flexor pollicis brevis (muscle)
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flex·or pol·li·cis bre·vis (mus·cle)[TA]
thenar muscle of palm; origin, superficial portion from flexor retinaculum of wrist, deep portion from ulnar side of first metacarpal bone; insertion, palmar aspect of base of proximal phalanx of thumb; action, flexes proximal phalanx of thumb; nerve supply, median (superficial head) and deep branch of ulnar (deep head). Some authors consider the deep head to be the first in a series of four palmar interossei muscles of the hand.