So now I'm going to talk about the infrahyoid muscles.
They've got the opposite action to the suprahyoid muscles.
They depress the hyoid bone.
So again, there are four muscles in the infrahyoid group.
The infrahyoid group of muscles are referred to as strap muscles because they look like
straps.
There are four muscles in this group.
You've got the sternohyoid, the omohyoid, the thyrohyoid and the sternothyroid.
I'll just run you through those.
The first muscle is this muscle which extends from the hyoid bone to the posterior aspect
of the sternoclavicular joint.
Because it attaches to the hyoid and the sternum, it's called the sternohyoid muscle.
What this muscle does is that it depresses the hyoid bone after swallowing.
So this is this muscle, which runs down from the hyoid to the posterior aspect of the sternum.
You can see that there.
That's the sternohyoid.
Annoyingly on this model, it doesn't have one of the muscles.
I'm going to whip up an old school grace anatomy image and show you that.
Here we go.
I just showed you on the 3D model the sternohyoid.
Lateral to that, you've got the omohyoid.
This muscle originates on the scapula here, the superior border of the scapula just medial
to the suprascapular notch.
It originates here and it runs through the posterior triangle into the anterior triangle
and inserts onto the hyoid bone.
This muscle depresses and fixes the hyoid bone.
It's got two bellies.
You've got this superior belly and you've got this inferior belly.
You've got this intermediate tendon here.
It isn't shown on this diagram, but there's actually a little fascial sling which attaches
the intermediate tendon to the medial end of the clavicle.
That separates the inferior belly from the superior belly of the omohyoid.
So the omohyoid lies lateral to the sternohyoid.
So just coming back to this 3D model, the position of that muscle I just showed you,
the omohyoid, would be lateral to this muscle, the sternohyoid.
It inserts here on the hyoid bone and run inferiorly and posteriorly to insert onto
the scapula back here.
So it runs up through the posterior triangle and into the anterior triangle to insert onto
the hyoid bone and it fixes and presses the hyoid bone.
So you've got two muscles which lie under the omohyoid and the sternohyoid bone.
You've got the thyrohyoid bone, which originates on the oblique line on the thyroid cartilage
(so on the oblique line of the lamina of the thyroid cartilage) and it attaches to the
hyoid bone.
I'll just actually remove this sternohyoid so we can have a look.
On this model, this muscle is shown as one muscle, but in real life, there are two muscles,
which are continuous.
This portion here which attaches to the thyroid cartilage and inserts onto the hyoid bone
is the thyrohyoid muscle.
Its action depends on which part is fixed.
So if the larynx is fixed, then it will depress the hyoid bone.
It generally does depress the hyoid bone.
But if the hyoid bone is fixed, it can elevate the larynx.
So it can raise the larynx.
And then you've got this muscle here.
In this diagram, it's shown as one, big muscle, but it's actually two.
You've got the upper bit, which is the thyrohyoid and this lower bit, which is called the sternothyroid
because it attaches to the sternum to the thyroid.
This muscle draws the larynx downwards.
Because it's attached to the sternum, it draws the larynx down.
So the sternothyroid and the thyrohyoid run in continuity just deep to the omohyoid and
the sternohyoid.
Just quickly, to recap, you've got four infrahyoid muscles.
You've got the sternohyoid and you've got the omohyoid, which is superficial.
So the omohyoid is lateral to the sternohyoid and it runs from the scapula through the posterior
triangle and inserts onto the hyoid bone and has an intermediate tendon, which inserts
onto the medial end of the clavicle.
And then deep to these, the omohyoid and the sternohyoid, you've got two muscles.
You've got the thyrohyoid and the sternothyroid muscles.
So four muscles, infrahyoid muscles that you need to know.