The forearm is divided into an anterior forearm and a posterior forearm.
Compartmentalized nicely.
When you learn about the nerves, you will learn about nerves of the anterior compartment
versus the posterior compartment.
The anterior forearm has muscles that mostly pass into the hand for movement of the wrist
or the fingers.
There are however a few muscles that are for movement of the forearm itself.
Notice that most of the forearm muscles come from the anterior surface of the medial epicondyle
of the humerus.
So they do across the elbow joint.
And we call that area the common flexor tendon.
Now on this model we can see one muscle that heads from medial to lateral across the front
of the forearm, where it will wrap around the posterior lateral aspect of the radius.
This muscle is called the pronator teres muscle.
As its name implies its function is pronation of the forearm.
Now if we look at the model carefully, running straight down the anterior surface of the
forearm and passing superficial to a band of connective tissue called the flexor retinaculum,
is a muscle known as the palmaris longus.
This muscles origin is off of that anterior medial epicondyle of the humerus, but it does
not insert anywhere on this model.
That is because its insertion is not a bony insertion.
It inserts into the skin of the hand, on the deep side of the connective tissue of the
skin of the hand.
And since this model is for showing muscles the skin has been removed.
So they have cut this muscle off right here.
The palmaris longus muscle.
This muscle is absent in roughly 16% of people.
If you don't have a palmaris longus it is really of no consequence.
To the lateral side of palmaris longus we can see running along the radius and inserting
just past the carpal bones of the wrist, is the flexor carpi radialis muscle.
Conversely, on the medial side, we see running along the length of the ulna and inserting
just past the carpal bones of the wrist, is the flexor carpi ulnaris.
They are both named for their action, flexion at the wrist.
Flexor carpi.
Radialis running along the radius.
And ulnaris running along the ulna.
Now if on this model we could peel away the palmaris longus muscle we would see a wider
muscle underneath it called the flexor digitorum superficialis muscle.
Now the flexor digitorum superficialis is named for both its position and its function.
Flexor digitorum tells you it's going to flex the digits.
Digits two, three, four, and five.
And it is more superficial than another muscle.
This is one of the muscles that divides into four tendons that will pass through the carpal
tunnel out to the digits of the fingers.
On this particular model they make it very hard to see the flexor digitorum superficialis
muscle.
It is this wide muscle here underneath the palmaris longus.
Now if I remove this part of the model and turn it around, on the backside you would
be looking at the flexor digitorum superficialis muscle.
Now if I put this back on just temporarily, if we have a flexor digitorum superficialis,
we know we must have a flexor digitorum profundus.
So if I remove the superficialis and the muscles that remove with it, the muscle immediately
beneath will be the flexor digitorum profundus.
It also flexes digits two, three, four, and five.
Now the insertion point of these two muscles is different.
But it is beyond the scope of what we are teaching in A&P I.
So flexor digitorum profundus flexes digits two, three, four, and five.
The muscle that we removed, the flexor digitorum superficialis, also flexes digits two, three,
four, and five.
Provides for a very strong group.
These are very large muscles in the forearm.
Notice we have talked about digits two, three, four, five, we have yet to address anything
with the thumb, or digit one.
The thumb is also known as the pollux.
Alongside, just lateral to the flexor digitorum profundus we will see the flexor pollicis
longus muscle.
And if we look carefully at the model it passes toward the wrist and then out into the hand
where we can see its tendon going all the way to the distal phalanx of the thumb.
The flexor pollicis longus muscle.
Now there is another pronator muscle, it cannot be seen on these models.
It is known as the pronator quadratus.
And it would be deep to these other muscles approximately here, about an inch from the
wrist spanning between the ulna and the radius.
So those are the muscles of the anterior compartment of the forearm.
Let's turn this around and look at muscles of the posterior compartment of the forearm.
Muscles of the posterior compartment of the forearm are all innervated by the radial nerve
or branches thereof.
If we look right along the lateral aspect of the forearm we will see a muscle known
as the brachioradialis.
Named because it comes from the brachium and inserts into the styloid process of the radius.
The brachioradialis muscle.
It is a very strong flexor of the elbow if the forearm is halfway between pronation and
supination.
