The abductor pollicis - named from its action upon the thumb - is a small thi triangular sheet.

Origin

• The tuberosity of the scaphoid bone ;
• the upper part of the ridge on the trapezium ;
• the outer part of the front of the anterior annular igament ;
• the outer or thenar division of the palmar fascia by which the ball of the thumb is covered ;
• and the slip from the tendon of the extensor ossis netacarpi pollicis which joins this part of the palmar fascia.

Insertion

With the outer tendon of the flexor brevis pollicis into the radial side of the base of the first phalanx of the thumb ; and into the outer edge of the aponeurosis of the extensor longus pollicis upon the back of the first phalanx.

Structure

Arising by fleshy or short tendinous fibres, the muscle converges upon a short tendon which blends with the adjacent tendon of the flexor brevis.

Nerve-supply

From the outer and inner cords of the brachial plexus (through the sixth cervical nerve), by a branch from the palmar division of the median nerve, which, after passing under the anterior annular ligament, goes upwards and outwards to enter the upper part of the deep aspect of the muscle near its ulnar border.

Action

To abduct, and to flex the first phalanx of the thumb. As the bones of the thumb are in a different plane to those of the fingers, this movement will draw the thumb forwards and at the same time slightly inwards. By its insertion into the aponeurosis of the extensor longus pollicis, this muscle will help to extend the last phalanx of the thumb.

Relations

Superficially, the thenar fascia, and the superficialis vol^e artery, which usually perforates the muscle ; deeply, the outer head of the flexor brevis pollicis and the opponens pollicis.

Variations

As already mentioned, it receives occasional slips from the radial extensors of the carpus and the extensor ossis metacarpi pollicis. It frequently receives a thin muscular slip from the skin over the trapezium ; it may also be supplemented by the opponens pollicis or a small slip from the styloid process of the radius.