Anatomy and Developmental Embryology of the Neck

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CHAPTER 181 Anatomy and Developmental Embryology of the Neck

Most descriptions of the neck divide the anatomy, for discussion purposes, into triangles. The use of triangles is simply an organizational approach to manage the volume of anatomic detail in the neck by parceling it into reasonable study units. When an incision is made over the carotid triangle, surgeons can thereby predict the structures they will encounter in precise order. Similarly, the embryologic correlates of the triangles are important in understanding the development and differential diagnosis of masses in the various triangles of the neck.

This chapter begins with descriptions of the posterior triangle and the subdivisions of the anterior triangle. Also presented is a description of the root of the neck that relates the anatomy of the two major triangles to the radical neck procedure. The chapter concludes with a discussion of the embryology of the branchial arches and the pharyngeal pouches and their contribution to various masses in the neck.

Strictly speaking, the term triangle connotes a planar form (i.e., a flat structure). The “triangles” in the neck, however, are three-dimensional spaces that should be visualized as shallow triangular boxes. These boxes have not only three sides but also a roof (top) and a floor (bottom). Most of the triangles are three-dimensional spaces bounded by bone and muscles, with distinct fascial layers forming the roof and floor of the space. In general terms, the triangular space contains blood vessels, nerves, lymphatic vessels, and lymph nodes. Using this somewhat contrived schema, the organization of the neck can be greatly simplified.

Posterior Triangle of Neck

The posterior triangle is bordered by the sternocleidomastoid muscle, the anterior border of the trapezius muscle, and the middle third of the clavicle (Fig. 181-1). Specific layers of deep fascia form the floor (medial wall) and roof (lateral wall) of the triangular box. An understanding of the fascial relationships in the neck is important not only because of the boundary relationships but also because fasciae form planes that provide routes of surgical access or pathways for hemorrhage and infection. For this reason, a brief discussion of the fascial planes is necessary before proceeding with the anatomy of the posterior triangle.

Deep Fascia

The deep fascia is divided into three layers, demonstrated best when the neck is viewed in cross section (Fig. 181-2). These are the external, middle, and internal layers of the deep cervical fascia. The external layer of deep fascia underlies the platysma muscle and completely invests or encircles all of the superficial neck structures. For these reasons, the external layer is also known as the superficial layer, or investing layer, of deep fascia. In the region of the sternocleidomastoid and trapezius muscles, it splits and envelopes the individual muscles. A middle layer of the deep cervical fascia encloses the visceral structures of the neck (the trachea and esophagus). Hence the synonym for the middle layer is the visceral fascia.

The third, or internal, layer of the deep cervical fascia surrounds the deep muscles of the neck and cervical vertebrae (Fig. 181-3). This layer also is known by its descriptive term prevertebral fascia (although “paravertebral fascia” would have been more appropriate). The muscles enclosed by the prevertebral fascia include the deep muscles of the neck (the cervical part of the erector spinae); the levator scapulae; the scalenus anterior, middle, and posterior muscles; and the longus colli and longus capitis muscles, which lie on the anterior aspect of the cervical vertebrae. The last-named pair of muscles serve as flexors of the vertebrae; the longus capitis also assists in flexion of the skull. The scalenus group underlies the prevertebral fascia in the region of transverse processes of the cervical vertebrae. The anterior tubercles of the transverse processes provide origin for the scalenus anterior muscle, whereas the posterior tubercles provide the origin for the scalenus medius and posterior muscles. Continuing posteriorly, in order, are the levator scapulae and deep cervical muscles already noted.

