Anterior Triangle — MRCS Part B Anatomy

What this station tests

A labelled dissection of the neck read from superficial to deep: the glands and muscles that bound the anterior triangle, the carotid sheath and its contents, and the nerves that cross it. Examiners move quickly from naming to function — which strap muscle the ansa does not supply, where the carotid divides, what is at risk in a carotid endarterectomy.

Boundaries

• Anterior: the midline of the neck.
• Posterior: the anterior border of sternocleidomastoid.
• Superior: the lower border of the mandible.
• Apex: the jugular notch of the manubrium.
• Roof: skin, platysma and the investing layer of deep cervical fascia. Floor: pharynx, larynx and thyroid gland.

Four smaller triangles

The two bellies of digastric and the superior belly of omohyoid divide it:

• Submental (unpaired) — between the anterior bellies of both digastrics and the hyoid, on a floor of mylohyoid. Submental lymph nodes.
• Submandibular — between the two bellies of digastric and the mandible. Submandibular gland, facial artery and vein, hypoglossal nerve, nerve to mylohyoid.
• Carotid — between the posterior belly of digastric, the superior belly of omohyoid and sternocleidomastoid. Carotid bifurcation, carotid sinus and body, internal jugular vein, vagus, hypoglossal nerve, ansa cervicalis.
• Muscular — between the superior belly of omohyoid, sternocleidomastoid and the midline. The infrahyoid muscles over the thyroid.

The strap muscles

Sternohyoid, sternothyroid, thyrohyoid and omohyoid, which steady and depress the hyoid and larynx. The ansa cervicalis (C1–C3) supplies all of them except thyrohyoid, which takes C1 fibres that travel with the hypoglossal nerve and leave it as the nerve to thyrohyoid. The ansa enters the muscles low down, so at thyroidectomy they are divided high to keep their nerve supply.

Carotid sheath

Common carotid artery medially, internal jugular vein laterally, vagus behind and between them; the ansa cervicalis lies on the front of the sheath. The common carotid divides at the level of the upper border of the thyroid cartilage, around C4. The carotid sinus, a dilatation at the start of the internal carotid, is a baroreceptor; the carotid body in the fork is a chemoreceptor; both report through the glossopharyngeal nerve. The internal carotid gives no branches in the neck and the external carotid does — the way to tell them apart at operation.

Veins

The facial vein crosses the carotid triangle to join the internal jugular. The external jugular forms behind the angle of the mandible, runs down across sternocleidomastoid under platysma, and pierces the deep fascia above the clavicle to reach the subclavian vein.

Nerves crossing the region

• Hypoglossal nerve loops forward over the internal and external carotid arteries, below the posterior belly of digastric, on its way to the tongue.
• Accessory nerve passes deep to or through sternocleidomastoid, which it supplies, then crosses the posterior triangle to trapezius — the nerve at risk in a posterior triangle node biopsy.
• Marginal mandibular branch of the facial nerve runs under platysma near the lower border of the mandible, which is why neck incisions are placed two finger-breadths below it.

Viva favourites

• Nerves at risk in carotid endarterectomy: hypoglossal, vagus and its recurrent laryngeal branch, marginal mandibular, great auricular, and the ansa cervicalis.
• Nerves at risk in submandibular gland excision: marginal mandibular superficially; lingual and hypoglossal deep to the gland.
• Which strap muscle is not supplied by the ansa cervicalis? Thyrohyoid.