Fascia of Neck

One of the earliest lessons in anatomy is that there are two types of fascia in the body: the superficial and the deep. In the region of the abdominal wall, superficial fascia consists of two layers—a fatty layer (Camper’s fascia) and a deeper, membranous layer (Scarpa’s fascia). The deep fascial layer of the abdominal wall is not subdivided but simply envelops the abdominal muscles. In the neck the superficial fascia is very thin and is not divided into layers, whereas the deep fascia is divided into three layers.¹ The names of these layers vary with different authors, resulting in a somewhat chaotic terminology. Regardless of the terminology used, the divisions are arbitrary at best. A simple approach can provide a workable solution for either the anatomist or the surgeon.

Superficial Layer of Cervical Fascia

As just noted, the superficial layer of cervical fascia is a single layer of fascia underlying the skin. It usually is thin, except in the obese person, in which case it is thickened by adipose tissue. Its primary surgical significance is that it provides a fascial pad that protects underlying structures when a skin incision is made. In exceptionally lean people, however, the paucity of this layer may not protect underlying structures, such as the accessory nerve, so the surgeon should be wary when operating on such patients.

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.

Figure 181-2. Divisions of the deep cervical fascia seen in transverse section at the level of cranial nerve VII.

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.

Figure 181-3. Structures contained by the deep cervical fascia seen in transverse section at the level of the cranial nerve.

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.

Figure 181-4. Nerves of the posterior triangle seen on the right side of the neck.

Figure 181-5. Distribution of the cutaneous branches of the cervical plexus.

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.

Figure 181-6. Arterial branches of the carotid triangle.

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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