We’ll end this section on the larynx by looking at the structures that are close to it in front and below: the infrahyoid muscles and the thyroid gland. In addition, we’ll see the parathyroid glands.
Here again are the thyroid cartilage, the cricoid cartilage, and the trachea. These are the rings of cartilage which reinforce the wall of the trachea. We’ll add the carotid sheaths to the picture
Here, just on each side of the trachea, are two of the four parathyroid glands. They’re recognizable by their brownish color. The other two parathyroid glands are further down.
Next, we’ll add the thyroid gland to the picture. Here it is. This is the left lobe of the thyroid gland, this is the right lobe. The two lobes are connected across the mid-line by the isthmus. The top of each lobe of the thyroid gland is level with the lower border of the thyroid cartilage. The top of the isthmus is about level with the third ring of the trachea.
Now we’ll add the four infrahyoid muscles to the picture, starting with the two deepest ones, the thyrohyoid, and the sternothyroid muscles.
In effect they’re one continuous muscle. The thyrohyoid arises from the back of the body of the hyoid bone, and inserts on the oblique line of the thyroid cartilage. The sternothyroid arises from the same oblique line, and passes down behind the upper end of the sternum. It inserts on the back of the sternum, down here.
Now we’ll add the other two infrahyoid muscles to the picture, the omohyoid, and the sternohyoid. The omohyoid muscle arises here, the sternohyoid here on the body of the hyoid bone.
The sternohyoid runs straight downwards, close to the mid-line, and inserts on the back of the sternum, here. The omohyoid muscle runs downwards, laterally and backwards.
It lies in front of the carotid sheath, and the brachial plexus, which is under here. The omohyoid muscle passes beneath the trapezius muscle, to insert on the upper border of the scapula.
Finally, we’ll return the clavicles, and the sternocleidomastoid muscles to the picture. In looking at the intact neck, it’s useful to remember that when the neck isn’t extended, the bottom of the cricoid cartilage imay be no higher than the top of the clavicle.