Human Anatomy - Kotsan I. Y. 2009

Muscles and fascia of the neck
Superficial neck muscles
Infrahyoid muscles

The infrahyoid Muscles also include four muscles: the omohyoid, sternohyoid, sternothyroid, and thyrohyoid.

The omohyoid Muscle (m. omohyoideus) is a long, slender muscle with two bellies—a superior belly (venter superior) and an inferior belly (venter inferior)—connected by an intermediate tendon. The inferior belly originates from the superior border of the scapula, medial to the scapular notch, and from the superior transverse scapular ligament. It courses upward and forward deep to the sternocleidomastoid muscle and continues into the intermediate tendon, from which the superior belly arises. The superior belly inserts into the body of the Hyoid bone. The intermediate tendon is fused externally with the carotid Sheath of the neck. Function: When the scapula is fixed, the omohyoid muscles pull the hyoid bone inferiorly and posteriorly; unilateral contraction shifts the hyoid bone inferiorly and posteriorly to the corresponding side. When the hyoid bone is fixed, the omohyoid muscles on both sides tense the pretracheal layer of the Cervical fascia, thereby preventing the compression of the deep Veins OF THE neck.

Innervation: Ansa cervicalis (C_I—C_II).

The sternohyoid muscle (m. sternohyoideus) originates from the posterior surface of the manubrium of the Sternum and the sternal end of the clavicle. It inserts into the inferior border of the body of the hyoid bone. A narrow, superiorly tapering triangular space remains between the medial borders of the sternohyoid muscles of both sides. Within this triangle, the superficial and middle (pretracheal) layers of the cervical fascia fuse to form the Linea Alba of the neck. Function: Upon contraction, the muscle pulls the hyoid bone, and consequently the Larynx, inferiorly.

Innervation: Ansa cervicalis (C_I—C_II).

The sternothyroid muscle (m. sternothyroideus) is somewhat broader than the preceding muscle. It originates from the posterior surface of the manubrium of the sternum and the Cartilage of the first rib. It inserts onto the lateral surface of the thyroid cartilage (at the oblique line). Function: Depresses the larynx.

Innervation: Ansa cervicalis (C_I—C_II).

The thyrohyoid muscle (m. thyrohyoideus) appears as a direct continuation of the sternothyroid muscle toward the hyoid bone. It originates from the oblique line on the lateral surface of the thyroid cartilage, courses superiorly, and inserts into the body and greater horn of the hyoid bone. Function: Depresses the hyoid bone and elevates the larynx. During upward and downward Movements of the thyroid cartilage, the sternothyroid and thyrohyoid muscles act as antagonists.

Innervation: Ansa cervicalis (C_I—C_II).

Occasionally, a levator glandulae thyroideae muscle (m. levator glandulae thyroideae) branches off from the thyrohyoid muscle and extends toward The Thyroid Gland.