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External carotid artery with carotid sheath

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Here is a detailed anatomical overview of the External Carotid Artery (ECA) and the Carotid Sheath, drawn from Gray's Anatomy for Students, Scott-Brown's Otorhinolaryngology, and K.J. Lee's Essential Otolaryngology.

External Carotid Artery (ECA)

Origin and Course

The ECA arises from the common carotid artery (CCA) at its bifurcation, typically at the level of the upper border of the thyroid cartilage (C3-C4). After the bifurcation, the ECA lies anteromedial to the internal carotid artery (ICA). It ascends, passes deep to (or enters) the parotid gland, and divides into its two terminal branches (superficial temporal and maxillary arteries) near the lower border of the ear, within the substance of the parotid gland.

Distinguishing ECA from ICA

The ECA is identified by the fact that it gives off branches in the neck, whereas the ICA has no branches in the neck.

Branches of the ECA

The ECA gives off 6 branches before its 2 terminal branches - a useful mnemonic is "Some Anatomists Like Freaking Out Poor Students" (Superior thyroid, Ascending pharyngeal, Lingual, Facial, Occipital, Posterior auricular, then Superficial temporal, Maxillary as terminals).
BranchOrigin (surface)Supplies
Superior thyroid arteryAnterior (1st branch)Thyroid gland, larynx (internal structures), sternocleidomastoid, cricothyroid muscles
Ascending pharyngeal arteryDeep/posterior (smallest, may actually arise 1st)Pharyngeal constrictors, palate, palatine tonsil, meninges of posterior cranial fossa, skull base, dura
Lingual arteryAnterior, at level of hyoid boneTongue, sublingual gland, floor of mouth, epiglottis, tonsil
Facial arteryAnterior (3rd anterior branch, just above lingual)Face from mandible to medial canthus, soft palate, tonsil, submandibular gland
Occipital arteryPosterior (near facial origin)Sternocleidomastoid, posterior scalp, deep back muscles, meninges of posterior cranial fossa, mastoid cells
Posterior auricular arteryPosterior (small, just above digastric)Parotid gland, muscles of ear, external ear and scalp posterior to ear, middle and inner ear
Superficial temporal arteryTerminal (upward continuation)Parotid gland and duct, masseter, lateral face, anterior external ear, temporalis muscle, parietal and temporal scalp
Maxillary arteryTerminal (passes deep to mandible)External acoustic meatus, TMJ, meninges, teeth, nasal cavity, palate, infratemporal fossa, sinuses
Note: When ligating the ECA for epistaxis, the ascending pharyngeal artery was traditionally preserved due to its supply to the skull base and dura. - Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol. 1

Relation to the Parotid Gland

The ECA enters/passes deep to the inferior border of the parotid gland and gives off the posterior auricular artery before dividing into its terminal branches within the gland. The retromandibular vein (formed by junction of superficial temporal and maxillary veins) also lies within the parotid. - Gray's Anatomy for Students

Carotid Sheath

Fascial layers of the carotid sheath showing the common carotid artery, internal jugular vein, and vagus nerve enclosed within superficial, middle, and deep layers of deep cervical fascia
Figure: Fascial layers of the carotid sheath - K.J. Lee's Essential Otolaryngology

Structure and Composition

The carotid sheath is a fascial tube derived primarily from the investing (superficial) layer of deep cervical fascia, medial to the sternocleidomastoid. It receives contributions from all three layers of deep cervical fascia:
  1. Investing layer
  2. Pretracheal (middle) layer
  3. Prevertebral (deep) layer
The sheath is thicker around the arteries but loose around the internal jugular vein, which allows the vein to expand during increased venous flow. The sheath connects to surrounding fascial layers via loose areolar tissue. - Scott-Brown's Otorhinolaryngology; Gray's Anatomy for Students

Contents

StructurePosition
Common carotid artery (below bifurcation) / Internal + External carotid aboveMedial
Internal jugular vein (IJV)Lateral/anterolateral
Vagus nerve [CN X]Posterior (between artery and vein)
Ansa cervicalisOn anterior surface

Extent

  • Superior: Base of skull
  • Inferior: Thorax (continues into the thoracic cavity)

Clinical Significance

The carotid sheath is known as "The Lincoln Highway of the Neck" because it is a potential avenue for rapid spread of infection from the neck down into the mediastinum. - K.J. Lee's Essential Otolaryngology
The carotid sheath space has specific boundaries:
  • Anterior: Sternocleidomastoid muscle
  • Posterior: Prevertebral space
  • Medial: Visceral space
  • Lateral: Sternocleidomastoid

