We have discovered muscles that elevate the hyoid bone and now will look at the muscles that perform the antagonistic function. These are known as the infrahyoid muscles. Unlike the suprahyoid muscles that used hyo- as a prefix, these can be distinguished by using hyoid as the suffix though like the suprahyoids one of them does not follow that rule. With one variation we will note they are all innervated by branches from the ansi cervicalis which has contributions from the first three cervical nerves.
The omohyoid is constructed like the digastric muscle with two bellies separated by an intermediate tendon. In the case of the omohyoid the central tendon is anchored in place on the superior surface of the clavicle. It originates on the superior surface of the scapula and inserts laterally on the inferior surface of the hyoid. It acts predictably to depress the hyoid.
As indicated by the name the sternohyoid connects the origin on the sternum to the hyoid bone. This strap like muscle when contracted will depress the hyoid bone which is necessary to complete the cycle of muscle movements involved in swallowing.
The thyrohyoid extends between its origin on the thyroid cartilage to insert into the hyoid. As the thyroid cartilage is not fixed in place, contraction of this muscle draws the hyoid and larynx together. If the thyroid is fixed this will result in depression of the hyoid bone and conversely if the hyoid is fixed in place it will elevate the larynx. This is the one muscle not innervated by the ansi cervicalis as it mirrors the innervation of the geniohyoid having fibers that originate in the first cervical nerve but travel on the hypoglossal nerve.
While the sternothyroid muscle does not directly connect to the hyoid bone it is included in this group of muscle because it influences the movement of hyoid bone and works in concert with the other muscles in this group. It originates on the sternum and inserts into the thyroid cartilage. It will, when contracted, depress the thyroid cartilage which is the main cartilage of the larynx. The cartilage is connected to the hyoid by the thyrohyoid muscle. This is the muscle working with the thyrohyoid will depress the hyoid bone.