The pretracheal layer consists of a collection of fascias that surround the trachea, esophagus, and thyroid gland (Fig. 8.162). Anteriorly, it consists of a pretracheal fascia that crosses the neck and encloses the infrahyoid muscles, and covers the trachea and the thyroid gland. The pretracheal fascia begins superiorly at the hyoid bone and ends inferiorly in the upper thoracic cavity. Laterally, this fascia encloses the thyroid gland and more posteriorly is continuous with fascia that surrounds the esophagus.
Posterior to the pharynx, the pretracheal layer is referred to as the buccopharyngeal fascia and separates the pharynx from the prevertebral layer (Fig. 8.163).
The buccopharyngeal fascia begins superiorly at the base of the skull and merges with fascia covering the esophagus that then continues inferiorly into the thoracic cavity.
Each carotid sheath is a column of fascia that surrounds the common carotid artery, the internal carotid artery, the internal jugular vein, and the vagus nerve as these structures pass through the neck (Fig. 8.162).
It receives contributions from the investing, prevertebral, and pretracheal layers, though the extent of each component’s contribution varies.
The arrangement of the various layers of cervical fascia organizes the neck into four longitudinal compartments (Fig. 8.160):
The first compartment is the largest, includes the other three, and consists of the area surrounded by the investing layer.
The second compartment consists of the vertebral column and the deep muscles associated with this structure, and is the area contained within the prevertebral layer.
The third compartment (the visceral compartment) contains the pharynx, the trachea, the esophagus, and the thyroid gland, which are surrounded by the pretracheal layer.
Finally, there is a compartment (the carotid sheath) consisting of the neurovascular structures that pass from the base of the skull to the thoracic cavity, and the sheath enclosing these structures receives contributions from the other cervical fascias.
Between the fascial layers in the neck are spaces that may provide a conduit for the spread of infections from the neck to the mediastinum.
(Fig. 8.163):
The first is the pretracheal space between the investing layer of cervical fascia covering the posterior surface of the infrahyoid muscles and the pretracheal fascia (covering the anterior surface of the trachea and the thyroid gland), which passes between the neck and the anterior part of the superior mediastinum.
The second is the retropharyngeal space between the buccopharyngeal fascia (on the posterior surface of the pharynx and esophagus) and the prevertebral fascia (on the anterior surface of the transverse processes and bodies of the cervical vertebrae), which extends from the base of the skull to the upper part of the posterior mediastinum.
The third space is within the prevertebral layer covering the anterior surface of the transverse processes and bodies of the cervical vertebrae. This layer splits into two laminae to create a fascial space that begins at the base of the skull and extends through the posterior mediastinum to the diaphragm.
The external jugular and anterior jugular veins are the primary venous channels for superficial venous drainage of the neck (Fig. 8.164).
The external jugular vein is formed posterior to the angle of the mandible as the posterior auricular vein and the retromandibular vein join:
The posterior auricular vein drains the scalp behind and above the ear.