Fascial Compartments of the Neck (Context)

The carotid sheath is the 4th longitudinal compartment of the neck created by cervical fascia:
  1. Outer compartment - investing layer
  2. Prevertebral compartment - deep muscles and vertebral column
  3. Visceral compartment - pharynx, trachea, esophagus, thyroid
  4. Carotid sheath - neurovascular structures
Infections spreading via these fascial spaces can pass from the neck to:
  • Anterior mediastinum (via pretracheal space)
  • Posterior mediastinum (via retropharyngeal space or prevertebral space)
Sources: Gray's Anatomy for Students | Scott-Brown's Otorhinolaryngology Head & Neck Surgery (Vol. 1 & 2) | K.J. Lee's Essential Otolaryngology

Ansa cervical

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Ansa Cervicalis

Ansa Cervicalis diagram showing C1, C2, C3 roots, superior and inferior roots, and the infrahyoid muscles supplied
Fig. 8.180 Ansa Cervicalis - Gray's Anatomy for Students

Definition

The ansa cervicalis (meaning "handle/loop of the neck") is a nerve loop formed by fibers from cervical nerves C1, C2, and C3 that innervates three of the four infrahyoid (strap) muscles in the anterior triangle of the neck. It lies anterior to the carotid sheath.

Formation: Two Roots

1. Superior Root (Descendens Hypoglossi) - C1 (± C2)

  • C1 fibers (sometimes also C2) join the hypoglossal nerve [CN XII] soon after it exits the skull via the hypoglossal canal
  • They "hitch a ride" on CN XII as it descends and arches forward across the internal and external carotid arteries
  • As CN XII turns forward, these fibers leave it and descend as the superior root, running between the internal jugular vein and the internal/common carotid arteries
  • The superior root innervates:
    • Superior belly of omohyoid
    • Upper parts of sternohyoid
    • Upper parts of sternothyroid

2. Inferior Root (Descendens Cervicalis) - C2, C3 (± C4 per Fischer's)

  • Arises directly from the cervical plexus (C2 and C3)
  • Descends either medial or lateral to the internal jugular vein before turning medially to join the superior root
  • At the junction, it gives branches to innervate:
    • Inferior belly of omohyoid
    • Lower parts of sternohyoid
    • Lower parts of sternothyroid

Muscles Supplied (Summary)

MuscleInnervated by
SternohyoidAnsa cervicalis (C1, C2, C3)
SternothyroidAnsa cervicalis (C1, C2, C3)
Omohyoid (both bellies)Ansa cervicalis (C1, C2, C3)
ThyrohyoidNOT ansa cervicalis - directly by C1 fibers via CN XII (nerve to thyrohyoid)
Key point: Thyrohyoid is the exception - it is innervated independently by C1 fibers that remain on the hypoglossal nerve and never join the ansa loop. - Fischer's Mastery of Surgery

Position / Relations

  • The ansa cervicalis loop forms on the anterior surface of the carotid sheath (anterior to the common carotid artery)
  • It is closely related to the internal jugular vein - the inferior root can pass either medial or lateral to it
  • The superior root travels closely with (and is often visible during) carotid endarterectomy and neck dissection

Clinical Significance

Ansa Cervicalis - Recurrent Laryngeal Nerve (RLN) Transfer

The ansa cervicalis has an important reinnervation role in laryngeal surgery:
  • Used for nerve transfer to the RLN in cases of unilateral vocal fold paralysis
  • First described by Frazier et al. (1924); current technique reported in 1986
  • An end-to-end anastomosis of the proximal ansa cervicalis to the distal stump of the RLN restores muscle tone, bulk, and position to the hemilarynx
  • No active movement of the vocal fold is expected - the goal is bulk and tone restoration
  • Ansa cervicalis is the preferred donor nerve over the hypoglossal nerve because sacrifice of the ansa results in significantly lower morbidity (the infrahyoid muscles are not critical for phonation or swallowing) - Cummings Otolaryngology

Neck Dissection

The ansa cervicalis is at risk during neck dissections, particularly when clearing the carotid sheath region. Identifying and preserving it helps maintain infrahyoid muscle tone and avoids postoperative neck contour deformity.
Sources: Gray's Anatomy for Students | Fischer's Mastery of Surgery, 8th ed. | Cummings Otolaryngology Head and Neck Surgery
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