Now we’ll look at the muscles that hold the hyoid bone in place, and cause it to move. There are seven pairs of them: two that pull the hyoid bone upwards and forwards, one that pulls it upwards and backwards, one that pulls it upwards by means of a pulley, and three that pull it downwards.
We’ll start with the two that pull upwards and forwards: the mylohyoid, and geniohyoid muscles. Here are the two mylohyoid muscles. Between them they form a continuous sling of muscle that forms the mobile floor of the oral cavity.
The mylohyoid muscle arises from the mylohyoid line on the mandible. Most of its fibers pass downwards and medially, joining in the midline with the fibers from the opposite side, all the way from the symphysis of the mandible, to the body of the hyoid bone.
The more posterior fibers of the mylohyoid insert here on the body of the hyoid bone. The mylohyoid muscle has a free posterior border which runs straight downwards when seen from the side, also a little inward when seen from behind.
Now we’ll add the two geniohyoid muscles to the picture: here they are: They lie above the mylohyoid: On each side the geniohyoid arises from the lower part of the mental spine. It inserts here, on the body of the hyoid bone.
Now we’ll bring the base of the skull into the picture, and add the muscle that pulls upwards and backwards, the stylohyoid. Here’s the stylohyoid. It’s a long, slender muscle. Just above its insertion there’s an opening in the stylohyoid. The digastric muscle passes through this opening as we’ll see.
The stylohyoid arises from the lateral aspect of the styloid process. It’s inserted on the base of the greater horn of the hyoid bone.
Next we’ll add the digastric muscle to the picture. Here it is. The digastric muscle is unusual in that it has two bellies, an anterior and a posterior, that are connected in the middle by a tendon.
The posterior belly of the digastric arises here, from the digastric notch on the underside of the temporal bone, and from the medial aspect of the mastoid process. The origins of the sternocleidomastoid and splenius muscles, which have been removed in this dissection, lie lateral to it.
The posterior belly narrows to a tendon which passes between the two slips of the stylohyoid. The digastric tendon then passes through a sling of fibrous connective tissue, by which it’s tethered to the hyoid bone, here.
The tendon then broadens out into the anterior belly of the digastric, which runs almost straight forward beneath the mylohyoid. It’s attached low down on the inner aspect of the body of the mandible, just lateral to the midline.
Lastly we’ll take a brief look at the attachments of the three muscles that pull the hyoid bone downwards. They’re the omohyoid, sterno-hyoid, and thyrohyoid muscles, known collectively as the infrahyoid muscles.
Here’s the body of the hyoid bone. Here’s the upper end of the omohyoid muscle, which goes all the way down to the scapula. Medial to it is the sternohyoid muscle, which goes to the sternum.
Behind these two is the short thyro-hyoid muscle, which goes down to a structure we haven’t seen yet, the thyroid cartilage. These muscles insert on the edge of the body of the hyoid bone, the thyrohyoid here, the omohyoid here, and the sternohyoid here. We’ll see these three muscles more fully, later in this tape.
The infrahyoid muscles pull the hyoid bone downwards. Acting together with the digastric muscle, the infrahyoid muscles assist in opening the jaw. The actions of the other hyoid muscles that we’ve seen are evident from the direction of their fibers.