The opponens pollicis, or flexor ossis metacarpi pollicis, is named from its action, as it helps in opposing the thumb to the other fingers and at the same time is a flexor of the first metacarpal bone. It is a short, thick, triangular sheet.

Origin

The front of the ridge on the trapezium below the preceding ; and the outer half of the lower border of the anterior annular ligament.

Insertion

The whole of the outer border of the anterior surface of the shaft of the first metacarpal bone.

Structure

Arising by a short tendinous or fleshy origin, the muscular fibres diverge fanwise to their insertion. The lower border of the muscle is often so blended Avith the outer head of the next muscle that the separation is somewhat artificial.

Nerve-supply

From the same source as the preceding ; the filaments entering the anterior surface of the muscle near to the upper part of its ulnar border.

Action

To flex the first metacarpal bone, which it draws forwards and inwards. This movement, on account of the shape of the carpo-metacarpal joint, is accompanied by a certain amount of rotation inwards, by which the palmar aspect of the thumb is made to look backwards and inwards.

Relations

Superficially, the abductor pollicis and the thenar fascia ; deeply, and upon its ulnar border, the flexor brevis pollicis. It also lies upon the joint between the metacarpal bone and the trapezium.