Chapter 5 The Neck
The neck (cervix) is the relatively narrow and flexible portion between the head above and the chest below. It transports a food tube, an air tube, and a neurovascular bundle between these two areas. In addition, the upper limbs originate from the cervical region during their embryological development. Their blood and nerves course through the base of the neck as they pass to and from the upper limb.
The skeleton of the neck consists of a vertebral unit and a visceral unit (Figure 5-1). In addition, bones of the upper limb girdle, inferior aspect of the skull, and the superior aspect of the thoracic skeleton help provide attachment for muscles of the neck.
Seven cervical vertebrae compose the vertebral unit of the neck. Descriptions of typical vertebrae C3 to C6 and atypical vertebrae (C1-atlas, C2-axis, and C7) are presented in Chapter 2. The cervical vertebrae should be reviewed at this point.
The skeleton of the visceral unit consists of the hyoid bone, the larynx, and trachea (Figure 5-2). These structures are briefly described here; more detailed descriptions are presented subsequently in Chapter 7 in the section dealing with the pharynx and larynx.
The hyoid bone is a floating bone just below the mandible (see Figure 5-2). It is a U-shaped bone with the prongs, or horns, of the U facing posteriorly. The hyoid bone consists of three parts: (1) a rectangular body to which is appended; (2) a pair of lesser horns, which project upward and backward; and (3) a pair of greater horns, which project posteriorly.
Immediately below the hyoid bone is the thyroid cartilage. It consists of two flat plates, or laminae, joined anteriorly. From above, the thyroid cartilage appears V-shaped, with the point of the V facing anteriorly and the deficiency posteriorly.
Attached to the posterolateral aspects of the cartilage are two pairs of horns. Superior horns project upward; inferior horns project downward. The external aspect of the laminae exhibits a ridge called the oblique line. The anterior union of the two laminae project forward as the thyroid prominence, or the Adam’s apple.
Additional bones that belong to other regions are noted here because they provide attachments for various cervical structures. These are the base of the skull and the mandible (see Chapter 6), the scapula and the clavicle of the upper limb girdle (see Chapter 9), and the manubrium of the sternum and the first rib (see Chapter 3).
Anteriorly is a series of important landmarks. Below the chin (submental) and below the inferior border of the mandible (submandibular) is the soft-tissue, muscu- lar diaphragm that forms the floor of the mouth. This area funnels downward toward a succession of palpable midline landmarks. The lower border of the mandible and chin can be palpated easily. Immediately below the chin is the body of the U-shaped hyoid bone. Below the hyoid bone, in descending order, are the thyrohyoid space, a dip immediately below the hyoid bone; the thyroid prominence (Adam’s apple) of the thyroid cartilage; the cricothyroid space, an important landmark for performing an emergency cricothyrotomy; the arch of the cricoid cartilage; and the trachea, descending from the cricoid cartilage down to where it disappears behind the jugular notch of the manubrium. The clavicles extend laterally on either side of the manubrium of the sternum to form the anterior aspect of the base of the neck.
Immediately deep to the skin of the neck is a layer of superficial fascia. Deeper still is an intricate covering of deep cervical fascia. These fasciae compartmentalize the structures of the neck (Figure 5-4). Between the various compartments are spaces occupied by loose areolar tissue. These fascial compartments are potential routes through which infection can spread from one site to another.
The superficial fascia is a subcutaneous layer that contains a variable amount of fat, superficial lymph nodes, cutane- ous nerves and vessels that supply the overlying skin, and a thin muscle (the platysma).
Platysma muscle is a thin, wide sheet of muscle that covers the anterior and lateral aspects of the neck (Figure 5-5). It is a superficial muscle related to the superficial muscles of facial expression, and therefore it lies within the superficial fascia of the neck. It originates from pectoral fascia below the clavicle and sweeps upward to the inferior border of the mandible. The more lateral fibers continue superiorly and medially to blend into the risorius muscle, which inserts into the angle of the mouth. The medial fibers decussate as they approach and insert into the inferior border of the mandible.
The platysma acts to tense the skin of the neck if the mandible is stabilized by the muscles of mastication. The platysma also purportedly helps depress the mandible. Its motor supply is from cervical branches of the facial nerve (cranial nerve VII), the cranial nerve that also supplies the muscles of facial expression.
Below the superficial fascial layer is a thin sheet of deep cervical fascia that wraps around the entire circumference of the neck—like a collar. This fibrous collar has superior attachments to the skull and inferior attachments to the sternum and pectoral girdle. As it encircles the neck, it splits to pass around and form the sheath of two large muscles, the sternocleidomastoid and the trapezius. The detailed attachments are rather complicated and are presented for reference purposes.*
* Superiorly the fascia attaches to the inferior border of the mandible, inferior border of the body of the hyoid bone, angle of the mandible, the inferior border of the zygomatic arch, and the mastoid and styloid processes. Because the fascia splits to enclose the sternocleidomastoid and trapezius muscles, it shares their attachment to the mastoid process, superior nuchal line, and external occipital protuberance of the skull. At the inferior aspect of the skull, the deep investing fascia also splits to pass around and help form the fibrous capsules of the parotid and submandibular glands.
