The flexor pollicis brevis (thumb muscle) is a small, narrow muscle consisting of two portions, the outer and inner. The outer portion is superficial and arises from the annular ~ligament~. The inner portion is deep and arises from the ulnar side of the metacarpal bone. These two portions work together to flex and rotate the thumb. The flexor pollicis brevis muscle is a part of the thenar eminence. The thenar eminence is a teardrop- or egg-shaped mass of muscle that constitutes the base of the thumb. It consists of the |abductor pollicis brevis|, the flexor pollicis brevis, and the |opponens pollicis|.
