The flexor longus pollicis - named from its action upon the thumb and its .ength as compared with the short muscles of the ball of the thumb- is a fusiform sheet.

Origin

1. The anterior surface of the radius below the oblique line, with the exception of the last two inches of the surface; and the adjacent surface of the interosseous membrane.
2. A second head arises from the inner border of the coronoid process of the ulna, but occasionally it may come from the internal condyle of the humerus.

Insertion

The front of the base of the last phalanx of the thumb.

Structure

A penniform muscle arising fleshy from the bone and the inter-osseous membrane. The tendon first appears upon the anterior surface near to its palmar border about the middle of the forearm. It receives the fleshy fibres upon its outer border and posterior surface and becomes free at the level of the wrist joint, where it enters the canal formed by the anterior annular ligament and the front of the carpus. It is here invested by a special compartment of the great synovial bursa and this compartment is continuous with the synovial sheath of the lendon as it lies in the thumb. After entering the palm of the hand, the tendon passes beneath the outer head of the flexor brevis pollicis, then through the groove formed by the two sesamoid bones which belong to the tendons of this muscle and after lying in close contact with the concave palmar surface of the first phalanx of the thumb, it is inserted into the front of the base of the second phalanx.

Nerve-supply

From the outer and inner cords of the brachial plexus (through the eighth cervical and first thoracic nerves) by means of the anterior interosseous branch of the median nerve, which sends filaments to it upon its anterior surface lear its ulnar border about the middle of the forearm.

Action

It is a powerful flexor of the last phalanx of the thumb. It will also flex its metacarpo-phalangeal joint and afterwards it will assist the other flexors of the wrist.

As the bones of the thumb are not in the same plane with those of the fingers, out rotated so that their palmar surface looks towards the rest of the hand, their flexion will be accompanied by adduction; as when the thumb meets the other fingers in picking up any small object, or in grasping anything between the fingers and thumb.

Relations

Superficially, the flexor sublimis digitorum, flexor carpi radialis, brachio-radialis (supinator longus) and the radial vessels; deeply, the pronator quadratus and wrist joint. In the hand, after passing beneath the anterior annular ligament, it is covered by the opponens pollicis and the outer head of the flexor brevis pollicis and it lies on the inner head of the flexor brevis pollicis.

Front of the forearm: third layer of muscles.

Variations

Besides the communication with the flexor profundus digitorum, we occasionally find slips passing from the tendon of the flexor longus pollicis to join the first Imubricalis muscle. The second head may be absent, as in the figure.