the wrist. And extensor carpi ulnaris can work
with flexor carpi ulnaris to adduct the wrist.
If we now move on to some deep muscles on the
posterior compartment, also known as these
outcropping muscles, then we can see we have
a whole series of them; abductor pollicis
longus, extensor pollicis longus, extensor
pollicis brevis, and extensor indicis.
Now these muscles are passing specifically to
the thumb, the pollicis muscles, and to the
index finger, extensor indicis. So these are
passing from the forearm going across the
wrist and attaching to features on digits
1 and 2. So if we look at abductor pollicis
longus, abductor pollicis longus, we can see
here, is passing all the way to the metacarpal
of the first digit. It’s coming from the
posterior surface of the radius and the ulna
and also the interosseous membrane. So it
has a widespread origin, and it inserts at
the metacarpal of the first digit. Extensor
pollicis longus, specifically, passes from
the posterior surface of the ulna, and also
the interosseous membrane, so not the radius
this time. And it passes to the dorsal surface
of the distal phalanx of the first digit;
so the distal phalanx of your thumb. Extensor
pollicis brevis, the posterior surface of
the radius, the interosseous membrane, and it
passes to the dorsal surface of the proximal
phalanx of the first digit. So here we have
extensor pollicis longus passing to the distal
phalanx, and extensor pollicis brevis passing
to the proximal phalanx. And this is where
they get their names, longus and brevis. Longus,
the longer of the muscle, passes most distally
to the distal phalanx. And brevis, the shorter
of the muscles, passing to the proximal phalanx.
All of these muscles are supplied by the posterior
interosseous nerve. This originates from the
deep radial nerve. And we can see they have
quite a widespread function. Abductor pollicis
longus extends the wrist because it crosses
the wrist joint, and it works on the thumb.
It abducts the thumb and extends it at the
carpometacarpal joint. Extensor pollicis longus
and extensor pollicis brevis are going to
work to extend the wrist, extend the distal
and proximal phalanx of the thumb at the interphalangeal
joints, and also extend the metacarpophalangeal
and carpometacarpal joints. So we can see
we have widespread function of these muscles.
Here, we can also see that we have what’s
known as supinator muscle we can see here.
And supinator muscles, as its names suggest,
is going to supinate the forearm. We can see
that supinator is running from the lateral
epicondyle of the humerus. We can see here
it’s one of the lateral epicondyle of the
humerus. It’s also running from the supinator
fossa on the proximal ulna, so a specific region
on the ulna and it passes to the posterior,
lateral, and anterior surfaces of the proximal
radius. Supplied by the deep branch of the
radial nerve, and as its name suggest, it’s
going to supinate the forearm. Finally, we
can just see extensor indicis. Extensor indicis
is important as it passes specifically to
the index finger, given the index finger,
some specific functionality. Extensor indicis
is coming from the posterior surface of the
ulna and also the interosseous membrane, and
it passes to the extensor expansion specifically
of the second digit, your index finger. It’s
supplied by the posterior interosseous nerve.
Again, this is coming from the deep radial nerve.
And it serves to extend the wrist and extend the
second digit. You can do this independently
from extensor digitorum. So the index finger
is allowed to have some independent function.
So here we can see the anatomy of the posterior
aspect of the forearm. We can see this is
a right forearm, and we have a lot of the muscles
all put together in their proper anatomical
position. So we can see most distally, a whole
series of tendons, and these tendons are coming
from the muscles on the posterior surface.
And here, we have the extensor retinaculum.
So most distally, we have the extensor retinaculum,
and that prevents bowstringing when the hand
is extended. When the hand is fully extended,
you don’t want these tendons from splaying
either side, and the extensor retinaculum
prevents that. Divided into superficial and
deep layers, the muscles in the posterior
compartment, the tendons pass over the wrist
and they’re covered in what are known as
synovial sheaths. These synovial sheaths prevent
friction as the tendons run over the wrist,
and they create these osseous tunnels formed
by the extensor retinaculum, and they help
to prevent friction. So you have these synovial
sheaths that prevent excess friction as the
tendons are trapped between the bones of the
wrist and the extensor retinaculum. And these
synovial sheaths prevent that excess friction.
The tendons of the extensor digitorum flatten,
as I mentioned previously, and form these
extensor expansions. These are tendinous aponeurosis
that pass over the dorsum and sides of metacarpal
and the proximal phalanx of each digit. So
instead of attaching directly to a phalanx,
a distal, a middle, or a proximal, they blend
with this tendinous aponeurosis that lies
on the dorsum and the sides of the metacarpals
and proximal phalanges. So the tendons actually
just blend with this aponeurosis. It means
that you actually lose extensor function.
Unlike the flexor compartment where the muscles
and the tendons inserted directly onto bony
points, you had a lot more flexion control.
For extension, you don’t have that level
of control because of this widespread insertion
into the extensor expansions. So if we dig
a bit deeper and remove some of these middle
and superficial layers, we can actually see
the deeper muscles. Here, we can see the tendons
of extensor digitorum have been cut away.
So we can look into this deep aspect. We can
see abductor pollicis longus here. We can
see extensor pollicis longus, extensor indicis.
We can see these outcropping muscles as well.
Extensor pollicis brevis is one of them. And
these are passing out going across the wrist
and heading towards the thumb, and for extensor
indicis, the index finger. Now, this forms
a very specific region at the wrist, known
as the anatomical snuff box. So passing laterally
towards digits 1 and 2 are these outcropping
muscles. They come from deep and they pass
outwards, out onto this lateral aspect heading
towards digits 1 and 2, and here are those
muscles that I mentioned. The tendons of extensor
pollicis longus, the tendons of extensor pollicis
longus, medially, and extensor pollicis brevis
and abductor pollicis longus laterally, form
the anatomical snuff box. So here, we’ve
got the fifth digit. So this is medially.
We can see we have extensor pollicis longus.
So if we follow this all the way around, we
can see that we’re forming this medial boundary
here. If we then look laterally, we can see
we have two tendons. We have extensor pollicis
brevis and we have the tendon of abductor
pollicis longus. And here, we can see a small
little region, this triangular region known
as the anatomical snuff box. It’s located
on the lateral surface of the wrist, bound
by these tendons. You can also observe that
the index finger receives a specific muscle
and this is where it separates and distinct to extensor
digotorum, and this is extensor indicis. We'll come
back to the anatomical snuff box when we look
at the dorsum of the hand. So if we then look
at the posterior compartment, we can appreciate
some important nerves which are located here.
Most of these arteries are going to be a direct
continuation of the brachial artery. So here,
we can see we have various tributaries running
along the posterior compartment. So we can
see the posterior interosseous artery, and
that’s coming from the ulnar artery.
And then we can see here, we’ve got the radial
nerve that’s passing into the posterior
compartment to supply all of these muscles.
They’re running in the posterior compartment.
So in this lecture, we’ve looked at the
forearm in cross-section. We’ve looked at
the antebrachial fascia and the compartments
that it forms. We then looked at a whole series
of muscles in the anterior compartment, and
we looked at their relations to one another
within the anterior compartment. And then
we looked at the posterior compartment.
We looked at the extensor muscles, where again,
we looked at a whole series of muscles. And
then briefly at the end, we looked at the
neurovascular relations. We’ll look at the
neurovascular relations in more detail when
we look at the overview of the blood supply
to the upper limb as a whole, and that’s
in a later lecture.