Origin and Insertion
The muscle's superficial part arises from the distal edge of the flexor retinaculum and the tubercle of the trapezium, a bone in the wrist. It passes along the radial side of the tendon of the flexor pollicis longus, and, becoming tendinous, is inserted into the radial side of the base of the proximal phalanx of the thumb; in its tendon of insertion there is a sesamoid bone.
The deeper (and medial) portion of the muscle is very small, and arises from the ulnar side of the first metacarpal bone between the oblique part of the adductor pollicis and the lateral head of the first dorsal interosseous muscle, and is inserted into the ulnar side of the base of the first phalanx with the adductor pollicis.
The deep (medial) part of the flexor brevis pollicis is sometimes described as the first palmar interosseous muscle. When this muscle is included, the total number of palmar interossei is four. Otherwise, there are only three palmar interossei.
Read more about this topic: Flexor Pollicis Brevis Muscle
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