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The extensor pollicis brevis muscle, or musculus extensor pollicis brevis in Latin, is an extrinsic muscle of the hand located within the posterior compartment of the forearm. It is often abbreviated as EPB. “Extensor” refers to the function of extension, “pollicis” is a Latin term referring to the thumb, and “brevis” is a Latin term for short, which is a reference to the short length of the muscle. As its name suggests, the functions of the extensor pollicis brevis muscle are to extend the proximal phalanx of the thumb at the metacarpophalangeal joint, to extend the carpometacarpal joint, and to aid in extension of the wrist. The origin, or proximal attachment, of the extensor pollicis brevis muscle is on the posterior surface of the distal radius, as well as along the distal interosseous membrane between the radius and ulna. The insertion, or distal attachment, is located at the base of the proximal phalanx of the thumb, or digit one. Innervation is provided by the posterior interosseous nerve, which arises from the deep branch of the radial nerve, from spinal nerve roots C7 and C8. Blood supply to the extensor pollicis brevis muscle comes from the posterior interosseous artery. The extensor pollicis brevis, the extensor pollicis longus, and the abductor pollicis longus muscles together are called the deep extensor muscles of the forearm. They are also referred to as the outcropping muscles of the thumb because in the distal forearm they emerge from between the more superficial extensor digitorum and extensor carpi radialis brevis muscles to act on the thumb. Note that while the extensor digitorum and extensor carpi radialis brevis muscles are more superficial, their distal tendons run posterior, or deep, to the extensor pollicis brevis and abductor longus muscle tendons. The outcropping muscles also form the boundaries of the anatomical snuff box on the posterior hand, which contains the radial artery. The extensor pollicis brevis and the abductor pollicis longus muscle tendons, which share a common fibrous sheath, form the anterior boundary, while the extensor pollicis longus muscle tendon forms the posterior boundary.
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