NEB Exam Q: What is Carotid Sheath? Name its contents? | Anatomy

1. Carotid sheath

1. Formed by local deep fascia, including superficial layer of deep cervical fascia, infrahyoid fascia, visceral fascia, and prevertebral fascia
2. Extends from base of skull surrounding jugular foramen and carotid canal to root of neck
3. Contains internal jugular vein laterally, common and internal carotid arteries medially, and vagus nerve behind and between the vessel

A. Carotid Sheath and its Contents. B. Neck, Computed Tomography Image, Transverse…

2. Contents of Carotid Sheath

1. Carotid arteries
   1. Common carotid
      1. Right: from brachiocephalic trunk
      2. Left: arises as second branch of aortic arch
      3. Both: course superiorly in carotid sheath with internal jugular vein laterally and vagus nerve posterolaterally
      4. 2 terminal branches at superior border of thyroid cartilage: internal and external carotid arteries
      5. Carotid body
         1. Small, flattened, ovoid nodule of specialized cells (0.5 cm) in outer connective tissue of bifurcation of common carotid artery
         2. Functions as chemoreceptor that monitors oxygen content of blood before it reaches brain, such that a decrease in partial pressure of oxygen, which can occur at high altitudes or in pulmonary disease, activates aortic and carotid chemoreceptors, increasing alveolar ventilation; also sensitive to changes in blood acidity (relative amount of carbon dioxide)
         3. Innervated by carotid sinus nerve (Hering’s nerve) of glossopharyngeal (CN IX), vagus (CN X), and sympathetics
   2. Internal carotid artery
      1. Continues in carotid sheath to enter carotid canal in base of skull
      2. No branches in neck; supplies brain and orbit
      3. Carotid sinus
         1. Slight dilation of terminal portion of common carotid and proximal part of internal carotid artery
         2. Functions as baroreceptor important in regulating arterial blood pressure
         3. Innervated mainly by carotid sinus nerve from glossopharyngeal (CN IX); also supplied by vagus (CN X) and sympathetic fibers
   3. External carotid artery
      1. Leaves carotid sheath by passing anterosuperiorly within carotid triangle
      2. 8 branches: 5 below posterior belly of digastric within carotid triangle, 3 above
         1. Superior thyroid artery: 1st branch; passes anteromedially to supply infrahyoid muscles, thyroid gland, larynx, and laryngopharynx; gives off superior laryngeal artery that passes through thyrohyoid membrane with internal branch of superior laryngeal nerve
         2. Ascending pharyngeal artery: often 2nd branch; arises from medial surface to pass superiorly to supply oro- and nasopharynx, prevertebral muscles, palatine branch to soft palate
         3. Lingual artery: 2nd anteromedially directed branch; passes above greater horn of hyoid to supply tongue, suprahyoid region, sublingual gland, and palatine tonsils
         4. Facial artery: 3rd anteromedial branch; passes deep to posterior belly of digastric and stylohyoid muscles and superficial portion of submandibular gland to reach mandibular margin and enter face anterior to masseter insertion; supplies submandibular gland and face up to orbit
         5. Occipital artery: arises posteriorly near lower border of posterior belly of digastric muscle, crosses hypoglossal nerve superiorly; passes posterosuperiorly roughly medial to posterior belly of digastric to eventually distribute on head posteriorly; supplies SCM muscle, meningeal branches, and scalp posteriorly
         6. Posterior auricular artery: posteriorly directed branch arising between posterior belly of digastric and stylohyoid muscles; ends in occipital and auricular branches; branches to parotid gland; enters stylomastoid foramen to medial wall of middle ear, mastoid air cells, stapedius muscle, and middle ear bones
         7. Maxillary artery: anteromedially directed terminal branch; supplies muscles of mastication, meninges, midface including upper teeth, nasal cavity, and hard and soft palates
         8. Superficial temporal artery: superiorly directed terminal branch; supplies scalp laterally

Figure| Carotid Arteries |​

Internal jugular vein

1. Origin: in jugular fossa, as continuation of sigmoid sinus
2. Course: in carotid sheath, first with internal and then common carotid artery
3. Termination: joins subclavian vein just lateral to sternoclavicular joint to form brachiocephalic vein
   1. Superior bulb: dilation at its origin
   2. Inferior bulb: dilation 1 in above termination, below pair of valves
4. Tributaries (variable)
   1. At origin: inferior petrosal sinus and a meningeal vein
   2. Pharyngeal plexus, near angle of jaw
   3. Common facial (formed from anterior division of retromandibular and facial vein) enters at level of hyoid bone
   4. Lingual, from tongue, may enter with or just below common facial, drains tongue and sublingual area
   5. Superior thyroid, from upper thyroid gland, enters with or just below common facial vein
   6. Middle thyroid vein, from the lateral portion of gland

Figure: | Internal Jugular Vein and Tributaries |​

1. Vagus nerve (CN X) in neck
   1. Exits skull at jugular foramen to enter carotid sheath and lie between and behind internal jugular vein and internal and common carotid arteries
   2. Gives off superior laryngeal nerve and superior cardiac branch below base of skull
   3. Gives multiple small branches to pharynx
   4. Gives off inferior cardiac branch in root of neck
   5. Passes anterior to right subclavian artery or lateral to left common carotid to enter thorax
2. Other contents of carotid sheath
   1. Superior laryngeal nerve (from CN X): passes medial to internal carotid artery to leave sheath and pass to larynx
   2. Accessory nerve (CN XI): leaves sheath near base of skull to pass laterally toward SCM muscle
   3. Glossopharyngeal nerve (CN IX): leaves sheath to pass inferoanteriorly onto posterior wall of pharynx
   4. Hypoglossal nerve (CN XII): passes inferoanteriorly between internal jugular vein and internal carotid artery to reach tongue
   5. Deep cervical lymph nodes: lie along internal jugular vein and between it and common carotid artery; not always easily visible, being small and scattered, but are important in carcinoma of mouth, larynx, or other head and neck structures.

How to remember CAROTID SHEATH ?​

Carotid sheath mnemonic

Carotid sheath contents mnemonic

I See 10 CC‘s in the IV:
I See (I.C.) = Internal Carotid artery
10 = CN 10 (Vagus nerve)
CC = Common Carotid artery
IV = Internal Jugular Vein​