Muscle · Forearm

Abductor pollicis longus

Musculus abductor pollicis longus

A deep muscle of the back of the forearm that comes to the surface on the way to the thumb. It arises from the backs of both forearm bones and the sheet between them, then winds around the outer edge of the radius so that a muscle of the extensor compartment ends up on the front of the wrist's radial border, inserting on the base of the thumb's metacarpal. It carries the whole thumb away from the palm and helps steady the wrist as it does. Its tendon runs beside that of extensor pollicis brevis in a single tight sheath at the styloid of the radius, which is the anatomical basis for the two of them being discussed as a pair.