The first thoracic vertebra lies at the highest part of the sloping thoracic inlet. From its upper border rises the cervical spinal column, gently convex forwards, and supporting the skull. A mass of extensor musculature lies behind the vertebrae, supplied segmentally by posterior rami of cervical nerves that emerge from intervertebral foramina. A much smaller amount of prevertebral flexor musculature, covered by prevertebral fascia, lies in front of the vertebrae and behind the pharynx, supplied segmentally by anterior rami.
Projecting forwards and downwards from the base of the skull in front of the upper part of the pharynx is the face. The hard palate lies on a level with the anterior arch of the atlas (C1 vertebra); the lower border of the mandible lies between C2 and 3 vertebrae. The pharynx extends from the base of the skull to the level of the cricoid cartilage (C6) and then continues on as the oesophagus.
In front of the lower pharynx and upper oesophagus lie the larynx and trachea. Lying above the larynx is the hyoid bone at C3 vertebra level. It is connected to the mandible by the mylohyoid muscles, which are at the upper limit of the anterior part of the neck and form the floor of the mouth. The hyoid bone is suspended by muscles from the skull and the larynx is suspended from the hyoid by a membrane and muscles. Inferiorly they are connected by muscles to the sternum and the scapula. Deep to these muscles the thyroid gland, enclosed in the pretracheal fascia, lies alongside the larynx and trachea.
On each side of the pharynx is the carotid sheath, containing the common and internal carotid arteries and the internal jugular vein, with the cervical sympathetic trunk behind it. The external carotid artery lies outside the sheath and gives several branches in the neck. Descending into the neck are the ninth, tenth, eleventh and twelfth cranial nerves; the ninth and twelfth pass forwards to the oropharynx and tongue, the eleventh runs backwards to the sternocleidomastoid and trapezius muscles, the vagus continues down in the carotid sheath.
Surrounding the whole neck is a collar of fascia, the investing layer of deep cervical fascia, which encloses the trapezius and sternocleidomastoid muscles; the fascia and the muscles are attached above to the base of the skull and below to the clavicle at the root of the neck.
Superficial structures
The platysma is a broad flat sheet of muscle that lies superficial to the investing layer of deep cervical fascia. It extends from the fascia over the upper parts of pectoralis major and deltoid to the lower border of the mandible; some fibres continue on to the face, blending with the muscles of facial expression. The muscle covers the external and anterior jugular veins. The two muscles are separated below, but converge above towards the midline just beneath the chin.
Nerve supply. By the cervical branch of the facial nerve; afferent (proprioceptive) fibres run with the transverse cervical nerve.
Action. It plays a part in facial expression and may assist in opening the mouth.
The anterior jugular veins commence beneath the chin and pass downwards, side by side beneath the platysma, to the suprasternal region (Fig. 6.5). Here they pierce the deep fascia and come to lie in the suprasternal space, where they are often connected by a short anastomotic vein. Each now angles laterally and passes deep to sterno-cleidomastoid, but superficial to the strap muscles, to open into the external jugular vein near its termination.
Deep cervical fascia
The deep cervical fascia consists of four parts: the investing layer, pretracheal fascia, prevertebral fascia, and the carotid sheath.
Investing layer
This fascia, comparable in every way to the deep fascia that underlies the subcutaneous fat in the limbs and elsewhere, surrounds the neck like a collar (Fig. 6.1). It splits around sternocleidomastoid and trapezius and posteriorly it blends with the ligamentum nuchae, which is attached to the spines of the cervical vertebrae. Anteriorly it is attached to the hyoid bone; and above to the lower border of the mandible and to the mastoid process, superior nuchal line and external occipital protuberance at the base of the skull.
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Figure 6.1 Deep cervical fascia of one half of the neck, showing its four components: the investing, pretracheal and prevertebral layers and the carotid sheath. |
Between the angle of the mandible and the tip of the mastoid process the investing layer is strong and splits to enclose the parotid gland. The superficial part extends superiorly as the parotidomasseteric fascia and reaches up to the zygomatic arch. The deep part extends to the base of the skull; between the styloid process and the angle of the mandible it is thickened as the stylomandibular ligament.
Below, the investing layer is attached to the spine and acromion of the scapula and the clavicle with the trapezius, and to the clavicle and the manubrium of the sternum with the sternocleidomastoid. In the intervals between these muscles, it is attached to both clavicles and to the jugular (suprasternal) notch by two layers into which it splits a short distance above them. The layers are attached to the anterior and posterior borders of the jugular notch, enclosing between them the suprasternal space which contains the lower parts of the anterior jugular veins, an anastomotic arch between them, the sternal heads of the sternocleidomastoids and sometimes a lymph node. Of the two layers that adhere to the middle third of the clavicle, the deeper splits around the inferior belly of the omohyoid, forming a fascial sling which keeps this muscle belly low down in the neck (see Fig. 2.2, p. 39). The two layers are pierced by the external jugular vein.
