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The abductor pollicis longus muscle, or musculus abductor pollicis longus in Latin, is an extrinsic muscle of the hand located within the posterior compartment of the forearm. It is often abbreviated as APL. “Abductor” refers to the function of abduction, or movement away from the midline, “pollicis” is a Latin term that refers to the thumb, and “longus” is a Latin term for long, which is a reference to the length of the muscle. The primary functions of the abductor pollicis longus muscle are to abduct the thumb and extend it at the carpometacarpal joint, and also to aid in extending the wrist joint. The abductor pollicis longus, the extensor pollicis brevis and the extensor pollicis longus muscles together are called the deep extensor muscles of the forearm. They are also referred to as the outcropping muscles of the thumb because in the distal forearm they emerge from between the more superficial extensor digitorum and extensor carpi radialis brevis muscles to act on the thumb. In addition, they form the boundaries of the anatomical snuff box on the posterior hand. The abductor pollicis longus and the extensor pollicis brevis tendons, which share a common fibrous sheath, form the anterior boundary, while the extensor pollicis longus tendon forms the posterior boundary.
The origin, or proximal attachment, of the abductor pollicis longus muscle is on the posterior surfaces of the proximal ulna and radius, as well as along the proximal, posterior portion of the interosseous membrane, which is the membrane that lies between the radius and ulna. The insertion, or distal attachment, of the abductor pollicis longus muscle is at the base of the first metacarpal bone. Additional sites of insertion can sometimes be found on the dorsal surface of the trapezium bone, as well as on the abductor pollicis brevis muscle, or occasionally the opponens pollicis muscle, which both lie deep to the abductor pollicis longus muscle. Innervation is provided by the posterior interosseous nerve, which arises from the deep branch of the radial nerve, from spinal nerve roots C7 and C8. Blood supply to the abductor pollicis longus arrives from the posterior interosseous artery.
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