[TA] outcropping muscle of posterior compartment of forearm; origin, interosseous membrane and posterior surfaces of radius and ulna; insertion, lateral side of base of first metacarpal bone; action, abducts and assists in extending thumb; nerve supply, radial. SYN: musculus abductor pollicis longus [TA], long abductor muscle of thumb, musculus extensor ossis metacarpi pollicis.