quite deep. So now let’s move on to the posterior
compartment. And the posterior compartment is
going to contain muscles that essentially
extend the wrist. But first of all, we can
see in this posterior compartment, brachioradialis.
Brachioradialis, as I just mentioned, was
that muscle that can also flex the elbow.
But it’s in the posterior compartment due
to its nerve supply, being supplied by the
radial nerve. So here we can see brachioradialis.
We can then see two muscles. These are known
as extensor carpi radialis. We have a longus
version and we have a brevis version. And
these are coming from the humerus and they’re
coming from the lateral epicondyle as we’ll
see. So we can see the brachioradialis, extensor
carpi radialis longus, and extensor carpi
radialis brevis. Here, we can see their attachment
sites. Brachioradialis is coming from the
proximal two-thirds of the lateral supracondylar
ridge. Here, we’d have the lateral supracondylar
ridge. That’s where brachioradialis is coming
from. And it passes to the lateral surface
of the distal radius. So it just crosses
the elbow. It doesn’t cross the wrist. It passes
to the lateral surface of the distal radius
and the pre-styloid process; so just a small
elevation before the styloid process.
It’s innervated via the radial nerve, and it’s
a weak flexor of the elbow. It’s a strong
flexor, however, when the forearm is mid-pronated.
So if the elbow is fully supinated like this,
it’s a very weak flexor. But in this mid-pronator
position, it is a very strong flexor of the
elbow joint. So, really important muscle,
brachioradialis. If we then look at the two
muscles slightly deeper. We have extensor
carpi radialis longus. We have extensor carpi
radialis brevis. We can see that these two
muscles also coming from the lateral supracondylar
ridge, extensor carpi radialis. And extensor
carpi radialis brevis is coming from the lateral
epicondyle. So we can see that they gradually
work their way down the humerus. From the
lateral supracondylar ridge all the way down
to the lateral epicondyle. Extensor carpi
radialis longus attaches to the second metacarpal.
Extensor carpi radialis brevis attaches to
the third metacarpal. And again, we can see
that here, attaching to the second metacarpal
and attaching to the third metacarpal, extensor
carpi radialis longus, attaching to the second,
brevis, attaching to the third. We can see
these two muscles are supplied by the radial
nerve; extensor carpi radialis longus by the
radial nerve itself, extensor carpi radialis
brevis via the deep branch of the radial nerve.
And these two muscles help to extend and abduct
or radially deviates the wrist, deviate the
wrist to the radial aspect, abduct the wrist.
If we look at the superficial layers, if we
carry on looking at the superficial layer
of the posterior compartment, then we can
see we have three more muscles here. We have
extensor digitorum, we have extensor digiti
minimi, and we have extensor carpi ulnaris.
And these muscles again are passing from this
lateral epicondyle, and they’re running
all the way down to the digits of the hand.
So here, if we see extensor digitorum and
extensor digiti minimi coming from the lateral
epicondyle of the humerus, and these pass towards
the digits. Now, unlike the flexor compartment,
these extensor muscles don’t kind of physically
attached to the distal or the middle or the
proximal phalanges of the digit. But they
attach to a connective tissue, fibrous band
over each of the digits, known as the extensor
expansion. And this is a sheath that surrounds
the interphalangeal joints of the digits.
So here we can see that extensor digitorum
is attaching to the extensor expansion of
the medial four digits. And extensor digiti
minimi is going specifically to the extensor
expansion of the fifth digit. These two muscles
are supplied by the posterior interosseous
nerve, and this is coming from the deep radial
nerve. They’re both associated with extending
the wrist, they extend the medial four digits
if your extensor digitorum, and the fifth
digit for extensor digiti minimi. They extend
the digits at the metacarpophalangeal and
interphalangeal joints. And they do this by
attaching to those extensor expansions.
If we look at the extensor carpi ulnaris, the
extensor carpi ulnaris is running down again
from the common extensor origin or at least
near to the common extensor origin, specifically,
the lateral epicondyle. It also is coming
from the posterior surface of the ulna.
This passes to the dorsal aspect of the fifth metacarpal
associated with the little finger. And again,
this is supplied by the posterior interosseous
nerve. This muscle is associated with extending
the wrist, just like the previous muscles.
It’s also associated with adducting
the wrist. And extensor carpi ulnaris can work
with flexor carpi ulnaris to adduct the wrist.