CHAPTER 28 Neck

The neck extends from the base of the cranium and the inferior border of the mandible to the thoracic inlet.

SKIN

The skin in the neck is normally under tension. The direction in which this is greatest varies regionally; in the living face these lines often coincide with wrinkle lines. Lines of greatest tension have been termed ‘relaxed skin tension lines’, and surgical incisions made along these lines are said to heal with minimal postoperative scarring. Susceptible individuals are more prone to keloid scarring in the head and neck region.

Cutaneous vascular supply and lymphatic drainage

The blood vessels supplying the skin of the neck are derived principally from the facial, occipital, posterior auricular and subclavian arteries. They form a rich network within platysma and in the subdermal plexus, and account for the viability of the various skin flaps raised during block dissection of the neck, irrespective of whether they include platysma: the latter is usually elevated in extensive incisions.

The anterior cervical skin is supplied mainly by the superior thyroid artery and the transverse cervical branch of the subclavian artery. The posterior skin is supplied by branches from the occipital artery and the deep cervical and transverse cervical branches of the subclavian artery. The superior skin is supplied by the occipital artery and its upper sternocleidomastoid branch, and the submandibular and submental branches of the facial artery. Inferiorly, the skin is supplied by the transverse cervical and/or suprascapular branches of the subclavian artery.

The pattern of venous drainage of the skin of the neck mirrors the arterial supply: the veins drain into the jugular and facial veins.

Many lymphatic vessels draining the superficial cervical tissues skirt the borders of sternocleidomastoid to reach the superior or inferior deep cervical nodes. Some pass over sternocleidomastoid and the posterior triangle to drain into the superficial cervical and occipital nodes (see Fig. 28.15). Lymph from the superior region of the anterior triangle drains to the submandibular and submental nodes. Vessels from the anterior cervical skin inferior to the hyoid bone pass to the anterior cervical lymph nodes near the anterior jugular veins, and their efferents go to the deep cervical nodes of both sides, including the infrahyoid, prelaryngeal and pretracheal groups. An anterior cervical node often occupies the suprasternal space.

Fig. 28.15 Superficial lymphatic system in the head and neck: the nodes are organized into a superficial group around the head, superficial cervical nodes along the external jugular vein, and deep cervical nodes along the internal jugular vein.

(From Drake, Vogl and Mitchell 2005.)

Cutaneous innervation

The cervical skin is innervated by branches of cervical spinal nerves, via both dorsal and ventral rami (see Fig. 43.6). The dorsal rami supply skin over the back of the neck and scalp, and the ventral rami supply skin covering the lateral and anterior portions of the neck, and the angle of the mandible (Fig. 28.1). The dorsal rami of the first, sixth, seventh and eighth cervical nerves have no cutaneous distribution in the neck. The greater occipital nerve mainly supplies the scalp; it comes from the medial branch of the dorsal ramus of the second cervical nerve. The medial branches of the dorsal rami of the third, fourth and fifth cervical nerves pierce trapezius to supply skin over the back of the neck sequentially. The ventral rami of the second, third and fourth cervical nerves supply named cutaneous branches (the lesser occipital, great auricular, transverse cutaneous and supraclavicular nerves), via the cervical plexus (Fig. 28.1) (see p. 456 for details of the motor branches of the cervical plexus).

Fig. 28.1 The cutaneous branches of the cervical plexus. The spinal part of the accessory nerve that supplies trapezius is also shown as it crosses the posterior triangle. Note that the interval between the upper attachments of sternocleidomastoid and trapezius is not normally as extensive as shown here.

(Adapted from Drake, Vogl and Mitchell 2005.)

BONES, JOINTS AND CARTILAGES

The bones and cartilages of the neck are the cervical vertebrae and the hyoid bone, and the cartilages of the upper respiratory tract, including the larynx. The cervical vertebrae, occipital bone and the atlantooccipital and atlanto-axial joints are described in Chapter 42, and the laryngeal cartilages are described in Chapter 34.

HYOID BONE

The U-shaped hyoid bone (Fig. 28.2) is suspended from the tips of the styloid processes by the stylohyoid ligaments. It has a body, two greater and two lesser horns, or cornua.

