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Anterior Triangle of the Neck

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Overview and Boundaries

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Subdivisions

1. Submental Triangle (unpaired, single)

• Boundaries: Body of hyoid bone (inferiorly), anterior belly of digastric on each side (laterally), mandibular symphysis (superiorly)
• Contents: Submental lymph nodes; tributaries forming the anterior jugular vein
• Floor: Mylohyoid muscles and their median raphe

2. Submandibular Triangle (paired)

• Boundaries: Lower border of mandible (superiorly), anterior belly of digastric (anteroinferiorly), posterior belly of digastric + stylohyoid (posteroinferiorly)
• Floor: Anteriorly - mylohyoid; posteriorly - hyoglossus; most posteriorly - middle constrictor of pharynx
• Contents:
  • Submandibular gland (the key content)
  • Submandibular lymph nodes
  • Hypoglossal nerve [CN XII]
  • Mylohyoid nerve and vessels
  • Facial artery and vein
  • (In the posterior part): External carotid artery, internal carotid artery, internal jugular vein, vagus nerve

3. Carotid Triangle (paired)

• Boundaries: Posterior belly of digastric + stylohyoid (superiorly), superior belly of omohyoid (anteroinferiorly), anterior border of SCM (posteriorly)
• Contents: The most clinically important triangle, containing:
  • Common carotid artery - bifurcates here into internal and external carotid arteries at the level of the superior border of the thyroid cartilage
  • Carotid sinus - dilation at the bifurcation; monitors blood pressure; innervated by CN IX
  • Carotid body - at the bifurcation; detects blood oxygen content; innervated by CN IX and X
  • Internal jugular vein
  • Vagus nerve [CN X] - in the carotid sheath, between internal jugular vein and carotid arteries
  • Hypoglossal nerve [CN XII]
  • Accessory nerve [CN XI]
  • Ansa cervicalis (superior and inferior roots)
  • Transverse cervical nerve
  • External carotid artery branches: superior thyroid, lingual, facial, ascending pharyngeal, occipital

4. Muscular Triangle (paired)

• Boundaries: Midline of neck (medially), superior belly of omohyoid (laterally), anterior border of SCM (posterolaterally), hyoid bone (superiorly)
• Contents: Strap muscles (infrahyoid), thyroid gland, parathyroid glands, trachea, pharynx/larynx

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Muscles of the Anterior Triangle

Suprahyoid Muscles (raise the hyoid; related to submental and submandibular triangles)

Infrahyoid Muscles (strap muscles - depress the hyoid; related to muscular triangle)

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Arteries

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Veins

• Internal jugular vein (IJV): Descends in the carotid sheath; the major venous drainage of the head and neck.
• Common facial vein: Drains into the IJV at the level of the hyoid bone.
• Anterior jugular vein: Runs in the submental and muscular triangles; communicates across the midline via the jugular venous arch above the suprasternal notch.
• Jugular venous pulse (JVP): Observed in the IJV; clinically reflects right heart function and venous pressure.

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Nerves

Cranial Nerves

Spinal Nerve Branches (Cervical Plexus)

• Ansa cervicalis (C1-C3): A loop formed by the superior root (C1, travels with CN XII) and inferior root (C2-C3); innervates all infrahyoid muscles except thyrohyoid
• Transverse cervical nerve (C2-C3): Sensory to the anterior neck skin

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Visceral Contents (Muscular Triangle)

Thyroid Gland

• Lies in the visceral compartment (pretracheal fascia), deep to the strap muscles
• Two lateral lobes + isthmus (crosses 2nd-3rd tracheal rings)
• A pyramidal lobe may extend superiorly in ~50% of people
• Blood supply: Superior thyroid artery (from external carotid) and inferior thyroid artery (from thyrocervical trunk of subclavian)
• Venous drainage: Superior and middle thyroid veins to IJV; inferior thyroid veins to brachiocephalic vein
• Lymphatics drain to deep cervical nodes
• Development: arises from foramen caecum of tongue; descends via thyroglossal duct; remnants may form thyroglossal cysts or ectopic thyroid tissue

Parathyroid Glands

• Usually 4 glands on the posterior surface of thyroid (2 superior, 2 inferior)
• Develop from 3rd and 4th pharyngeal pouches
• Superior parathyroids are more constant in position; inferior ones are more variable (may be ectopic in the mediastinum)
• Blood supply: inferior thyroid artery

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Summary Table - Structures by Subdivision

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Clinical Points

• Carotid endarterectomy and carotid artery ligation are performed via the carotid triangle, with care to protect CN IX, X, XI, XII and ansa cervicalis.
• Carotid sinus massage (used in SVT) stimulates baroreceptors in the carotid sinus; pressure transmitted via CN IX to the vagal nuclei.
• Thyroglossal duct cysts present as midline neck swellings that move on swallowing and on tongue protrusion (they are attached to the hyoid bone).
• Submandibular gland excision risks the marginal mandibular branch of CN VII (runs deep to platysma near the gland) and CN XII.
• Central venous catheter placement in the IJV is performed within the carotid triangle; the tip should rest at the junction of the superior vena cava and right atrium.
• Ansa cervicalis is used surgically as a source of reinnervation for laryngeal muscles (nerve-to-larynx reinnervation procedures).