Lateral to the sternohyoid muscle is the omohyoid muscle (Fig. 8.168). This muscle consists of two bellies with an intermediate tendon in both the posterior and anterior triangles of the neck:
The inferior belly begins on the superior border of the scapula, medial to the suprascapular notch, and passes forward and upward across the posterior triangle ending at the intermediate tendon.
The superior belly begins at the intermediate tendon and ascends to attach to the body of the hyoid bone just lateral to the attachment of the sternohyoid.
The intermediate tendon is attached to the clavicle, near its medial end, by a fascial sling.
The omohyoid depresses and fixes the hyoid bone. It is innervated by the anterior rami of C1 to C3 through the ansa cervicalis.
The thyrohyoid muscle is deep to the superior parts of the omohyoid and sternohyoid (Fig. 8.168). Originating at the oblique line on the lamina of the thyroid cartilage it passes upward to insert into the greater horn and adjacent aspect of the body of the hyoid bone.
The thyrohyoid muscle has variable functions depending on which bone is fixed. Generally, it depresses the hyoid, but when the hyoid is fixed it raises the larynx (e.g., when high notes are sung). It is innervated by fibers from the anterior ramus of C1 that travel with the hypoglossal nerve [XII].
Lying beneath the sternohyoid and in continuity with the thyrohyoid, the sternothyroid is the last muscle in the infrahyoid group (Fig. 8.168). It arises from the posterior surface of the manubrium of the sternum and passes upward to attach to the oblique line on the lamina of the thyroid cartilage.
The sternothyroid muscle draws the larynx (thyroid cartilage) downward and is innervated by the anterior rami of C1 to C3 through the ansa cervicalis.
Passing through the anterior triangle of the neck are the common carotid arteries and their branches, the external and internal carotid arteries. These vessels supply all structures of the head and neck.
Associated with this arterial system are the internal jugular vein and its tributaries. These vessels receive blood from all structures of the head and neck.
The common carotid arteries are the beginning of the carotid system (Fig. 8.169):
The right common carotid artery originates from the brachiocephalic trunk immediately posterior to the right sternoclavicular joint and is entirely in the neck throughout its course.
The left common carotid artery begins in the thorax as a direct branch of the arch of the aorta and passes superiorly to enter the neck near the left sternoclavicular joint.
Both right and left common carotid arteries ascend through the neck, just lateral to the trachea and esophagus, within a fascial compartment (the carotid sheath). They give off no branches as they pass through the neck.
Near the superior edge of the thyroid cartilage each common carotid artery divides into its two terminal branches—the external and internal carotid arteries (Fig. 8.170).
The superior part of each common carotid artery and its division into external and internal carotid arteries occurs in the carotid triangle (Fig. 8.170), which is a subdivision of the anterior triangle of the neck (see Fig. 8.166).
At the bifurcation, the common carotid artery and the beginning of the internal carotid artery are dilated. This dilation is the carotid sinus (Fig. 8.171) and contains receptors that monitor changes in blood pressure and are innervated by a branch of the glossopharyngeal nerve [IX].
Another accumulation of receptors in the area of the bifurcation is responsible for detecting changes in blood chemistry, primarily oxygen content. This is the carotid body and is innervated by branches from both the glossopharyngeal [IX] and vagus [X] nerves.