Flexor pollicis longus muscle

The flexor pollicis longus (FPL, Latin flexor, bender; pollicis, of the thumb; longus, long) (/ˈflɛksər ˈpɒlᵻsᵻs ˈlɒŋɡəs/)) is a muscle in the forearm and hand that flexes the thumb. It lies in the same plane as the flexor digitorum profundus.

This muscle is unique to humans, being "either rudimentary or absent" in other primates.