The neck extends from the inferior border of the mandible to the superior border of the clavicle. It contains neurovascular structures and part of the aerodigestive tract. A thorough understanding of these anatomic relationships is essential when undertaking surgery or addressing injury in this area, because these vital structures occur in close proximity within the narrow confines of the neck.
Cervical Fascial Planes
The structures of the neck are invested by layers of fascia, which are essential to understanding the anatomy of the neck. The superficial cervical fascia surrounds the platysma and muscles of facial expression. Deeper structures are enveloped by the deep cervical fascia, which is subdivided into three layers: superficial, middle, and deep. The most superficial layer of the deep cervical fascia is also referred to as the investing fascia; it surrounds the entire neck, attaching at the sternum and clavicle anteriorly and the superior nuchal line of the skull and cervical vertebrae posteriorly. The investing fascia splits to completely enclose the sternocleidomastoid and trapezius muscles as well as the parotid and submandibular glands within it. The middle layer of the deep cervical fascia is also known as the pretracheal fascia. It surrounds the pharynx, larynx, trachea, esophagus, thyroid gland, and strap muscles of the neck. The deep layer of the deep cervical fascia consists of the prevertebral fascia and an alar layer. The prevertebral fascia envelops the cervical vertebrae and paraspinous muscles. The alar layer is superficial (anterior) to the prevertebral layer and separates it from the pretracheal fascia. The carotid sheath, which envelops the common carotid artery, internal jugular vein, and vagus nerve, is composed of contributions from all three deep cervical fascial layers.
The neck contains several muscles that function to rotate, extend, and flex the neck, as well as to elevate and depress the larynx and hyoid bone when swallowing or phonating.
The platysma is a broad, thin muscle that overlies the anterolateral neck. It is the most superficial muscle in the neck and covers portions of the sternocleidomastoid and trapezius. The platysma originates from fascia surrounding the clavicle and extends superiorly to attach to the mandible and around the mouth.
The sternocleidomastoid (SCM) muscle runs obliquely from its dual inferior attachments to the sternum (medial head) and clavicle (lateral head) to its insertion on the mastoid process superiorly. It is enveloped by the superficial layer of the deep cervical fascia. Contraction of the SCM flexes and rotates the head toward the contralateral side. In addition, the SCM serves as an accessory muscle of respiration. Lying posterior to the SCM, the trapezius is a large triangular muscle with fibers originating from the occiput and spinous processes of the cervical and thoracic vertebrae; both muscles are supplied by the spinal accessory nerve. Trapezius has multiple insertion points on the clavicle and scapula; functions include elevation and retraction of the shoulder, as well as scapular rotation.
The cervical strap muscles are located over the anterior neck and can be divided into the suprahyoid and infrahyoid muscles. The suprahyoid muscles include the mylohyoid, stylohyoid, geniohyoid, and digastric muscles, which elevate the hyoid. The infrahyoid muscles include the omohyoid, sternohyoid, sternothyroid, and thyrohyoid muscles, which depress the hyoid. Concurrent action of the suprahyoid and infrahyoid muscles serves to hold the hyoid in place while the mandible is depressed, causing the mouth to open.
There are three pairs of scalene muscles (anterior, middle, and posterior) deep to the prevertebral fascia in the neck, and deep to the SCM and trapezius. The scalene muscles originate from the transverse process of the cervical vertebrae to insert on the first rib. Their functions include lateral flexion of the neck and serving as accessory muscles of respiration (due to their ability to elevate the rib).
The common carotid arteries branch off the aorta on the left and the brachiocephalic artery on the right. They are deep to the SCM and contained (along with the internal jugular vein and vagus nerve) within the carotid sheath, which is part of the deep cervical fascia. At the level of the thyroid cartilage, the common carotids bifurcate into the internal and external carotid arteries. The internal carotid artery lies posterior and lateral to the external carotid artery; it enters the skull through the carotid canal without branching in the neck. In contrast, the external carotid artery gives off several branches in the neck, including the superior thyroid, ascending pharyngeal, posterior auricular, occipital, facial, and lingual arteries. The external carotid artery ends in two terminal branches in the parotid gland: the superficial temporal and maxillary arteries. Branches of the subclavian artery that supply or traverse the neck include the vertebral artery and thyrocervical trunk, which gives off the inferior thyroid and transverse cervical arteries.
