Boundaries and subdivisions
The anterior triangle is bounded by the midline of the neck in front, the anterior border of sternocleidomastoid behind and the lower border of the mandible above, with its apex at the jugular notch of the manubrium. Its roof is skin, platysma and the investing layer of deep cervical fascia; its floor is the pharynx, larynx and thyroid gland.
The anterior and posterior bellies of digastric and the superior belly of omohyoid cut it into four smaller triangles.
| Triangle | Boundaries | Main contents |
|---|---|---|
| Submental (unpaired) | Anterior bellies of both digastrics, body of hyoid; floor of mylohyoid | Submental lymph nodes, small veins forming the anterior jugular vein |
| Submandibular (digastric) | Anterior and posterior bellies of digastric, lower border of mandible | Submandibular gland and nodes, facial artery and vein, hypoglossal nerve, nerve to mylohyoid |
| Carotid | Posterior belly of digastric, superior belly of omohyoid, sternocleidomastoid | Carotid bifurcation, carotid sinus and body, internal jugular vein, vagus, hypoglossal nerve, ansa cervicalis |
| Muscular | Superior belly of omohyoid, sternocleidomastoid, midline | Infrahyoid muscles, thyroid, larynx, trachea |
Strap muscles and the ansa cervicalis
The strap (infrahyoid) muscles are sternohyoid, sternothyroid, thyrohyoid and omohyoid, and they depress the hyoid and larynx after swallowing and speech. Omohyoid has two bellies joined by an intermediate tendon that a fascial sling ties down towards the clavicle: the superior belly runs up to the body of the hyoid, and the inferior belly runs back across the posterior triangle to the upper border of the scapula near the suprascapular notch.
The ansa cervicalis is a loop of C1–C3 fibres lying on or in the front of the carotid sheath. Its superior root is C1 fibres that travel with the hypoglossal nerve and then leave it; its inferior root comes from C2 and C3. The ansa supplies sternohyoid, sternothyroid and omohyoid. Thyrohyoid is the exception: its C1 fibres stay with the hypoglossal nerve and leave it as the nerve to thyrohyoid. The ansa enters the strap muscles in their lower halves, so surgeons divide them high when they need access to a large thyroid.
Carotid sheath, arteries and veins
The carotid sheath holds the common and then internal carotid artery medially, the internal jugular vein laterally and the vagus nerve behind and between them. The common carotid divides at the upper border of the thyroid cartilage, usually at C3–C4.
- Carotid sinus: a dilatation of the terminal common carotid and the start of the internal carotid. It is a baroreceptor, served mainly by the carotid sinus branch of the glossopharyngeal nerve.
- Carotid body: a small chemoreceptor in the fork of the bifurcation, with the same nerve supply.
- Internal carotid artery: starts posterolateral to the external and gives no branches in the neck.
- External carotid artery: starts anteromedial and gives superior thyroid, ascending pharyngeal, lingual, facial, occipital and posterior auricular branches before ending in the parotid as the maxillary and superficial temporal arteries.
The facial vein crosses the carotid triangle to join the internal jugular vein at about the level of the hyoid. The external jugular vein forms near the angle of the mandible from the posterior auricular vein and the posterior division of the retromandibular vein. It runs down across sternocleidomastoid under platysma and pierces the deep fascia just above the clavicle to end in the subclavian vein.
Relations: nerves crossing the triangle
Four nerves cross the anterior triangle at predictable points, and each is a surgical landmark.
- Hypoglossal nerve (CN XII): curves forward across the lateral side of the internal and external carotid arteries just below the posterior belly of digastric, hooks under the sternocleidomastoid branch of the occipital artery, and runs on to hyoglossus towards the tongue.
- Superior laryngeal nerve: from the vagus, passes deep to both carotids and splits into the internal laryngeal nerve, which pierces the thyrohyoid membrane, and the external laryngeal nerve, which runs with the superior thyroid artery to cricothyroid.
- Accessory nerve (CN XI): crosses the internal jugular vein high in the neck and enters the deep surface of sternocleidomastoid, which it supplies, before crossing the posterior triangle to trapezius.
- Marginal mandibular branch of the facial nerve: runs under platysma close to the lower border of the mandible, and can dip below it.
Clinical relevance
The anterior triangle is the approach for carotid, thyroid and submandibular surgery, so its nerves are the ones examiners ask about.
- Carotid endarterectomy: the hypoglossal nerve, vagus, superior and recurrent laryngeal nerves, marginal mandibular branch, great auricular nerve and ansa cervicalis are all at risk. The facial vein is ligated to expose the bifurcation.
- Submandibular gland excision: the incision sits two finger-breadths below the mandible to protect the marginal mandibular nerve. The lingual and hypoglossal nerves lie deep to the gland.
- Carotid body tumour: a pulsatile mass at the bifurcation that splays the internal and external carotids apart.
- Branchial cyst: presents at the anterior border of sternocleidomastoid, classically at the junction of its upper and middle thirds.
On the specimen
This prosection is read from superficial to deep, and it extends behind sternocleidomastoid, so posterior triangle muscles appear alongside the anterior triangle contents.
- Internal or external carotid: the one with branches is the external; trace a branch back to its parent. The internal is the posterolateral vessel and carries the swollen carotid sinus at its origin.
- Internal or external jugular vein: the internal is the large vein in the sheath lateral to the artery; the external is superficial, on sternocleidomastoid.
- Ansa or hypoglossal nerve: the hypoglossal runs horizontally forward across both carotids high up; the ansa is a thin loop hanging on the front of the sheath lower down.
- Superior or inferior belly of omohyoid: the superior belly climbs to the hyoid beside sternohyoid; the inferior belly crosses the posterior triangle.
- Deep muscles: scalenus anterior carries the phrenic nerve on its front, scalenus medius lies behind the brachial plexus, levator scapulae runs down to the scapula, semispinalis is the deep vertical muscle near the midline behind, and pectoralis major appears at the lower edge.