the thumb.

Directly under or deep to these two muscles lies the opponens pollicis . Its oblique fibers insert into the first metacarpal to more effectively rotate the first metacarpal during thumb opposition.

See if you can find the median nerve innervating any of these three muscles.

The very deep and transversely running adductor pollicis is slightly distal and medial to the true thenar group. Although it receives motor innervation from the ulnar nerve and does not contribute to the thenar eminence, it's still considered part of the thenar group.

Move now to the little finger side and locate the hypothenar muscles. They are nearly identical in attachment and function to the three similarly named thenar muscles. The difference is they act on the fifth digit instead of the first digit. The most lateral and superficial abductor digiti minimi, and the more medial flexor digiti minimi, lie in approximately the same plane and insert on the proximal phalanx of the fifth digit. The more deeply placed opponens digiti minimi, with its more oblique fibers, inserts on the fifth metacarpal. The little finger has no "adductor."

A deeper dissection into the palm of the hand reveals two very important sets of rather small muscles. The first and more superficial of these is the lumbricals. These muscles arise from the radial side of the tendons of the flexor digitorum profundus and insert into the dorsal digital expansion mechanism of the extensor digitorum. They look like small "worms" as their name implies.

Deeper yet, under the tendons of the flexor digitorum profundus and the lumbricales, lies the deep palmar arch (or the terminal end of the radial artery ), and, deeper still, the three palmar interossei. Remember these muscles attach proximally to the metacarpals and insert on the dorsal digital expansion and adduct the M-P joints. These muscles are very difficult to see. Do not tear up other palmar structures in order to see them!

You can, however, see the four posterior interossei on the dorsum of the hand as there is very little tissue superficial to them. These muscles also run from adjacent metacarpals to the dorsal digital expansion. All of the posterior interossei are abductors of the M-P joints. Take special note of the first posterior interossei. Can you distinguish this muscle from the adductor pollicis?