Anterior Cervical Triangle – Comprehensive Study Notes

Anterior Cervical Triangle – General Overview

• Region of the neck bounded by
  • Sternocleidomastoid (SCM) – posterolateral border
  • Anterior midline of the neck – medial border
  • Inferior border of the mandible – superior border
• Roof
  • Superficial fascia (subcutaneous tissue) containing the platysma
• Floor
  • Viscera and walls of upper aerodigestive tract: pharynx, larynx, thyroid gland
• Sub-divided into four smaller triangles
  • Submandibular (a.k.a. Digastric)
  • Submental
  • Carotid
  • Muscular

Suprahyoid & Infrahyoid Muscle Tables (concise extract)

• Suprahyoid group (form the “floor of mouth”)
  • Mylohyoid – \text{CN V}_3 (nerve to mylohyoid); elevates hyoid, floor of mouth & tongue
  • Geniohyoid – C1 via \text{CN XII}; pulls hyoid anterosuperiorly, broadens pharynx
  • Stylohyoid – facial nerve (stylohyoid branch); elevates & retracts hyoid
  • Digastric
  • Anterior belly – nerve to mylohyoid
  • Posterior belly – facial nerve (digastric branch)
  • Together: depress mandible against resistance, elevate & steady hyoid
• Infrahyoid (“strap”) group
  • Sternohyoid – C1–C3 via ansa cervicalis; depresses hyoid after swallowing
  • Omohyoid (superior & inferior bellies) – C1–C3 via ansa cervicalis; depresses, retracts, steadies hyoid
  • Sternothyroid – C2–C3 via ansa cervicalis; depresses hyoid & larynx
  • Thyrohyoid – C1 via \text{CN XII} (nerve to thyrohyoid); depresses hyoid, elevates larynx

Submandibular (Digastric) Triangle

• Boundaries
  • Superior: body of mandible
  • Anteroinferior: anterior belly of digastric
  • Posteroinferior: posterior belly of digastric & stylohyoid
• Floor (muscular)
  • Mylohyoid
  • Hyoglossus
  • Styloglossus
  • Middle pharyngeal constrictor
• Roof
  • Skin, superficial fascia & platysma, investing fascia of neck
• Contents
  • Gland
  • Submandibular gland fills most of the space
    • Superficial part – superficial to mylohyoid
    • Deep part – deep to mylohyoid, superficial to hyoglossus
    • Submandibular duct (~5\text{ cm})
    • Emerges from deep part, hooks around lingual nerve, opens at sublingual papilla beside frenulum
  • Nerves
  • Hypoglossal nerve (CN XII) – passes forward deep to posterior belly of digastric
  • Glossopharyngeal nerve (CN IX) – briefly crosses region deep to styloid muscles
  • Nerve to mylohyoid
  • Lingual nerve (CN V$_3$)
  • Arteries
  • Facial artery (winds deep–superficial around submandibular gland)
  • Submental artery (branch of facial)
  • Lingual artery (deep to hyoglossus)
  • Veins
  • Common facial vein (union of facial & anterior division of retromandibular)
  • Lymphatics
  • Submandibular lymph nodes (drain face, oral cavity, anterior nasal cavity)
• Nerve supply of gland
  • Sensory: lingual nerve
  • Parasympathetic (secretomotor): presynaptic fibers from chorda tympani (CN VII) → join lingual nerve → synapse in submandibular ganglion → postsynaptic fibers follow arteries
  • Sympathetic: postsynaptic fibers from superior cervical ganglion travel on peri-arterial plexuses

Submental Triangle

• Only unpaired triangle
• Boundaries
  • Lateral: right & left anterior bellies of digastric
  • Inferior: body of hyoid bone
  • Apex: mandibular symphysis
• Floor: two mylohyoid muscles meeting at a median raphe
• Contents
  • Nerve to mylohyoid (motor & sensory to skin over chin)
  • Submental artery (branched from facial)
  • Commencement of anterior jugular vein (if present)
  • Submental lymph nodes (part of pericervical ring)
• Functional note: suprahyoid muscles here help depress mandible in concert with infrahyoids

Muscular Triangle

• Boundaries
  • Superior belly of omohyoid (superior & lateral)
  • Anterior border of SCM (inferolateral)
  • Anterior midline of neck (medial)
• Roof & Floor
  • Similar to general anterior triangle roof; floor formed largely by infrahyoid (“strap”) muscles positioned on pretracheal fascia
• Contents
  • Infrahyoid muscles: sternohyoid, omohyoid, sternothyroid, thyrohyoid
  • Thyroid & parathyroid glands, larynx, trachea, esophagus (deep)
  • Vessels: superior & inferior thyroid vessels, anterior jugular veins
  • Nerves: ansa cervicalis loops embedded in / anterior to carotid sheath; nerve to thyrohyoid (C1)

