Contents of the Carotid Sheath
The carotid sheaths form part of the fascial layers of the neck (pp 324, 325) and extend from the base of the skull into the superior mediastinum. Each sheath contains the common, the internal and part of the external carotid arteries, the internal jugular vein and parts of the glossopharyngeal (IX), vagus (X) and accessory (XI) nerves.
The common carotid artery arises on the left from the aortic arch and on the right from the brachiocephalic artery, then ascends within its sheath into the neck. At the upper border of the thyroid cartilage it divides into internal and external carotid arteries (Fig. 7.17). At its origin the internal carotid artery is dilated to form the carotid sinus. The vessel ascends within the carotid sheath without branching and, in company with its sympathetic nerve plexus, enters the carotid canal. In the cranial cavity the internal carotid artery terminates as the anterior and middle cerebral arteries (p. 377), having given rise to the ophthalmic artery, which supplies the contents of the orbit.
The external carotid artery leaves the carotid sheath, inclines superolaterally and enters the deepest part of the parotid gland, where it divides into terminal branches at the level of the neck of the mandible. Sepa- rating the external and internal carotid arteries are the styloid process, stylopha- ryngeus, the glossopharyngeal (IX) nerve and the pharyngeal branch of the vagus (X) nerve.
The external carotid artery usually has eight branches (Fig. 7.17), supplying much of the extracranial portion of the head and upper part of the neck. The ascending pharyngeal artery arises on the medial aspect of the external carotid artery and ascends on the lateral surface of the pharynx. The next branch, the superior thyroid artery, inclines downwards and forwards from the anterior surface of the external carotid to supply the thyroid gland and the larynx. The lingual artery passes forwards from the anterior surface of the external carotid artery into the base of the tongue (p. 351). The facial artery, another anterior branch, supplies the tonsil and descends under the body of the mandible, grooving the submandibular gland and emerging from beneath the mandible at the anterior border of masseter to supply the face (Fig. 7.35).
Arising from the posterior surface of the external carotid artery, the occipital artery passes upwards and backwards deep to sternocleidomastoid (Figs 7.17 & 7.24). The vessel crosses the apex of the posterior triangle of the neck to supply the posterior part of the scalp. The posterior auricular artery arises below the pinna and passes upwards and backwards to supply the pinna and the scalp.
The terminal branches of the external carotid artery, the superficial temporal and maxillary arteries, arise within the parotid gland. The former supplies the lateral part of the scalp, while the maxillary artery supplies the infratemporal fossa, pterygopalatine fossa and lateral wall of the nose. Postganglionic sympathetic nerve fibres accompany the external carotid artery and its branches.
The internal jugular vein (Figs 7.16 & 7.17) is formed below the jugular foramen after the union of the sigmoid and inferior petrosal dural venous sinuses. The vein descends through the neck within the carotid sheath and receives the facial, lingual, pharyngeal, and superior and middle thyroid veins. The internal jugular vein also communicates with the external jugular system via the anterior branch of the retromandibular vein. It terminates by uniting with the subclavian vein to form the brachiocephalic vein.
Within the carotid sheath lie the glossopharyngeal (IX), vagus (X) and accessory (XI) nerves, which leave the skull via the jugular foramen. The glossopharyngeal nerve passes inferiorly, leaves the sheath and winds around the posterolateral surface of stylopharyngeus to enter the posterior third of the tongue. The nerve supplies stylopharyngeus and gives a sensory branch, which innervates the carotid sinus (p. 352).
The vagus (X) nerve (Fig. 7.17) bears two sensory ganglia, one in the jugular foramen and one below the base of the skull, between which the nerve receives the cranial part of the accessory nerve. The vagus nerve descends in the posterior part of the carotid sheath between the carotid artery and internal jugular vein and gives rise to pharyngeal, superior laryngeal and cardiac branches before traversing the superior thoracic aperture. The pharyngeal branch passes forwards between the internal and external carotid arteries to the outer surface of the pharynx, contributing to the pharyngeal plexus. The superior laryngeal nerve accompanies the superior thyroid artery and vein supplying the larynx. The cardiac branches of the vagus join those from the sympathetic trunk and descend into the thorax. The right vagus gives a recurrent laryngeal branch in the neck which curves around the right subclavian artery and passes superiorly to reach the larynx (p. 331). The left recurrent laryngeal nerve arises from the left vagus nerve in the thorax and passes around the aortic arch before ascending into the neck.
The accessory (XI) nerve (Fig. 7.17) is formed by the fusion of cranial and spinal roots in the posterior cranial fossa. In the jugular foramen the nerve divides into a cranial part joining the vagus nerve and a spinal part which supplies sternocleidomastoid and trapezius.
Lymphatic vessels from structures in the head and neck accom- pany the vascular supply and drain into superficial or deep groups of lymph nodes.
Superficial lymph nodes
Several groups of superficial nodes form an incomplete ring around the lower part of the head. The submental nodes lie between the anterior bellies of the two digastric muscles, draining lymph from the tip of the tongue and the mental region. The submandibular group (Fig. 7.16) lies on the superficial surface of the submandibular gland and receives lymph from the submental nodes, from the remainder of the anterior two-thirds of the tongue, from the floor of the mouth (including the gums and teeth), and from the nose, face and anterior part of the scalp. The parotid nodes lie on or within the parotid gland, the mastoid group (Fig. 7.16) on the lateral surface of the mastoid process, and the occipital lymph nodes at the apex of the posterior triangle of the neck. These three groups drain the remainder of the face and scalp, including the external ear. Lymph from all the superficial nodes drains into the deep cervical nodes.
Deep cervical lymph nodes
The deep cervical or jugular lymph nodes (Fig. 7.16) form a chain in and around the carotid sheath. Two of the deep cervical nodes are of particular clinical significance. The jugulodigastric node receives lymph mainly from the tongue and tonsil and the jugulo-omohyoid node drains the tongue. From the deep cervical nodes lymph drains inferiorly, via the jugular lymphatic trunk, usually into the thoracic duct on the left and into the right lymphatic duct on the right. Removal of these nodes by careful surgical dissection can successfully treat locally metastatic cancer.