This is a tutorial on the muscles of the neck which lie on the anterior triangle. The anterior triangle of the neck is defined by a few boundaries. The anterior border of the sternocleidomastoid, the midline and the inferior border of the mandible. This is the anterior triangle we’re looking at here. There are several muscles which lie in this triangle.
You’ve got the suprahyoid muscles, which lie above the hyoid bone and the infrahyoid muscles which lie below the hyoid bone.
The infrahyoid muscles are known as strap muscles presumably because they look like a strap. I don’t know, something like that.
The suprahyoid muscles lie above the hyoid bone. The hyoid bone is important because it serves as a point of attachment for several muscles. The muscles of the tongue attach to the hyoid bone and it’s important in swallowing.
There are four suprahyoid muscles. You’ve got the digastric, the stylohyoid, the mylohyoid (this big, fan-shaped one) and superior to this you’ve got the geniohyoid muscle, which I’ll show you in a moment.
The stylohyoid muscle is this muscle here, which connects from the styloid process of the skull to the lateral surface of the hyoid bone (lateral of the body of the hyoid). The name really gives away its origin and insertion, which is quite useful. It originates on the styloid process and inserts onto the lateral area of the body of the hyoid bone.
The hyoid bone lies at rest roughly at the level of the base of the mandible and lies just a little bit above the thyroid cartilage. Several muscles attach here.
This is the stylohyoid muscle. Obviously, you’ve got these on both sides. The next muscle is the digastric muscle. This is this muscle which loops all the way here, attaches to the hyoid bone and also attaches on the inside surface of the mandible. It attaches in the digastric fossa and also on the medial surface of the mastoid process.
The digastric muscle is called ‘digastric’ because di- means two and –gastric means bellies, so it’s a two bellied muscle. You’ve got the anterior belly and the posterior belly. the posterior belly originates on the medial surface of the mastoid process and inserts via this tendon onto the hyoid bone and the anterior belly originates on the digastric fossa on the inside surface of the mandible and inserts via the same tendon.
This muscle has various actions depending on which bone is fixed. The hyoid bone can move up and down. The mandible obviously has several movements. It can protrude, retract, depress and elevate. If the hyoid bone is fixed in place, then the digastric muscle will act to open the mouth. It’ll pull the jaw, the mandible down opening the mouth. You can imagine this action. The mastoid process is obviously fixed. If the hyoid bone is fixed in place, then the anterior belly of the digastric muscle will pull the jaw bone down and open the mouth.
If the mandible is fixed however (and obviously, the mastoid process is fixed), then the digastric muscle, both bellies will cause the hyoid bone to elevate. It depends which bones are fixed. It’s quite an unusual muscle because it has three attachments.
The next muscle is the mylohyoid bone. I’m just going to rotate the model around and we’re going to look at it from underneath. You can see the digastric muscle. It lies inferior to the mylohyoid, which is this fan-shaped muscle here. This muscle forms the floor of the mouth and this muscle can elevate the hyoid bone. It’s attached on the inside surface of the mandible and it originates on the mylohyoid line on the mandible and it inserts obviously onto the hyoid bone.
These muscles, several muscles blend in the midline and originate on the inside surface of the mandible on the mylohyoid line. It lies superior to the gastric muscle and it supports and elevates the floor of the mouth and it also elevates the hyoid bone. That’s the mylohyoid muscle.
Just superior to the mylohyoid muscle, you’ve got the final muscle, the suprahyoid muscle, which is the geniohyoid. If I just remove the mylohyoid, you can see there’s a muscle, a thin muscle, which runs from the hyoid bone to the inside surface of the mandible again.
This muscle originates on the inferior mental spine of the mandible. This is a little thing that lies on the inside surface of the mandible. And it inserts onto the hyoid bone.
Again, the action of this muscle depends on which bone is fixed. I’ll just remove the mylohyoid on this side just so we can have a look. You can see this thin muscle. It lies superior to the mylohyoid muscle medially. You can see this muscle above it. This is the muscle of the tongue. This is the genioglossus muscle, but ignore that for now. Just looking at this, then you can see this thin muscle running from the hyoid to the inside surface of the mandible.
It depends on which bone is fixed as to what the action of this muscle is. If the hyoid bone is fixed, then the geniohyoid will open the jaw. It will pull it downwards and inwards. Whereas if the mandible is fixed, it will elevate the hyoid.
The word ‘genio’ refers in Greek or something to ‘chin’. If you see the prefix genio-, it refers to chin. The genioglossus, this tongue muscle here, attaches inside on the chin on the inside surface of the mandible. The geniohyoid runs from the inside of the mandible to the hyoid bone.
Those are the four suprahyoid muscles in the anterior triangle of the neck. You’ve got the stylohyoid, the digastric, the fan-shaped mylohyoid and you’ve got the geniohyoid.
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.
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. 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.
The omohyoid lies lateral to the sternohyoid.
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. 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.
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. 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. 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.
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. 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. Four muscles, infrahyoid muscles that you need to know.