Carotid Sheath Anatomy

• The carotid sheath's lower part contains the common carotid artery, internal jugular vein, vagus nerve, and ansa cervicalis.
• It allows movement over the prevertebral fascia posteriorly.
• It connects to the fascia on the deep surface of sternocleidomastoid anteriorly via fibrous tissue.
• Pus from around the pharynx passes behind the sheath, potentially pointing in the posterior triangle.
• The sheath is thin over the internal jugular vein, allowing vein dilation.
• The 'dead space' around the internal jugular vein houses anterior and posterior groups of inferior deep cervical lymph nodes, the latter extending into the posterior triangle as the supraclavicular group.
• The carotid sheath extends from the sternoclavicular joint to the point where the common carotid artery splits into the external and internal carotids.
• Carotid artery bifurcation typically occurs at the upper border of the thyroid cartilage (C4 vertebra) but can be near the tip of the greater horn of the hyoid bone (C3 vertebra).
• The terminal portion of the common carotid artery often dilates into the carotid sinus.
• The common carotid artery originates from the arch of the aorta on the left, anterior to the subclavian artery up to the sternoclavicular joint.
• On the right, the brachiocephalic trunk bifurcates behind the sternoclavicular joint into the common carotid and subclavian arteries.
• The common carotid artery has no branches proximal to its bifurcation.
• Within the carotid sheath, the artery is medial, the internal jugular vein is lateral, and the vagus nerve lies deep between the two.
• The sympathetic trunk is behind the artery and outside the sheath.
• The infrahyoid muscles and sternocleidomastoid overlap the sheath superficially.
• The carotid pulse can be felt by pressing the common carotid artery against the transverse process tubercle of the C6 vertebra.
• The surface marking of the artery is a line from the sternoclavicular joint to the bifurcation near the greater horn of the hyoid bone.
• Surgical exposure involves retracting the sternocleidomastoid and incising the carotid sheath.

Ansa Cervicalis

• The ansa cervicalis is located on the front of the internal jugular vein.
• It innervates the infrahyoid muscles.
• This nerve is within the carotid sheath's anterior wall.
• It is formed by the union of superior and inferior roots.

Superior Root

• The superior root, also known as descendens hypoglossi, branches off the hypoglossal nerve.
• It originates where the nerve loops below the posterior belly of digastric, near the occipital, external carotid, and lingual arteries.
• It runs down on the front of the internal jugular vein.
• This root contains only C1 fibers.

Inferior Root

• The inferior root, or descendens cervicalis, is formed by branches from the C2 and C3 anterior rami in the cervical plexus.
• The single nerve spirals from behind around the internal jugular vein.
• It joins the superior root at a variable level.
• It may form a wide loop over the lower part of the vein, with branches arising from the loop.
• Alternatively, the two nerves may join high up in a Y-shape.
• The branches from these configurations distribute segmentally to the infrahyoid muscles.
• The distribution is C1, C2, and C3 from top to bottom to sternohyoid, sternothyroid, and omohyoid muscles.

Hypoglossal Nerve

• In the submandibular region, it emerges between the internal carotid artery and internal jugular vein, deep to the posterior belly of digastric.
• It hooks around the occipital artery and curves forward over the external carotid and the loop of the lingual artery.
• In dissections, look for it below digastric, behind its tendon, and above the tip of the greater horn of the hyoid bone.
• At this point, it gives off the superior root of the ansa cervicalis.
• It passes forward on the surface of hyoglossus, separating it from the lingual artery.
• Then, it gives off the branch to thyrohyoid (C1 fibers) and runs deep to mylohyoid to enter the mouth.

The carotid sheath contains major structures like the common carotid artery, internal jugular vein, and vagus nerve. Its connections and thinness allow for vein dilation and potential pus passage. Understanding its anatomy is crucial for medical professionals.