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Triangles of the Neck — Full Anatomy (20 Marks)

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Overview

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I. ANTERIOR TRIANGLE OF THE NECK

A. Boundaries

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B. Subdivisions of the Anterior Triangle

1. Submental Triangle (Unpaired)

• Boundaries: Mandibular symphysis superiorly; anterior belly of digastric on each side; body of hyoid bone inferiorly
• Contents:
  • Submental lymph nodes
  • Tributaries forming the anterior jugular vein

2. Submandibular Triangle (Digastric Triangle) — Paired

• Boundaries: Lower border of mandible superiorly; anterior belly of digastric anteriorly; posterior belly of digastric posteriorly
• Contents:
  • Submandibular gland
  • Submandibular lymph nodes
  • Hypoglossal nerve [XII]
  • Mylohyoid nerve
  • Facial artery and vein

3. Carotid Triangle — Paired

• Boundaries: Posterior belly of digastric + stylohyoid superiorly; superior belly of omohyoid anteroinferiorly; anterior border of SCM posteriorly
• Contents (clinically most important):
  • Common carotid artery (bifurcating into internal & external carotid)
  • Carotid sinus & carotid body
  • Internal jugular vein
  • Superior thyroid, lingual, facial, occipital, and ascending pharyngeal arteries
  • Vagus nerve [X], accessory nerve [XI], hypoglossal nerve [XII]
  • Superior and inferior roots of ansa cervicalis
  • Transverse cervical nerve
  • Tributaries to common facial vein; cervical branch of facial nerve [VII]

4. Muscular Triangle (Strap Triangle) — Paired

• Boundaries: Midline of neck medially; superior belly of omohyoid laterally; anterior border of SCM posterolaterally; hyoid bone superiorly
• Contents:
  • Infrahyoid strap muscles: sternohyoid, omohyoid, sternothyroid, thyrohyoid
  • Thyroid and parathyroid glands
  • Larynx, trachea, pharynx, esophagus

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C. Muscles of the Anterior Triangle

Suprahyoid Muscles (superior to hyoid bone)

Infrahyoid Muscles (inferior to hyoid bone)

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D. Vascular Contents of the Anterior Triangle

• Right CCA: from brachiocephalic trunk, entirely in neck
• Left CCA: branches from arch of aorta, enters neck at left sternoclavicular joint
• Both ascend within the carotid sheath, lateral to trachea and esophagus
• Give no branches until bifurcation at the level of superior edge of thyroid cartilage (C3–C4)

• Carotid sinus — dilatation at bifurcation; baroreceptors for blood pressure; innervated by glossopharyngeal nerve [IX]
• Carotid body — chemoreceptor for O₂ levels; innervated by glossopharyngeal [IX] and vagus [X] nerves

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II. POSTERIOR TRIANGLE OF THE NECK

A. Boundaries

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B. Subdivisions of the Posterior Triangle

1. Occipital Triangle (larger, superior): Contains accessory nerve [XI], cervical plexus branches, occipital artery, transverse cervical artery
2. Omoclavicular (Subclavian) Triangle (smaller, inferior): Contains 3rd part of subclavian artery, subclavian vein, brachial plexus (lower trunks), suprascapular artery & vein

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C. Muscles — Posterior Triangle

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D. Vessels of the Posterior Triangle

• Formed near angle of mandible (posterior retromandibular + posterior auricular veins)
• Crosses SCM superficially → enters posterior triangle → drains into subclavian vein
• Important landmark for estimating jugular venous pressure

• Found in the omoclavicular triangle
• Branches: transverse cervical artery (crosses base of posterior triangle to trapezius) and suprascapular artery (from thyrocervical trunk, crosses lowest part of posterior triangle)

• Continuation of axillary vein from lateral border of rib I
• Passes anterior to anterior scalene muscle
• Receives external jugular vein; joins internal jugular vein → brachiocephalic vein

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E. Nerves of the Posterior Triangle

1. Accessory Nerve [XI]

• Exits cranial cavity via jugular foramen
• Descends posteriorly; innervates SCM then enters posterior triangle
• Crosses the posterior triangle in the investing cervical fascia
• Innervates the trapezius muscle
• Superficially placed → vulnerable to surgical injury (e.g., lymph node biopsy)

2. Cervical Plexus (C1–C4)

• Lesser occipital nerve (C2): Skin of scalp posterior to ear
• Great auricular nerve (C2, C3): Skin of auricle and parotid region
• Transverse cervical nerve (C2, C3): Anterior skin of neck
• Supraclavicular nerves (C3, C4): Skin over clavicle and shoulder

• Phrenic nerve (C3, C4, C5) — Descends over anterior scalene → diaphragm (motor and sensory)
• Ansa cervicalis (C1–C3) — innervates infrahyoid muscles
• Branches to prevertebral muscles (rectus capitis, longus colli, longus capitis)

3. Brachial Plexus (C5–T1)

• Roots emerge between anterior and middle scalene muscles
• Trunks visible in the posterior triangle (lower part)
• Important for supraclavicular block in regional anaesthesia

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III. CLINICAL SIGNIFICANCE

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Summary Diagram Reference

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