The arteries of the neck are paired with corresponding veins, which provide drainage of the head and neck. Some of the larger of these veins include the retromandibular, external jugular, internal jugular, and inferior thyroid veins. The internal jugular vein receives drainage from the regions of the head and neck supplied by the internal carotid artery. It also receives drainage from the superior and middle thyroid veins. At the root of the neck, the internal jugular veins join with the subclavian veins to form the brachiocephalic veins, which then form the superior vena cava.
Major nerves in the neck include the spinal accessory, vagus, and phrenic nerves, as well as the sympathetic trunk. The spinal accessory nerve (SAN, cranial nerve XI) originates from C1-5 spinal nerve roots, which enter the skull through the foramen magnum. The SAN exits the skull via the jugular foramen and provides motor innervation to the SCM and trapezius muscles. In the neck, it runs lateral to the internal jugular vein and medial to the posterior belly of digastric and the stylohyoid. It then follows an oblique course as it enters the SCM, exits posteroinferiorly, and finally enters the trapezius. During neck dissection, the SAN can be identified approximately 1 cm superior to the point where the great auricular nerve crosses the posterior edge of the SCM.
The vagus nerve (cranial nerve X) exits the skull via the jugular foramen . In the neck, it travels within the carotid sheath, giving off branches to the larynx and pharynx. The recurrent laryngeal nerve (RLN) is one such branch that provides motor innervation to the laryngeal muscles (except cricothyroid, which is innervated by the external branch of the superior laryngeal nerve). In the neck, the RLN runs in the tracheoesophageal groove. On the right, the RLN loops around the right subclavian artery and changes course to run back superiorly and innervate the larynx. On the left, the RLN descends into the thorax and loops around the aortic arch to return to the larynx. The superior laryngeal nerve (SLN) is another branch of the vagus nerve. It has two branches, internal and external, which provide sensory and motor innervation to the larynx, respectively. Both the RLN and SLN run in close proximity to the thyroid gland; therefore they must be identified and preserved during thyroid surgery to avoid compromising laryngeal function.
The hypoglossal nerve (CN XII) exits the posterior cranial base through the hypoglossal canal and descends in the neck, traveling in a plane between the internal carotid artery and the internal jugular vein, outside of the carotid sheath. Inferior to the mandible, the hypoglossal nerve passes deep and medial to the posterior belly of the digastric muscle and into the submandibular triangle. It then passes inferior to the deep portion of the submandibular gland and runs along the lateral aspect of the genioglossus muscle before ending in terminal branches that supply the tongue.
The marginal mandibular branch of the facial nerve (CN VII) supplies the depressor muscles of the mouth and the mentalis muscle. After branching off of the facial nerve trunk at the pes anserinus, the marginal mandibular branch descends toward the angle of the mandible, where it curves to run either along or just below the inferior border of the mandibular body. The nerve runs deep to the platysma. As it nears the corner of the mouth, the nerve becomes more superficial and gives off terminal branches to supply the lower lip and chin. Injury to the marginal mandibular nerve results in paresis of the ipsilateral corner of the mouth, which can be observed as an asymmetric smile on examination.
The phrenic nerve arises from C3-5 spinal nerves and lies deep to the prevertebral fascia as it descends in the neck anterior to the anterior scalene muscle and lateral to the carotid sheath. It provides motor innervation to the diaphragm, as well as sensation to the thoracic viscera (such as the pericardium and pleura). The sympathetic trunk carries autonomic information to the cervical spinal nerves, thyroid gland, pharynx, and carotid arteries. It lies posteromedial to the carotid sheath and anterior to the longus muscles of the neck. The cervical portion of the sympathetic trunk contains three sympathetic ganglia: the inferior, middle, and superior cervical ganglia. Additional sensory and motor innervation to the neck is via branches of the cervical spinal nerves. The cervical plexus is formed by C2-C4; its cutaneous branches include the transverse cervical, supraclavicular, great auricular, and lesser occipital nerves. The great auricular nerve (GAN) supplies the skin of the ear, upper neck, and face overlying the parotid. It traverses the sternocleidomastoid and may be transected during parotid surgery or neck dissection, resulting in paresthesia. The ansa cervicalis is a nerve loop formed by branches of the C1-3 spinal nerves; it is located superficial to the internal jugular vein and provides motor innervation to the sternohyoid, sternothyroid, and omohyoid muscles.
