This is a tutorial on the muscles of the neck
which lie on the anterior triangle. So the anterior triangle of the neck is defined by
a few boundaries. So the anterior border of the sternocleidomastoid, the midline and the
inferior border of the mandible. So 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. So 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. So 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. So it originates on the styloid process and inserts onto the lateral
area of the body of the hyoid bone. So 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. So 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. So 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. So the hyoid bone can move up and down. The mandible obviously
has several movements. It can protrude, retract, depress and elevate. So if the hyoid bone
is fixed in place, then the digastric muscle will act to open the mouth. So it’ll pull
the jaw, the mandible down opening the mouth. So 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. So it depends which bones are fixed. It’s quite an unusual muscle because
it has three attachments. So the next muscle is the mylohyoid bone.
So I’m just going to rotate the model around and we’re going to look at it from underneath.
So 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. So 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. So 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. So 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.
So 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. So 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’. So if you see the prefix genio-, it refers to chin. So the genioglossus, this
tongue muscle here, attaches inside on the chin on the inside surface of the mandible.
So the geniohyoid runs from the inside of the mandible to the hyoid bone. So 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.