Carotid sheath
The carotid sheath is an anatomical term for the fibrous connective tissue that surrounds the vascular compartment of the neck. It is part of the deep cervical fascia of the neck, below the superficial cervical fascia meaning the subcutaneous adipose tissue immediately beneath the skin. The deep cervical fascia of the neck includes four parts: Structure. The carotid sheath is located at the lateral boundary of the retropharyngeal space at the level of the oropharynx on each side of the neck and deep to the sternocleidomastoid muscle. It extends from the base of the skull to the first  rib and sternum, varying between C7 and T4. It merges with the axillary sheath when it reaches the subclavian vein. Contents. The four major structures contained in the carotid sheath are: In the upper part, the carotid sheath also contains the glossopharyngeal nerve (IX), the accessory nerve (XI), and the hypoglossal nerve (XII), which pierce the fascia of the carotid sheath. Relations. Posterior to common carotid artery is sympathetic trunk. The sympathetic trunk is separated from the transverse processes of C4 to C6 vertebrae by prevertebral muscles. Medially are trachea and oesophagus with recurrent laryngeal nerve in between them. At  higher levels, larynx, pharynx, recurrent laryngeal nerve, and thyroid gland are located medially to common carotid artery. Anterolaterally, the artery is covered by sternocleidomastoid and strap muscles. Above the cricoid cartilage, the artery is only covered by skin and fascia. Posterior to internal carotid artery are sympathetic trunk, transverse processes of C1 to C3 vertebrae, and prevertebral muscles. Medial to the internal carotid artery is the lateral wall of pharynx. Anterolaterally, it is covered by sternocleidomastoid muscle throughout its length. The upper part of internal carotid artery is separated from external carotid artery by styloid process and its muscles. At  the level of base of skull, internal carotid artery passes in front of internal jugular vein and enters the carotid canal. Meanwhile, the internal jugular vein and the vagus nerve enter the jugular foramen. The ansa cervicalis is embedded in the anterior wall of sheath. It is formed by "descendens hypoglossi" (C1) and "descendens cervicalis" (C2-C3). The cervical part of the sympathetic trunk is embedded in prevertebral fascia immediately posterior to the sheath. Layers. The three major fascial layers in the neck contribute to the carotid sheath: the investing fascia, the pretracheal fascia, and the prevertebral fascia. The carotid  sheath has limited loose connective tissue. Clinical significance. The carotid sheath may act as a conduit for infections, although this is rare due to the limited connective tissue.