Summary Flexor Pollicis Brevis Origin Superficial head: Flexor retinaculum and tubercles of scaphoid and trapezium Deep head: Trapezoid and Capitate (more medial than superficial head) Insertion Lateral side of base of proximal phalanx of thumb Action Flexes thumb Innervation Superficial head: recurrent branch of the median nerve (C8, T1) Deep head: deep branch of ulnar nerve. Arterial Supply Superficial palmar branch of the radial artery