In head and neck oncology, the neck is divided into levels that correlate with lymph node groups receiving drainage from specific areas of the head and upper aerodigestive tract. Level I includes the submandibular and submental lymph nodes, as well as the submandibular gland. It is bounded by the body of the mandible, the posterior belly of the ipsilateral digastric muscle, and the anterior belly of the contralateral digastric muscle. Levels II through IV contain the lymph nodes of the jugular chain, from superior to inferior. Laterally, these three levels extend to the posterior border of the SCM, while, medially, they are bounded by the sternohyoid muscle. Level II contains the upper jugular lymph nodes, from the level of the skull base superiorly to the level of the carotid bifurcation or inferior border of the hyoid inferiorly. Level III contains the middle jugular lymph nodes, extending from the level of the carotid bifurcation down to the inferior border of the hyoid bone or the intersection of the omohyoid muscle with the internal jugular vein. Level IV contains the lower jugular lymph nodes and extends inferiorly to the clavicle. Level V contains the lymph nodes in the posterior triangle of the neck; these include the lymph nodes that accompany the spinal accessory nerve and transverse cervical artery, as well as the supraclavicular nodes. Level V is bounded medially by the posterior border of the SCM, laterally by the anterior border of the trapezius muscle, and inferiorly by the clavicle. Level VI contains the midline lymph nodes in the anterior compartment of the neck, including the paratracheal and perithyroid nodes. These nodes are the primary site of drainage for the midline viscera, such as the larynx, thyroid gland, and cervical esophagus. Level VI extends from the inferior border of the hyoid bone down to the suprasternal notch. Laterally, it is bounded by the medial border of the carotid sheath. Level VII contains the superior mediastinal lymph nodes, although it is located in the thorax rather than the neck.
The arrangement of fascial layers in the neck creates several potential spaces that can harbor infection. The prevertebral space is the space between the vertebrae and the prevertebral fascia. It extends along the entire length of the vertebral column, from the base of the skull to the coccyx. The danger space is the space between the prevertebral and alar layers of the deep layer of deep cervical fascia. It extends from the base of skull down to the diaphragm. It is referred to as the danger space because it provides a pathway for spread of infection from the head and neck into the mediastinum, where mediastinitis and sepsis can occur. The retropharyngeal space is the space between the alar fascia and the middle layer of deep cervical fascia surrounding the pharynx and esophagus. It extends from the base of skull to the level of the tracheal bifurcation at T4. Because this space extends into the superior mediastinum, mediastinitis is also a potential complication of retropharyngeal infection. The prevertebral, danger, and retropharyngeal spaces are collectively referred to as the postvisceral spaces, due to their location posterior to the middle layer of the deep cervical fascia. The pretracheal space lies anterior to the postvisceral spaces, between the retropharyngeal space and the investing fascia (superficial layer of deep cervical fascia). It is the space in which the pharynx and larynx are contained. It, too, extends into the superior mediastinum. The parapharyngeal space is a pyramidal space that extends from the base of skull down to the level of the hyoid. It lies lateral to the pharynx (bounded medially by the superior constrictor muscle). The parapharyngeal space is divided into a prestyloid compartment (anterior to the styloid process) and a poststyloid compartment. The prestyloid compartment contains the internal maxillary artery and branches of the mandibular branch of the trigeminal nerve (cranial nerve V3). The poststyloid compartment contains the carotid artery, internal jugular vein, sympathetic chain, and multiple cranial nerves (IX, X, XI, and XII).