The internal layer of deep fascia is described by some authors as enveloping the carotid and jugular vessels. Hence, the carotid sheaths are included as part of the definition of the internal layer of deep fascia. An effective means of visualizing the spatial relationships of these layers of fascia is to examine a cross section of the neck. This view not only is informative in defining the three layers of deep fascia but also serves to relate them to the posterior triangle of the neck (see Fig. 181-2). Placing a finger over the middle of the posterior triangle (i.e., between the trapezius and sternocleidomastoid muscles) will clarify that the roof (lateral wall) of the triangle is formed by the superficial layer of deep fascia. Palpation deeper into the triangle will bring the tip of the finger into contact with the prevertebral fascia that forms the floor of the posterior triangle and invests the prevertebral muscles. If the superficial layer of the deep fascia is incised, a finger inserted into the space, exploring anteriorly between the sternocleidomastoid and prevertebral muscles, will encounter the carotid sheath. This is a surgical approach to the retropharyngeal area or to the carotid vessels for vascular surgery.

Contents of Posterior Triangle

The major contents of the posterior triangle are the cutaneous branches of the cervical plexus, the accessory nerve (cranial nerve XI), two arterial branches of the thyrocervical trunk, and an abundant number of lymph nodes associated with the veins of the region.

Cutaneous Branches of Cervical Plexus

The sensory branches of the cervical plexus contained within the posterior triangle are four cutaneous nerves, which supply the skin of the head and neck from the area of the posterior scalp to the supraclavicular region. These cutaneous nerves are the (1) lesser occipital, (2) great auricular, (3) anterior, and (4) supraclavicular (Fig. 181-4). The first three of these nerves contain cervical segments C2 and C3 and the supraclavicular nerves C3 and C4. The topographic relations of these nerves and their distribution to the skin are illustrated in Figure 181-5. The important landmark for the cervical nerves is the accessory nerve at the point where it enters the posterior triangle from under cover of the sternocleidomastoid muscle. If this point is viewed as the center of a clock face, the cervical nerves mimic the hands pointing in different directions (see Fig. 181-4, inset). For instance, on the right side, the lesser occipital nerve is approximately at the 11 o’clock position, the great auricular is at the 12 o’clock position, and the anterior cutaneous nerve of the neck can be found at the 3 o’clock position. The supraclavicular nerves consist of three or four bundles of filaments scattered between the 5 and 7 o’clock positions on the clock’s face. Continuing this analogy, the accessory nerve can be seen to traverse the posterior triangle along the line of the 8 o’clock position until it enters the deep surface of the trapezius muscle. Viewed in this manner, the accessory nerve serves as a focus for organizing the pathways of the cutaneous nerves, as well as for emphasizing cranial nerve XI as a motor nerve in the posterior triangle.

Accessory Nerve

The accessory nerve supplies both the sternocleidomastoid and trapezius muscles; however, in the posterior triangle, the only fibers remaining in the nerve are those destined for the trapezius. The accessory nerve should be preserved whenever possible because paralysis of the trapezius muscle produces devastating effects on shoulder joint function. The range of shoulder abduction is diminished by at least 50 degrees when scapular rotation is impaired because of a trapezius palsy.

The accessory nerve is the only motor nerve in the posterior triangle as just defined. However, if the prevertebral fascia is incised, it is possible to encounter the motor nerves of the deep cervical muscles, the phrenic nerve, or the brachial plexus. With regard to its innervation of the sternocleidomastoid and trapezius muscles, cranial nerve XI is in fact a spinal nerve. Cell bodies of motor neurons innervating these muscles lie near the ventral horn in the upper cervical spinal cord, in the region of segments C2 through C4. The axonal fibers exit the anterior lateral portion of the cord as a series of fine filaments before becoming bundled as a nerve fiber within the subarachnoid space of the vertebral canal. The nerve ascends in the subarachnoid space, enters the posterior fossa through the foramen magnum, and joins with the cranial root of the accessory nerve before exiting the skull through the jugular foramen on the deep surface of the internal jugular vein. As the nerve descends the neck adjacent to the carotid sheath, motor or sensory filaments may join the nerve directly from segments C2 through C4.2 After crossing the deep surface of the sternocleidomastoid muscle, the nerve enters the posterior triangle, as already described.

