[TA] muscle of posterior (extensor) compartment of forearm; origin, dorsal surface of distal radius and adjacent interosseous membrane; insertion, posterior aspect of base of proximal phalanx of thumb; action, extends and abducts the thumb at metacarpophalangeal joint; nerve supply, radial (posterior interosseous). SYN: musculus extensor pollicis brevis [TA], musculus extensor brevis pollicis, short extensor (muscle) of thumb.