Carotid Triangle

• Boundaries
  • Superior belly of omohyoid (anteroinferior)
  • Posterior belly of digastric & stylohyoid (superior)
  • Anterior border of SCM (posterolateral)
• Key vascular relations
  • Common carotid artery ascends within carotid sheath; bifurcates at superior border of thyroid cartilage (= C3\text{–}C4 ) into internal & external carotids
  • Carotid sinus (baroreceptor) – dilatation at bifurcation and initial internal carotid; innervation \text{CN IX},\; CN X, sympathetic fibers
  • Carotid body (chemoreceptor) – reddish-brown mass on deep side of bifurcation; same innervation as sinus
• Carotid sheath contents (lateral → medial mnemonic “V I C” reversed in cross-section):
  • Internal jugular vein – lateral
  • Vagus nerve (CN X) – posterior/intermediate
  • Carotid arteries – medial (common ↓, internal ↑; external is anteromedial outside sheath for most branches)
  • Deep cervical lymph nodes along sheath
• Ansa cervicalis
  • Usually embedded in anterior wall of sheath
  • Superior root (C1–C2) briefly runs with hypoglossal nerve then descends
  • Inferior root (C2–C3); both join → loop that supplies omohyoid, sternohyoid, sternothyroid
  • Thyrohyoid gets C1 fibers directly via hypoglossal (nerve to thyrohyoid)
• External Carotid Artery Branches (inferior → superior mnemonic “Some Little Fat Man Stole Papa’s Only Apple”)
  • Superior thyroid (gives superior laryngeal branch)
  • Ascending pharyngeal
  • Lingual
  • Facial (gives submental)
  • Occipital
  • Posterior auricular
  • Maxillary (terminal)
  • Superficial temporal (terminal)
• Nerves encountered
  • Vagus nerve within sheath gives superior laryngeal nerve
  • Internal laryngeal nerve (sensory) pierces thyrohyoid membrane with superior laryngeal artery
  • External laryngeal nerve (motor) descends with superior thyroid artery → innervates cricothyroid
  • Cervical sympathetic trunk (posterior to sheath)
  • Superior, middle, inferior cervical ganglia (postsynaptic fibers to head/neck viscera via arteries; gray rami to cervical nerves; cardiac branches to thorax)

Cervical Plexus (C1–C4)

• Location: deep to SCM, superficial to levator scapulae & middle scalene
• Cutaneous branches (all emerge at posterior border of SCM ~ midpoint “nerve point of neck”)
  • Lesser occipital (C2)
  • Great auricular (C2–C3)
  • Transverse cervical (C2–C3)
  • Supraclavicular (C3–C4)
• Muscular/deep branches
  • Ansa cervicalis (C1–C3) – to strap muscles
  • Phrenic nerve (C3–C5, mainly C4)
  • Segmental branches to prevertebral & scalene muscles

Lymphatics of the Neck (clinically relevant)

• Pericervical collar (superficial nodes)
  • Submental, Submandibular, Parotid, Mastoid, Occipital
• Superficial lymphatic vessels follow superficial veins → drain to deep cervical nodes
• Deep lymphatic vessels follow arteries → drain directly to deep cervical chain
• Deep cervical lymph nodes: form vertical chain along internal jugular vein
  • Receive all lymph from head & neck; inferior nodes drain into right lymphatic duct (right side) or thoracic duct (left side)

Hyoid Bone & Related Attachments (orientation points)

• Lies at C3 vertebral level
• No bony articulations; suspended by muscles & stylohyoid ligaments
• Serves as anchoring point for suprahyoid & infrahyoid groups
• Clinical: fracture (“strangulation fracture”) suggests throttling

Key Named Nerves in Region – Summary Table

• Hypoglossal nerve (CN XII)
  • Motor to intrinsic & extrinsic tongue except palatoglossus
  • Carries C1 fibers to geniohyoid & thyrohyoid, and to superior root of ansa cervicalis
• Glossopharyngeal nerve (CN IX)
  • Sensory to carotid sinus/body, stylopharyngeus motor branch, taste/sensory posterior 1/3 tongue
• Vagus nerve (CN X)
  • Within carotid sheath; gives superior laryngeal (internal & external), pharyngeal branches, recurrent laryngeal
• Cervical sympathetic chain
  • No white rami at cervical levels; receives pre-ganglionics from T1–T5

Applied / Clinical Correlations

• Carotid sinus hypersensitivity → syncope when stimulated (tight collar)
• External laryngeal nerve can be injured in thyroidectomy → monotone voice (paralysis of cricothyroid → inability to tense vocal cords)
• Submandibular stone (sialolith) may obstruct submandibular duct at sublingual papilla; pain & swelling during meals
• Fascial planes: infection in submandibular space can spread posteriorly into parapharyngeal space → airway compromise
• Penetrating injury in anterior triangle may endanger carotid sheath contents (bleeding, vagus injury, air embolism via IJV)

Review Questions (from slides)

1. Digastric (submandibular) triangle boundaries:
   • Correct answer: C – anterior digastric, mandible, posterior digastric
2. Parotid gland portion lies within:
   • Correct answer: B – Digastric triangle (posterior/upward part under mandible)
3. Carotid sinus & body vertebral level:
   • Correct answer: C3\text{–}C4 (superior border of thyroid cartilage)
4. Omohyoid innervation:
   • Correct answer: B – Ansa cervicalis (C1–C3)