The flexor digitorum superficialis is a large, wide muscle that lies partially deep and partially superficial on the forearm. It originates from all three arm bones: the humerus, the ulna, and the radius. The muscle extends down the arm narrowing into a tendon that inserts in the palmar side of the phalanges of the index, middle, ring, and little fingers. It does not have an attachment to the thumb. This muscle is used to bend the fingers.
