Deep Structures of the Neck

Deep Cervical Fascia - Neck's Scaffolding

• Dense connective tissue; forms fascial planes, compartmentalizes neck structures.
• Main Layers:
  • Investing Layer: Encloses Sternocleidomastoid (SCM) & Trapezius.
  • Pretracheal Layer: Anterior neck; encloses thyroid, trachea, esophagus. Has muscular & visceral parts.
  • Prevertebral Layer: Surrounds vertebral column & associated muscles (scalenes, longus colli/capitis).
  • Carotid Sheath: Vascular compartment; contains common/internal carotid artery, internal jugular vein (IJV), vagus nerve (CN X). 📌 Mnemonic: "I See 10 CCs in an IV" (IJV, CN X, CCA, Carotid Sheath).
• Clinical Importance: Defines spaces for infection spread.

  ⭐ Retropharyngeal space (Danger Space) infections can spread to the posterior mediastinum.

Posterior Triangle (Deep Contents) - Nerve & Vessel Passageway

• Nerves:
  • Accessory Nerve (CN XI): Crosses superficially; motor to trapezius & SCM.
  • Cervical Plexus: Sensory branches (e.g., supraclavicular); motor contributions (e.g., phrenic nerve roots C3, C4, C5).
  • Brachial Plexus Trunks: Emerge between anterior & middle scalene muscles.
• Vessels:
  • Subclavian Artery (Third Part): Major arterial component.
  • Subclavian Vein: Anterior to anterior scalene; may arch into triangle.
  • Transverse Cervical & Suprascapular Arteries: Branches of thyrocervical trunk.
  • External Jugular Vein: Terminates in this region.

⭐ The accessory nerve (CN XI) is vulnerable to injury during procedures in the posterior triangle, leading to trapezius muscle weakness.

Anterior Triangle (Deep Contents) - Carotid Kingdom

• Carotid Sheath: Encloses Common Carotid Artery (CCA), Internal Carotid Artery (ICA), Internal Jugular Vein (IJV), Vagus Nerve (CN X).
• CCA Bifurcation: At C3-C4 vertebral level (upper border of thyroid cartilage).
  • Carotid Sinus: Baroreceptor (CN IX, X).
  • Carotid Body: Chemoreceptor (CN IX, X).
• External Carotid Artery (ECA) Branches: (📌 SALFOPMS)
  • Superior thyroid
  • Ascending pharyngeal
  • Lingual
  • Facial
  • Occipital
  • Posterior auricular
  • Maxillary
  • Superficial temporal
• Internal Carotid Artery (ICA): No branches in the neck; enters skull via carotid canal.
• Nerves: Ansa cervicalis (C1-C3) supplies infrahyoid muscles; Hypoglossal nerve (CN XII) crosses ECA/ICA.

⭐ The carotid sinus, innervated by CN IX (Hering's nerve) and CN X, acts as a baroreceptor.

Root of the Neck - Gateway Junction

• Transition zone: Neck ↔ Thorax (aka Superior Thoracic Aperture).
• Boundaries:
  • Anterior: Manubrium sterni.
  • Posterior: T1 vertebra.
  • Lateral: 1st ribs & costal cartilages.
• Key Contents:
  • Arteries: Subclavian, Common Carotid.
  • Veins: Subclavian, Internal Jugular.
  • Nerves: Vagus (CN X), Phrenic, Sympathetic trunk, Brachial plexus (roots C5-T1).
  • Lymphatics: Thoracic duct (L), Right lymphatic duct (R).
  • Viscera: Apices of lungs, Trachea, Esophagus.

⭐ Pancoast tumor at the lung apex can compress structures at the root of the neck, causing Horner's syndrome (due to sympathetic trunk involvement).

Deep Neck Viscera & Lymphatics - Central Command

• Thyroid Gland:
  • Anterior neck (C5-T1). Two lobes, isthmus.
  • Blood: Sup. & Inf. thyroid arteries. Veins: Sup, Mid, Inf. thyroid.
• Parathyroid Glands (Usually 4):
  • Posterior to thyroid. Secrete PTH ($Ca^{2+}$ regulation).
• Cervical Lymphatics:
  • Deep cervical nodes: Along Internal Jugular Vein (IJV).
  • Thoracic duct: Drains to L. venous angle (L. IJV & L. subclavian v. jnct.).
    • Right lymphatic duct: Drains to R. venous angle.

⭐ The thoracic duct drains into the junction of the left internal jugular and subclavian veins; Virchow's node (left supraclavicular) enlargement can indicate metastatic gastric cancer.

High-Yield Points - ⚡ Biggest Takeaways

• Carotid sheath: CCA/ICA (medial), IJV (lateral), Vagus nerve (posterior); Ansa cervicalis anteriorly.
• Prevertebral fascia encloses vertebrae, prevertebral/scalene muscles, phrenic nerve, brachial plexus roots, sympathetic trunk.
• Retropharyngeal space: between buccopharyngeal & prevertebral fascia; infection can reach mediastinum.
• Phrenic nerve (C3,C4,C5) runs on Scalenus Anterior.
• Sympathetic trunk: posterior to carotid sheath, anterior to prevertebral muscles.
• Thoracic duct (left) arches over subclavian artery to brachiocephalic vein.
• Recurrent laryngeal nerve: in tracheo-esophageal groove; vulnerable in thyroid surgery.