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bjbjT~T~ Forearm & Hand 2 Okay I am leaning in close again to see the detail. Here we
are still on the left forearm, but moving to the back or posterior or dorsal side so
pronating the left forearm. We are again looking at the deepest muscles first those will be
some thumb muscles - and then adding more superficially. Deepest, and coming from the
radius and ulna, are three muscles going to the thumb. These are first the abductor pollicis
longus running to the base of the first metacarpal, and, traveling with it, the extensor pollicis
brevis. These two muscles travel together passing through one narrow connective tissue
compartment, the first dorsal compartment, and as such are involved in an overuse syndrome
called de Quervain s tenosynovitis. That has to do with both of these tendons gliding through
the same small passage with too much repetition of use, leading to swelling, pain and more
swelling and pain. The third thumb muscle, separate a little bit from the other two,
is the extensor pollicis longus, and it goes all the way to the distal phalanx of the thumb.
Those three muscles together form the deepest layer. You will see a view of these three
where we go from looking at them with the forearm horizontal, to being viewed in a vertical
orientation, so you can see the way the three tendons line up. A little bit more superficial
and over more toward the ulnar side we see the extensor digitorum some people call this
the extensor digitorum communis. It is like the flexor muscles in that it is one belly
that divides and gives rise to four tendons that travel to digits 2, 3, 4, and 5. These
tendons travel to insert on the distal phalanx of each finger, attaching to other parts of
the finger along the way. But these don t have to interact with another set of tendons
going to the digits as we saw on the flexor side. And now over all the way to the ulna
side, and this is more ulnar than it is posterior or dorsal, is the extensor carpi ulnaris.
So this is the extensor of the wrist on the ulnar side, inserting onto the base of the
fifth metacarpal, and coming from the common extensor tendon on the lateral epicondyle.
And now coming back over toward the radial side, and more superficial, we have the extensor
carpi radialis longus going to the base of the second metacarpal, and extensor carpi
radialis brevis going to the base of the third metacarpal. Those arise from the lateral epicondyle.