Anterior Triangle of Neck

The anterior triangle of the neck is complementary to the posterior triangle and is bounded by the sternocleidomastoid muscle, the body of the mandible, and the midline of the neck. This space can be further subdivided into smaller triangular units, such as the submandibular triangle, and carotid and muscular triangles, which are included in this discussion (see Fig. 181-1).

Carotid Triangle

The carotid triangle is bordered by the posterior belly of the digastric muscle, the superior belly of the omohyoid muscle, and the midportion of the sternocleidomastoid muscle. The roof (lateral wall) of the carotid triangle is bounded by the superficial layer of deep fascia, whereas the floor (medial wall) is formed by the prevertebral fascia of the vertebral column and the visceral fascia covering the pharynx and larynx.

The contents of the carotid triangle are the carotid sheath structures. These include the common carotid with its internal and external branches, the last four cranial nerves (cranial nerves IX, X, XI, and XII), and the ansa cervicalis. In the carotid triangle, only a small branch of cranial nerve IX is left to supply the carotid sinus, because its main portion exits the sheath to enter the posterior third of the tongue. Cranial nerve XI, which also is in the upper portion of the carotid sheath, enters the sternocleidomastoid muscle at the apex of the carotid triangle, where the posterior belly of the digastric crosses the sternocleidomastoid muscle. The vagus nerve travels the entire length of the carotid sheath between the internal jugular vein and the carotid artery. Whereas the cranial nerves just mentioned lie in the posterior aspect of the carotid sheath, cranial nerve XII lies on the lateral and anterior aspect of the carotid sheath. At the level of the hyoid bone, cranial nerve XII courses anteroinferiorly to pass deep to the posterior belly of the digastric onto the surface of the hyoglossus muscle before entering the substance of the tongue.

The major focal point of the triangle is the bifurcation of the common carotid artery (Fig. 181-6). The internal carotid artery ascends the neck without branching. The external carotid artery provides several branches, many of which are important landmarks. The external carotid artery has anterior, posterior, and terminal branches.

Three anterior branches are common. In ascending order, they are the superior thyroid, lingual, and facial arteries. In some persons, the lingual and facial arteries form a common linguofacial trunk. Another important variation is the origin of the superior thyroid from the common carotid rather than the external carotid. The latter variation is important, because the external carotid occasionally is ligated (for chronic epistaxis) at its origin from the common carotid. If the superior thyroid artery is used as a landmark in patients with this particular anatomic variation, it is possible to ligate the common carotid inadvertently instead of the external carotid.

The posterior branches of the external carotid are the occipital and the posterior auricular. The occipital is noteworthy for its small sternocleidomastoid branch, which supplies the sternocleidomastoid muscle. The origin of this small artery hooks cranial nerve XII as it courses anteroinferiorly and prevents it from ascending further in the neck. Thus, the sternocleidomastoid artery can be used to trace the position of cranial nerve XII. Similarly, the sternocleidomastoid artery enters its muscle near the point of cranial nerve XI, thereby also providing a means of locating the accessory nerve.

Ansa Cervicalis

The ansa cervicalis is part of the cervical plexus. Its essential role is to supply the infrahyoid muscles: the sternohyoid, sternothyroid, and omohyoid muscles (Fig. 181-7). C1 fibers, which descend the neck, enter the hypoglossal nerve and travel as far as the occipital artery. At this point the hypoglossal nerve curves anteroinferiorly, but C1 fibers descend the neck to form the branch of cranial nerve XII, termed the superior root of the ansa. Fibers from C2 and C3 form an inferior root, which descends the carotid sheath on the surface of the internal jugular vein before turning anteroinferiorly to join the superior root. The ansa cervicalis is thus formed by the union of the two roots of the ansa. Three muscles are supplied directly by the ansa: the sternohyoid, sternothyroid, and omohyoid. At the origin of the superior root of the ansa, some C1 fibers do not follow the superior root but rather continue in the hypoglossal nerve transversely across the neck, before leaving cranial nerve XII to supply either the thyrohyoid muscle or the geniohyoid muscle (see Fig. 181-7

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