Prevertebral fascia
This is a firm, tough membrane that lies in front of the prevertebral muscles (Fig. 6.1). It extends from the base of the skull, in front of the longus capitis, rectus capitis lateralis and longus colli muscles, downwards to blend with the anterior longitudinal ligament on the body of T4 vertebra. It extends sideways across the scalenus anterior, scalenus medius and levator scapulae muscles (Fig. 6.8), getting thinner further out and fading under cover of the anterior border of trapezius. It covers the muscles that form the floor of the posterior triangle of the neck and all the cervical nerve roots (thus the cervical plexus and trunks of the brachial plexus lie deep to it). The lymph nodes of the posterior triangle and the accessory nerve lie superficial to it. The third part of the subclavian artery lies deep to the fascia, which becomes prolonged over the artery and the brachial plexus below the clavicle as the axillary sheath to a varying extent in the axilla. It does not invest the subclavian or axillary vein; these lie in loose areolar tissue anterior to it, free to dilate during times of increased venous return from the upper limb. The fasica is pierced by the four cutaneous branches of the cervical plexus (great auricular, lesser occipital, transverse cervical and supraclavicular nerves).
Pretracheal fascia
This thin fascia lies deep to the infrahyoid strap muscles (sternothyroid, sternohyoid and omohyoid) so that its upward attachment is limited by the respective attachments of those muscles, namely, the body of the hyoid bone and the oblique line of the thyroid cartilage. It splits to enclose the thyroid gland, to which it is not adherent except to the back of the isthmus where it is also attached to the second, third and fourth rings of the trachea. Laterally, it fuses with the front of the carotid sheath on the deep surface of the sternocleidomastoid and inferiorly it passes behind the brachiocephalic veins to blend with the adventitia of the arch of the aorta and the fibrous pericardium. The pretracheal fascia is also described in some accounts as being part of a cervical visceral fascia that surrounds the pharynx, oesophagus, larynx and trachea.
Carotid sheath
This is not a fascia in the sense of a demonstrable membranous layer, but consists of a feltwork of areolar tissue that surrounds the common and internal carotid arteries, internal jugular vein, vagus nerve and some deep cervical lymph nodes (Fig. 6.1). It is thin where it overlies the internal jugular vein, allowing the vein to dilate during increased blood flow. The sheath is attached to the base of the skull at the margins of the carotid canal and jugular fossa, and is continued downwards along the vessels to blend with the adventitia of the aortic arch. In front the lower part of the sheath fuses with the fascia on the deep surface of the sternocleidomastoid. Where they lie alongside, the sheath blends with the pretracheal fascia. Behind the carotid sheath there is a minimum of loose areolar tissue between it and the prevertebral fascia; the cervical sympathetic trunk lies here in front of the prevertebral fascia (Fig. 6.8). The carotid sheath is described further on page 366.
Tissue spaces of the neck
Behind the prevertebral fascia is the closed prevertebral space from which an anterior escape can only be made by a perforation in the fascia. Hence pus from an abscess in a cervical vertebra can lift the prevertebral fascia as far down as the superior mediastinum.
Immediately in front of the prevertebral fascia is a space that extends from the base of the skull to the diaphragm passing through the superior into the posterior mediastinum as it does so (see Fig. 4.9, p. 189). Its upper part is the retropharyngeal space, which is continuous laterally with a parapharyngeal space at the side of the pharynx; the upper part of this space is in the infratemporal fossa (see p. 361), bounded laterally by the pterygoid muscles and the parotid sheath.
In the upper part of the neck is the submandibular space below the mylohyoid muscle and deep to the investing layer of fascia between the hyoid bone and the mandible. This space communicates around the posterior border of mylohyoid with a sublingual space under the mucous membrane of the floor of the mouth. Ludwig's angina is a rare but severe form of cellulitis that involves these spaces and spreads backwards into the parapharyngeal space.
Three or four small submental lymph nodes lie beneath the chin, some superficial and others deep to the investing layer of deep cervical fascia (Fig. 6.6). They drain, across the midline, a wedge of tissue in the floor of the mouth opposite the four lower incisor teeth, including those teeth, gums and lip, and the tip of the tongue (Fig. 6.34). In their turn they drain to submandibular nodes or directly to the upper deep cervical group.
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Figure 6.34 Lymph drainage of the tongue. Of the deep cervical nodes, only the jugulodigastric and jugulo-omohyoid nodes are shown; other channels drain to other nodes of the group. |
About half a dozen submandibular lymph nodes lie on the surface of the submandibular gland, some embedded within the gland (Fig. 6.6). They drain the submental nodes, the lateral parts of the lower lips, all the upper lip and external nose, and the anterior part of the tongue, mainly but not exclusively from their own side. They also receive lymph from the anterior half of the nasal walls and the paranasal sinuses that drain there (frontal, anterior and middle ethmoidal, and maxillary), and from all the teeth (except lower incisors).
