Neck Muscles

Neck Muscles

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The Role of Fasciae in the Neck

The numerous components of the neck, from the muscles to the neurovascular structures, are supported and compartmentalised by a series of fasciae. The superficial cervical fascia is found deep to the dermis. It comprises a thin layer of adipose tissue and superficial neurovascular structures, as well as the Platysma. The deep cervical fascia, described as a complex arrangement of dense connective tissue, accounts for the most superficial component, the investing fascia. This invests the deep structures in the neck and on its circumference meets and encases trapezius and sternocleidomastoid.

Deep to the investing fascia are the pretracheal fascia, the prevertebral fascia and the carotid sheaths. The pretracheal fascia is composed of two parts, the muscular component which surrounds the infrahyoid muscles, and a visceral part which encircles the trachea, oesophagus, and the thyroid gland. Additionally, the buccopharyngeal fascia which wraps the pharynx contributes to the posterior part of the pretracheal fascia.

The vertebral column and its associated muscles are surrounded by the prevertebral fascia, extending downward from the skull base to fuse with the endothoracic fascia medially and form the axillary sheath laterally. The axillary sheath houses the brachial plexus and axillary artery. Situated bilaterally between the edges of the prevertebral and pretracheal fasciae are the carotid sheaths, forming columns which harbour the common carotid artery, internal jugular vein, vagus nerve and lymph nodes as they course between the skull base and the mediastinum. The carotid sheaths are formed from contributions from the other components of the deep cervical fascia.

From a clinical perspective, the cervical fascia and the potential spaces they form are important for understanding the spread of infection through the neck. To protect the pharynx from potential pathogens, groups of mucosa-associated lymphatic tissue are in place. These include the pharyngeal tonsil, the palatine tonsils, the tubal tonsils, and the lingual tonsils. These structures are roughly arranged in a circle surrounding the oropharynx, known collectively as Waldeyer's ring. The lymph from the tonsils all drains via the deep cervical chains of lymph nodes.

Cervical Fascia anatomically

Anatomically, the cervicofacial fascia is composed of three layers, being superficial, intermediate, and deep. The superficial fascia is the most superficial layer and is composed of thin, loose connective tissue containing fat-laden adipocytes, and houses neurovascular structures. The intermediate fascia is composed of thick, fibrous connective tissue, while the deepest layer is the prevertebral fascia, which is a condensation of fibrous tissue that is attached to the transverse processes of the cervical vertebrae. This fascia wraps around the neck and its structures, including the vertebral column, the sternocleidomastoid, and the trapezius muscles.

Within the deep facial layers is the carotid sheath, a structure that houses the carotid artery, and is composed of densely packed connective tissue. It is bounded laterally by the prevertebral fascia, medially by the pretracheal fascia, and the anteriorly by the investing fascia. The carotid sheath further consists of two compartments, an anterior and a posterior, where the posterior compartment houses the vagus nerve, internal jugular vein, lymphatics, and the phrenic nerve.

Clinical Significance of Cervical Fascia

The cervical fascia is of clinical significance, as spread of infection through the neck may occur in this area. For example, during a thyroidectomy the pre-tracheal, pre-vertebral and carotid sheaths may become infected and result in a rare but lethal condition known as the 'lethal triangle'. To prevent this, surgeons must ensure complete hemostasis in these compartments, and take extra care when dissecting in the deep cervical fascia.

The cervical fascia is also important in understanding the spread of parasitic worms, which are often found in the histological sections of this region. It has been found that the parasite Trichurus caniche spread through the cervical fascia, and also has a preference for the carotid sheath. Additionally, the cervical fascia is also important in understanding the spread of maligntant tumours, due to its extensive network of lymphatics.

Protective Structures of the Pharynx

In addition to the cervical fascia, several protective structures of the pharynx are in place to protect the pharynx from potential pathogens. These are found in the form of lymphoid tissue arranged roughly in a circular formation encircling the oropharynx, better known as Waldeyer's ring. This formation includes the pharyngeal tonsil, palatine tonsils, tubal tonsils, and the lingual tonsils, all of which drain their lymph to the deep cervical lymph nodes.

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