[TA] muscle of posterior (extensor) compartment of forearm; origin, posterior surface of middle of shaft of ulna; insertion, dorsal aspect of base of distal phalanx of thumb; action, extends distal phalanx of thumb; nerve supply, radial (posterior interosseous). SYN: musculus extensor pollicis longus [TA], long extensor (muscle) of thumb, musculus extensor longus pollicis.