The structure indicated is the abductor pollicis brevis muscle of the hand.

The abductor pollicis brevis muscle is one of the intrinsic muscles of the hand that belongs to the thenar group.

The thenar group of intrinsic hand muscles consist of three muscles:

1. Abductor pollicis brevis
2. Flexor pollicis brevis
3. Opponens pollicis

All the intrinsic muscles of the hand, except the thenar muscles and the lateral two lumbrical muscles are innervated by the deep branch of the ulnar nerve. The thenar muscles and the lateral two lumbrical muscles are innervated by the median nerve.  A useful mnemonic for remembering this is MEATLOAF. “MEAT” refers to the Median nerve, and LOAF refers to the muscles which it innervates: Lateral two lumbricals, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis.

Origin: Flexor retinaculum, tubercles of scaphoid and trapzeium

Insertion: Lateral base of proximal phalanx of thumb

Innervation: Recurrent branch of median nerve: C8, T1

Action: Abducts thumb

Learn all about the anatomy of the muscles of the hand in this tutorial.