Immediately adjacent to the flexor, or excuse me, the brachioradialis muscle, is the extensor
carpi radialis longus and the extensor carpi radialis brevis.
Extensor carpi radialis longus is in the extensor compartment, the posterior forearm.
And runs along the radius to insert into the carpal bones.
And right next to it, the extensor carpi radialis brevis.
Inserting also into the carpal bones.
So extending at the wrist, running along the radius.
If we look to the medial side of the forearm we will see a muscle running along the ulna
that inserts just past the carpal bones.
The extensor carpi ulnaris muscle.
Now in this case there is no longus or brevis.
Just extensor carpi ulnaris.
Immediately medial and slightly higher than the origin for the extensor carpi ulnaris
we see a small triangular shaped muscle which goes from the posterior lateral epicondyle
of the humerus into the posterior shaft of the ulna.
This muscle is known as the anconeus muscle.
If you watched the video on the muscles of the arm, I said that some books consider the
posterior arm to have two muscles.
This is the muscle that is sometimes considered to be posterior arm, and in other books it
is considered to be posterior forearm.
The anconeus muscle.
Now if we look at the remainder of the posterior forearm we can see a large muscle running
along the back of the posterior forearm that will divide into four tendons.
These tendons go to digits two, three, four, and five.
Since this is the extensor compartment of the forearm and we are going to the digits,
this is called the extensor digitorum muscle.
It will extend digits two, three, four, and five.
And if you look carefully on the model you can see that breaking away from the extensor
digitorum there is a smaller muscle, very narrow, which goes only to the little finger.
That is the extensor digiti minimi muscle.
Extensor digiti minimi.
Now the posterior forearm, like the anterior forearm, was divided into layers.
So if I remove the different muscle layers as the model allows, I will remove the muscle
bellies mostly of extensor digitorum, extensor digiti minimi, it extensor carpi ulnaris,
and we can see underneath, some of the other muscles we need to discuss.
And I'm also going to remove the muscles that are over on the lateral aspect of the forearm,
which were the brachioradialis, the extensor carpi radialis longus and brevis.
Now we can see the deep compartment of the posterior forearm.
Wrapping around from the olecranon process of the ulna and the lateral epicondyle of
the humerus and wrapping around the radius, we see the muscle known as the supinator muscle.
As its name implies the supinator muscle supinates the forearm.
Immediately below the insertion of the supinator muscle we have a muscle which wraps laterally
around to insert at the base of the first metacarpal bone, the metacarpal that goes
to the thumb.
This is called the abductor pollicis longus muscle.
Abductor pollicis longus.
This is the muscle that allows us to take our thumb away from the rest of the hand.
Abduction of the thumb.
Immediately distal to that muscle we see the extensor pollicis brevis muscle.
And next to that, on the model we can't see it cutting across the remainder of the muscles
that we weren't able to remove, but we can see the tendon of the extensor pollicis longus
muscle.
And very poorly indicated on these models is yet one more muscle.
It passes underneath the other muscles that were not removable, the extensor digitorum
and the extensor digiti minimi.
And its tendon comes out to join only to the index finger, and that is the extensor indices
muscle.
So with the extensor indices muscle we can extend only the index finger while keeping
the other fingers flexed.
With the extensor digiti minimi we can extend the only the small finger, the digiti minimi,
while keeping the other fingers flexed.
With extensor digitorum we can extend all of the digits of the hand.
Now one more thing I want to put in, I want to add these different muscle pieces back
in.
I want to show a small trick for learning the muscles leading from the lateral part
of the posterior forearm around to the posterior forearm.
Remember we have the brachioradialis.
Then we have the extensor carpi radialis longus and the brevis and then the abductor pollicis
longus.
Then the extensor pollicis brevis and the extensor pollicis longus.
If we ignore the brachioradialis then we simply alternate longus and brevis.
Extensor carpi radialis longus, then there's a brevis, that there's a longus, then there's
a brevis, then there's another longus.
Extensor carpi radialis longus, extensor carpi radialis brevis.
Abductor pollicis longus.
Extensor pollicis brevis.
Extensor pollicis longus.
The muscles of the posterior forearm.