Superficial structures of the neck: Anterior triangle

The neck is the anatomical region between the base of the cranium superiorly and the clavicles inferiorly and it joins the head to the trunk and limbs, serving as a major conduit for structures passing between them.

The neck is divided in two major triangles: anterior and posterior, based mainly on the borders of the sternocleidomastoid, or SCM, and trapezius muscles, as well as other muscular and bony structures found in the neck.

These regions provide a clear location regarding the structures, injuries or pathologies involving the neck. Now, the anterior triangle, like any respectable triangle, has three sides, called boundaries.

The anterior boundary is formed by the median line of the neck, the posterior boundary is formed by the anterior border of the sternocleidomastoid muscle, or SCM for short, and the superior boundary is formed by the inferior border of the mandible.

The triangle has a superficial boundary or a roof formed by subcutaneous tissue containing the platysma, a deep boundary or a floor formed by the pharynx, larynx and thyroid gland and an apex located at the jugular notch in the manubrium.

And if that wasn’t enough, the anterior triangle is further subdivided by the omohyoid and digastric muscles into four smaller triangles: submental, submandibular, carotid and muscular.

The submental triangle sits right below the chin and contains several small submental lymph nodes and small veins that unite to form the anterior jugular vein.

Inferiorly, the submental triangle is bounded by the hyoid body, and laterally by the right and left anterior bellies of the digastric muscles.

The floor is formed by the two mylohyoid muscles which meet in a median fibrous raphe, the apex is located at the mandibular symphysis, and the base is formed by the hyoid bone.

Next, the submandibular triangle contains the submandibular gland, submandibular lymph nodes, cranial nerve XII which is the hypoglossal nerve, mylohyoid nerve and parts of the facial artery and facial vein.

In addition, in 20% of individuals the marginal mandibular branch of the facial nerve extends below the inferior border of the mandible into the submandibular triangle.

The submandibular triangle is bounded superiorly by the inferior border of the mandible, anteroinferiorly by the anterior belly of the digastric muscle and posteroinferiorly by the posterior belly of the digastric muscle.

Its floor is formed by the mylohyoid and hyoglossus muscles, and its roof by the investing layer of deep cervical fascia.

The hypoglossal nerve enters the submandibular triangle by passing deep to the posterior belly of the digastric muscle and deep to the mylohyoid muscle within the submandibular triangle.

It provides motor innervation to the intrinsic and extrinsic muscles of the tongue. Finally, there are also some spinal nerves in the mix.

C1 fibers travel with the hypoglossal nerve and give off the thyrohyoid branch for the thyrohyoid muscle.

Then there’s the superior root of the ansa cervicalis, which is composed of fibers from C1 and C2.

The superior root of the ansa cervicalis unites with the inferior root of the ansa cervicalis, which derives from spinal nerves C2 and C3.

These superior and inferior roots form a loop which gives off branches to supply the omohyoid, sternothyroid, and sternohyoid muscles.

Next, the carotid triangle is bound inferomedially by the superior belly of the omohyoid muscle, superiorly by the posterior belly of the digastric muscle and inferolaterally by the anterior border of the SCM.

The carotid triangle contains blood vessels such as the common carotid artery with its branches, the external carotid artery and some of its branches, the internal jugular vein and its tributaries, and also nerves like branches of the vagus nerve, the hypoglossal nerve and superior root of ansa cervicalis, the spinal accessory nerve and branches of the cervical plexus and viscera such as the thyroid gland, larynx, pharynx and deep cervical lymph nodes.

The neurovascular structures in the carotid triangle are surrounded by the carotid sheath. In the carotid sheath, the carotid arteries lie medially, the internal jugular vein lies laterally and the vagus nerve lies posteriorly.

Superiorly, the common carotid artery is replaced by the internal carotid artery and on the anterolateral aspect of the sheath you can find the ansa cervicalis.

Lets take a quick break and see if you can identify all of the different triangles in this image!

Speaking of carotids, there are two common carotid arteries: the right common carotid artery which begins at the bifurcation of the brachiocephalic trunk, forming it and the right subclavian artery, and the left common carotid artery which begins at the arch of aorta and ascends into the neck.

Each common carotid artery splits into the internal and external carotid arteries at the upper border of the thyroid cartilage.

The internal carotid arteries give off no named branches in the neck, and they enter the cranium through the carotid canals in the petrous parts of the temporal bones. Here, they become the main arteries for the brain and structures in the orbits.

The proximal part of each internal carotid artery contains the carotid sinus which is a baroreceptor that reacts to changes in arterial blood pressure and is innervated by the glossopharyngeal nerve through the carotid sinus nerve and also by the vagus nerve.

The carotid body is located in a septum on the deep side of the bifurcation of the common carotid artery in close relation with the carotid sinus.

It is also innervated primarily by the carotid sinus nerve, a branch of the glossopharyngeal nerve, and it acts as a chemoreceptor that monitors the level of oxygen in the blood.

This means that when there are low levels of oxygen, that sends a signal to the brain to increase the respiration rate.

The external carotid arteries supply most structures external to the cranium and they run posterosuperiorly to the region between the neck of the mandible and the lobule of the auricle, where they split in two terminal branches: the maxillary artery, which has 3 major branches that supply numerous structures, and the superficial temporal artery, which enters the temporal fossa and supplies regions of the scalp.

Before its termination, it gives off six arteries: the superior thyroid artery, ascending pharyngeal artery, lingual artery, facial artery, occipital artery and posterior auricular artery.

A good mnemonic to remember the branches of the external carotid artery in order from which branches first is “Some Anatomists Like Freaking Out Poor Medical Students”, where the first four branches arise from the anterior aspect, and the last four, from the posterior aspect.

So first, there’s the superior thyroid artery, which runs deep into the infrahyoid muscles to supply the thyroid gland.

It also supplies the infrahyoid muscles and SCM and gives a branch called the superior laryngeal artery which supplies the larynx.

Second, there’s the ascending pharyngeal artery, which ascends to the pharynx giving branches to the pharynx, prevertebral muscles, middle ear and cranial meninges.

Third, the lingual artery runs deep to the hypoglossal nerve, the stylohyoid muscle and the posterior belly of the digastric muscle and gives branches to the posterior tongue.

Then it turns superiorly at the anterior border of the hyoglossus muscle and splits into two branches: the deep lingual artery and the sublingual artery.

Fourth, the facial artery gives rise to the ascending palatine artery and the tonsillar artery.

Then it passes obliquely beneath the digastric and stylohyoid muscles and the angle of the mandible, and it gives rise to the submental artery to the floor of the mouth and hooks around the middle of the inferior border of the mandible to enter the face.