Inferiorly the deep investing fascia attaches to the manubrium of the sternum, clavicles, and spines of the scapula along with the sternocleidomastoid and trapezius muscles, which it ensheathes. As it descends to the manubrium, it divides into two sheets: an anterior one attaching to the anterosuperior aspect of the manubrium and a posterior one attaching to the posterosuperior aspect of the manubrium. This creates a space, the suprasternal space (of Burns), that contains some areolar tissue, fat, and lymph nodes. It also contains some portions of the inferior thyroid veins and the anterior jugular venous arch.
Posteriorly the deep investing fascia gains attachment to the ligamentum nuchae, a membranous extension of the cervical spines.
The visceral fascia lies deep to the deep investing fascia and forms a sheath around the visceral unit of the neck. Contained within the visceral unit are the pharynx, which continues below vertebral level C6 as the esophagus; the larynx, which continues below vertebral level C6 as the trachea; and the thyroid gland, which is found lateral to the trachea and larynx as two lateral lobes joined by an isthmus across the midline. Visceral fascia that surrounds the pharynx is called buccopharyngeal fascia; the visceral fascia surrounding the trachea and esophagus is referred to as the pretracheal fascia, despite the fact that it surrounds the visceral unit.
The prevertebral fascia surrounds the cervical vertebral unit. The vertebral unit includes the following components: the seven cervical vertebrae, the cervical portion of the spinal cord and eight pairs of spinal nerves (C1 to C8), anterior vertebral muscles that flex the neck, and posterior vertebral muscles that extend the neck. The prevertebral fascia extends laterally on either side to surround the brachial plexus and subclavian vessels as they pass from the neck to the axilla. This covering is called the axillary sheath.
Superiorly the prevertebral fascia extends from the base of the skull, forming a sheath around the vertebral column and its musculature as they descend through the neck. Inferiorly the posterior portion of the sheath blends with the investing fascia of the musculature of the back, and the anterior portion blends with the anterior longitudinal ligament of the thoracic vertebrae.
The alar fascia is formed by a division of the anterior component of the prevertebral fascia to form two potential spaces between the posterior vertebral unit and the anterior visceral unit. The alar fascia binds to the transverse processes on either side to limit the space laterally.
Lateral to the visceral unit are bilateral tubes of fascia extending from the base of the skull to the thoracic inlet. Contained within the carotid sheaths are the common carotid artery, the internal jugular vein, and the vagus nerve (cranial nerve X).
Massive infections of dental origin can break through into the retropharyngeal and alar spaces and track down to the mediastinum below, resulting in infection and inflammation of the mediastinum (mediastinitis). A more detailed description of the spread of dental infection is presented in Figure 11-32.
Emphysema is a pathological condition in which air or gas is abnormally present within or between tissues. Inappropriate oral surgery procedures can introduce air under pressure into the fascial issue planes. If it occurs near the surface, a characteristic “crackling” is produced on palpation. If the air is forced into the retropharyngeal or alar fascial space, it can rapidly track to the mediastinum. This condition is described in Figure 11-32.
Superiorly the carotid sheath attaches to the base of the skull around the carotid canal, which transmits the internal carotid artery, and the jugular foramen, which transmits the internal jugular vein and the vagus nerve. The carotid sheath blends anteriorly with the visceral fascia and posteriorly with the prevertebral fascia as it descends in the neck; its inferior limits are the visceral and prevertebral fascia.
Loose areolar connective tissue fills the spaces between the various layers of deep cervical fascia. They are potential spaces and become actual spaces only when invaded and displaced by infective material (pus) or occasionally by air (surgical emphysema). There are two important fascial spaces to consider.
The retropharyngeal space is a potential space between the visceral unit anteriorly and the vertebral unit posteriorly. It extends from the base of the skull down to the superior mediastinum. It is packed with loose connective tissue that allows a degree of up-and-down movement between the visceral and vertebral units during swallowing.
To facilitate the study of a seemingly complicated area, the neck is divided into two major areas, or triangles, by the sternocleidomastoid muscle (Figure 5-6). The area anterior to this muscle and below the inferior border of the mandible is the anterior triangle of the neck. The area posterior to the sternocleidomastoid muscle is the posterior triangle of the neck, which is limited posteriorly by a second large muscle, the trapezius.
The sternocleidomastoid and trapezius muscles developed from a single muscular sheet during prenatal development and therefore share the same nerve supply. During development a cleft develops between them, and they separate to form the borders of the posterior triangle of the neck.
The sternocleidomastoid muscles, acting bilaterally, flex the neck. Singly they act to flex the head laterally and rotate the head to the opposite side. During forced inspiration the sternocleidomastoid muscles pull upward on the sternum to further increase the intrathoracic volume.
The nerve supply to the sternocleidomastoid muscle is primarily from the spinal accessory nerve (cranial nerve XI). In addition, motor and proprioceptive fibers arise from anterior rami (AR) of spinal nerves C2 and C3.
