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.
Carotid arteries
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.
Nerves
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.
