The anterior triangle of the neck contains structures that enter the cranial vault and the thorax.
Boundaries and Divisions of the Anterior Triangle
The borders, or boundaries, of the anterior triangle are as follows (Figure 26-2A):
- Anteriorly. The midline down the neck.
- Posteriorly. The sternocleidomastoid muscle.
- Superiorly. The lower border of the mandible.
A. Boundaries of the anterior triangle of the neck. B. The anterior triangle of the neck further divided into the submandibular, carotid, submental, and muscular triangles. C. Contents of the submandibular and carotid triangles.
The anterior triangle is subdivided by three structures—the omohyoid muscle, the hyoid bone, and the digastric muscle—into submandibular, carotid, submental, and muscular triangles (Figure 26-2B). The submental and muscular triangles have little clinical value and, therefore, will not be discussed in this text.
The submandibular triangle is bounded by the mandible, digastric, stylohyoid, and mylohyoid muscles (Figure 26-2C). The triangle consists of the following structures:
- Submandibular salivary gland. Fills most of the space of the triangle and is one of three paired salivary glands. The facial vein courses superficially to the submandibular salivary gland, whereas the facial artery courses deep to the gland. The submandibular gland is innervated by parasympathetic fibers from the facial nerve (CN VII).
- Hypoglossal nerve (CN XII). Provides motor innervation to the tongue muscles. The nerve courses deep to the posterior digastric and stylohyoid muscles and deep to the hyoglossus muscle.
The superior belly of the omohyoid and the posterior belly of the digastric and sternocleidomastoid muscles form the boundaries of the carotid triangle (Figure 26-2B). The triangle contains the superior portion of the common carotid artery (hence, the triangle's name), which bifurcates at the thyroid cartilage into the external and internal carotid arteries.
The internal carotid artery gives off no branches in the neck as it ascends to the carotid canal in the base of the skull. However, the following structures are related to the internal carotid artery in the neck:
- Carotid sinus. A swelling in the origin of the internal carotid artery containing baroreceptors that monitor blood pressure. The carotid sinus is innervated by visceral sensory neurons from the glossopharyngeal nerve (CN IX).
- Carotid body. A chemoreceptor at the bifurcation of the common carotid artery that monitors the partial pressure of oxygen. The carotid body is innervated by visceral sensory neurons from CN IX and the vagus nerve (CN X).
The external carotid artery sends off the following branches (Figure 26-2C):
- Superior thyroid artery. Courses inferiorly to the thyroid gland.
- Lingual artery. Courses anteriorly, deep to the hyoglossus muscle to reach the tongue.
- Facial artery. Courses deep to the submandibular gland, within the submandibular triangle, to exit anterior to the masseter muscle en route to the face.
- Occipital artery. Courses deep to the posterior belly of the digastric muscle en route to the occipital region.
- Ascending pharyngeal artery. Arises posteriorly from the external carotid artery and ascends vertically between the internal carotid artery and side of the pharynx.
Veins within the carotid triangle include the following:
- Internal jugular vein, which courses lateral to the common carotid artery.
- Tributaries of the internal jugular vein are the superior thyroid, lingual, common facial, ascending pharyngeal, and the occipital veins.
Nerves within this space include the following:
- Glossopharyngeal nerve (CN IX). Before entering the base of the tongue, CN IX sends off visceral sensory fibers to the carotid sinus and the carotid body.
- Hypoglossal nerve (CN XII). CN XII crosses both the internal and external carotid arteries en route to the tongue muscles through the submandibular triangle.
- Ansa cervicalis. This motor loop from the cervical plexus is found within the fascia of the carotid sheath and innervates the infrahyoid muscles. The superior limb originates from the ventral rams of C1, whereas the inferior limb originates from ventral rami of C2 and C3.
- Vagus nerve (CN X). CN X exits the jugular foramen and courses inside the carotid sheath, deep between the internal jugular and common carotid vessels. CN X gives off a superior laryngeal nerve, which divides into internal and external branches. The internal branch courses through the thyrohyoid membrane to provide sensory innervation to the mucosa of the larynx superior to the vocal folds. The external branch provides motor innervation to the cricothyroid muscle of the larynx.