Congenital torticollis results from damage to the sternocleidomastoid muscle following a difficult delivery. Damaged muscle fibers and a hematoma (localized hemorrhage producing a swelling) develop into scar tissue that causes spasm. The neck is pulled to the ipsilateral side, and the face is turned to the contralateral side.
The sternocleidomastoid and trapezius muscles divide the neck into two major areas, or triangles. Other key muscles further subdivide these triangles into smaller component triangles: (1) the anterior triangle of the neck and its component muscular, carotid, submandibular, and submental triangles and (2) the posterior triangle of the neck and its component occipital and subclavian triangles.
The anterior triangle occupies the anterior portion of the neck as an inverted triangle, its base consisting of the inferior border of the mandible and its apex directed downward toward the manubrium of the sternum (Figure 5-7). Like the posterior triangle the anterior triangle has depth and should be considered a region. Therefore, in addition to three boundaries, it has a roof, a floor, and several contents.
The anterior boundary is the midline of the neck (i.e., straight line running from the base of the chin above to the jugular notch of the sternum below). The posterior boundary is formed by the anterior border of the sternocleidomastoid muscle. The superior border is the bony inferior border of the mandible. For ease of description the anterior triangle of the neck is further divided into smaller component triangles (see Figure 5-6).
The muscular triangle occupies the anterior aspect of the neck below the hyoid bone. It is bounded by the superior belly of the omohyoid muscle, the sternocleidomastoid muscle, and the midline of the neck. It contains the infrahyoid strap muscles of the neck.
The carotid triangle is bounded by the superior belly of the omohyoid muscle, the posterior belly of the digastric muscle, and the sternocleidomastoid muscle. It contains the common carotid artery and its branches; the internal jugular vein and its tributaries; cranial nerves X, XI, and XII; and several branches of the cervical plexus.
The boundaries of the submandibular triangle are the inferior border of the mandible and the upper borders of the posterior and anterior bellies of the digastric muscle. This region contains the submandibular gland, submandibular lymph nodes, the lingual and facial arteries, cranial nerve XII, and the nerve to the mylohyoid muscle.
The detailed venous drainage of the face is described with the study of the face in Chapter 7. To summarize, the anterior portion of the face is drained by the facial vein, and the posterior portion of the face is drained by the retromandibular vein. Both veins leave the face and drain inferiorly to the neck. The retromandibular vein at the angle of the jaw divides into anterior and posterior divisions (Figure 5-8).
The posterior division of the retromandibular vein unites with the posterior auricular vein to form the external jugular vein just below the lobe of the ear. The external jugular vein passes obliquely downward over the sternocleidomastoid muscle to enter the posterior triangle.
The anterior jugular vein originates in the submental region, drains the anterior aspect of the neck, and descends on either side of the midline to a point just above the jugular notch of the manubrium. Here it dives deep to the origin of the sternocleidomastoid muscle, emerging in the posterior triangle, where it empties to the external jugular vein. A communicating vein joins the common facial vein above to the anterior jugular vein below. The right and left anterior jugular veins occasionally may be joined across the midline by the anterior jugular arch.
The transverse cervical nerve of the neck originates in the posterior triangle and passes across the sternocleidomastoid muscle to supply skin overlying the anterior triangle of the neck (see Figure 5-5). The cervical branch of the facial nerve (cranial nerve VII) passes inferiorly from the parotid region above to supply the platysma muscle.
The floor of the anterior triangle of the neck is formed by the pharynx, the larynx, and the thyroid gland. These structures are posteroinferior extensions and relations of the oral and nasal cavities and are therefore described in Chapter 7.
The muscles of the anterior triangle are grouped according to position and function (see Figures 5-7, 5-9, and Table 5-1). Suprahyoid muscles originate above the hyoid bone: infrahyoid muscles originate below the hyoid bone. Both sets of muscles insert directly or indirectly into the hyoid bone.
The omohyoid muscle consists of a superior and an inferior belly. The inferior belly of the omohyoid muscle is actually an upper limb girdle muscle and originates from the superior border of the scapula. It runs anteriorly and medially across the posterior triangle of the neck and narrows down to an intermediate tendon. The tendon passes through a sling of fascia attached to the clavicle that deflects the muscle upward and medially as the superior belly of the omohyoid muscle. The superior belly passes deep to the sternocleidomastoid muscle, emerges into the anterior triangle, and inserts into the body of the hyoid bone.
The sternohyoid muscle arises from the posterior aspect of the manubrium of the sternum and the head of the clavicle. The fibers pass upward, over the anterior aspect of the trachea and larynx, and insert into the body of the hyoid bone above.
The infrahyoid and suprahyoid muscles always contract bilaterally, never singly, unless a motor nerve lesion renders a muscle of one side paralyzed. The infrahyoid muscles act to depress the hyoid bone and the larynx, as occurs during swallowing. They also aid indirectly in depressing the mandible.
All of the infrahyoid muscles are supplied by AR of spinal nerves C1, C2, and C3 via the cervical plexus and ansa cervicalis (see Figure 5-12). The cervical plexus is described as one of the nerve contents of the anterior triangle.