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1.3.11 Long (extrinsic) muscles of the thumb

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(2.38)

Now let’s move on, to look at the long muscles of the thumb. The thumb has a long flexor, a long abductor, and two extensors, a long one and a short one.

The long flexor, flexor pollicis longus, lies deep in the forearm. We’ll remove flexor digitorum superficialis to see it. Here’s flexor pollicis longus, lying alongside flexor digitorum profundus. It arises from the anterior surface of the radius, and from the interosseous membrane.

Its tendon passes through the carpal tunnel with the finger flexors. Here’s the tendon of flexor pollicis longus emerging. It enters the fibrous flexor sheath of the thumb, and inserts on the base of the distal phalanx. Flexor pollicis longus flexes both the MP joint and the IP joint of the thumb.

The other three long thumb muscles lie on the extensor aspect of the forearm. They lie deep to extensor digitorum, which we’ll remove.

This is the long abductor, abductor pollicis longus, and these are the extensors, extensor pollicis brevis, and longus. The abductor arises from the radius here, and also from the interosseous membrane. The two extensors arise a little more distally, the short one here, the long one here.

Now we'll put extensor digitorum back in the picture. Here it is. The three thumb muscles emerge obliquely from beneath the extensor digitorum. Their tendons pass beneath the extensor retinaculum, extensor longus by itself, the other two together.

The tendon of abductor pollicis longus inserts round here, on the base of the first metacarpal. Extensor pollicis brevis inserts on the base of the proximal phalanx, and extensor pollicis longus inserts on the base of the distal phalanx.

The movement produced by abductor pollicis longus is a combined abduction and extension occurring at the CMC joint. It's the reverse of opposition.

Extensor pollicis longus ...

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(2.38)

Now let’s move on, to look at the long muscles of the thumb. The thumb has a long flexor, a long abductor, and two extensors, a long one and a short one.

The long flexor, flexor pollicis longus, lies deep in the forearm. We’ll remove flexor digitorum superficialis to see it. Here’s flexor pollicis longus, lying alongside flexor digitorum profundus. It arises from the anterior surface of the radius, and from the interosseous membrane.

Its tendon passes through the carpal tunnel with the finger flexors. Here’s the tendon of flexor pollicis longus emerging. It enters the fibrous flexor sheath of the thumb, and inserts on the base of the distal phalanx. Flexor pollicis longus flexes both the MP joint and the IP joint of the thumb.

The other three long thumb muscles lie on the extensor aspect of the forearm. They lie deep to extensor digitorum, which we’ll remove.

This is the long abductor, abductor pollicis longus, and these are the extensors, extensor pollicis brevis, and longus. The abductor arises from the radius here, and also from the interosseous membrane. The two extensors arise a little more distally, the short one here, the long one here.

Now we'll put extensor digitorum back in the picture. Here it is. The three thumb muscles emerge obliquely from beneath the extensor digitorum. Their tendons pass beneath the extensor retinaculum, extensor longus by itself, the other two together.

The tendon of abductor pollicis longus inserts round here, on the base of the first metacarpal. Extensor pollicis brevis inserts on the base of the proximal phalanx, and extensor pollicis longus inserts on the base of the distal phalanx.

The movement produced by abductor pollicis longus is a combined abduction and extension occurring at the CMC joint. It's the reverse of opposition.

Extensor pollicis longus extends the IP joint and the MP joint of the thumb. Extensor pollicis brevis extends only the MP joint.

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