Chapter 5 The Neck

1 Skeleton and Surface Anatomy

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 CERVICAL SKELETON

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.

Figure 5-1 Skeleton of cervical region. A, anterior view. B, posterior view. C, right lateral view. D, anterior view of larynx and trachea in relation to cervical skeleton.

Skeleton of the Vertebral Unit

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.

Skeleton of the Visceral Unit

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.

Figure 5-2 Skeleton of larynx. A, Frontal view. B, Posterior view.

Hyoid Bone

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.

Thyroid Cartilage

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.

Other Bones

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).

SURFACE ANATOMY

Posteriorly, the neck is an upward extension of the back, that is, a group of back extensor muscles overlying the cervical vertebrae (see Chapter 2) (Figure 5-3).

Figure 5-3 Surface features of anterior aspect of the neck.

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.

2 Coverings and Regions

FASCIAL COVERINGS

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.

Figure 5-4 Fascia of neck. A, Cross section to demonstrate fascial compartments of neck. B, Sagittal section to show continuity of cervical fascia and fascial spaces with mediastinum.

Superficial Fascia and Platysma Muscle

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.

Figure 5-5 Platysma muscle.

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.

Deep Cervical Fascia

Deep Investing Fascia

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.

REGIONS

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.

Figure 5-6 Key muscles of neck that delineate anterior and posterior triangles. A, Anterior view. B, Right lateral view.

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.

Nerve Supply

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.

CLINICAL NOTES

Torticollis (Wry Neck)

The term torticollis literally means a twisted neck.

Spasmodic torticollis occurs in adults and is characterized by unilateral spasms of the sternocleidomastoid and trapezius muscles and possibly the deeper muscles of the neck.

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.

3 The Anterior Triangle

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.

Figure 5-7 Anterior triangle of neck and its contents. A, Anterior view. B, Right lateral view.

BOUNDARIES

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).

Muscular Triangle

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.

Carotid Triangle

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.

Submandibular Triangle

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.

Submental Triangle

As the name implies, the submental triangle is below the chin (mens) and is bounded by the right and left anterior bellies of the digastric muscle and the body of the hyoid bone. The submental triangle is a midline structure. The other triangles are bilateral.

ROOF

The posterior triangle and anterior triangle are covered by skin, superficial fascia containing platysma muscle, and deep investing fascia of the neck.

Superficial Veins

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).

Figure 5-8 Superficial venous drainage of neck.

The anterior division of the retromandibular vein unites with the facial vein to form the common facial vein, which drains to the internal jugular vein below the sternocleidomastoid muscle.

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.

Superficial Nerves

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.

FLOOR

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 anterior triangle of the neck contains a number of muscles, arteries, veins, and nerves.

Muscles

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.