[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. SYN: musculus flexor pollicis brevis [TA], short flexor (muscle) of thumb.