Fig. 28.2 The hyoid bone: A, B, anterosuperior aspect; C, lateral aspect. B shows the positions of muscular attachments.

Body

The body is irregular, elongated and quadrilateral. Its anterior surface is convex, faces anterosuperiorly, and is crossed by a transverse ridge with a slight downward convexity. A vertical median ridge often bisects the upper part of the body, but rarely extends to the lower part. The posterior surface is smooth, concave, faces posteroinferiorly, and is separated from the epiglottis by the thyrohyoid membrane and loose areolar tissue. There is a bursa between the hyoid bone and the membrane.

Geniohyoid is attached to most of the anterior surface of the body, above and below the transverse ridge; the medial part of hyoglossus invades the lateral geniohyoid area. The lower anterior surface gives attachment to mylohyoid, the line of attachment lying above sternohyoid medially and omohyoid laterally. The lowest fibres of genioglossus, the hyoepiglottic ligament and (most posteriorly) the thyrohyoid membrane are all attached to the rounded superior border. Sternohyoid is attached to the inferior border medially and omohyoid is attached laterally. Occasionally the medial fibres of thyrohyoid and, when present, of levator glandulae thyroideae, are attached along the inferior border.

Greater cornua

In early life, the greater cornua are connected to the body by cartilage, but after middle age they are usually united by bone. They project backwards (curving posterolaterally) from the lateral ends of the body. They are horizontally flattened, taper posteriorly, and each ends in a tubercle. When the throat is gripped between finger and thumb above the thyroid cartilage, the greater cornua can be identified and the bone can be moved from side to side.

The middle pharyngeal constrictor and, more laterally (i.e. superficially), hyoglossus, are attached along the whole length of the upper surface of each greater cornu. Stylohyoid is attached near the junction of the cornu with the body. The fibrous loop for the digastric tendon is attached lateral and a little posterior to hyoglossus. The thyrohyoid membrane is attached to the medial border and thyrohyoid is attached to the lateral border. The oblique inferior surface is separated from the thyrohyoid membrane by fibroareolar tissue.

Lesser cornua

The lesser cornua are two small conical projections at the junctions of the body and greater cornua. At its base, each is connected to the body by fibrous tissue and occasionally to the greater cornu by a synovial joint which occasionally becomes ankylosed.

The middle pharyngeal constrictors are attached to the posterior and lateral aspects of the lesser cornua. The stylohyoid ligaments are attached to their apices and are often partly calcified, and the chondroglossi are attached to the medial aspects of their bases.

Ossification

The hyoid bone develops from cartilages of the second and third pharyngeal arches, the lesser cornua from the second, the greater cornua from the third and the body from the fused ventral ends of both. Chondrification begins in the fifth fetal week in these elements, and is completed in the third and fourth months. Ossification proceeds from six centres, i.e. a pair for the body and one for each cornu. Ossification begins in the greater cornua towards the end of intrauterine life, in the body shortly before or after birth, and in the lesser cornua around puberty. The greater cornual apices remain cartilaginous until the third decade and epiphyses may occur here. They fuse with the body. Synovial joints between the greater and lesser cornua may be obliterated by ossification in later decades.

JOINTS

The atlanto-occipital and atlanto-axial joints are described on pages 733–734.

TRIANGLES OF THE NECK

Anterolaterally the neck appears as a somewhat quadrilateral area, limited superiorly by the base of the mandible and a line continued from the angle of the mandible to the mastoid process, inferiorly by the upper border of the clavicle, anteriorly by the anterior median line, and posteriorly by the anterior margin of trapezius. This quadrilateral area can be further divided into anterior and posterior triangles by sternocleidomastoid, which passes obliquely from the sternum and clavicle to the mastoid process and occipital bone (Fig. 28.3). It is true that these triangles and their subdivisions are somewhat arbitrary, because many major structures – arteries, veins, lymphatics, nerves, and some viscera – transgress their boundaries without interruption, nevertheless they have a topographical value in description. Moreover, some of their subdivisions are easily identified by inspection and palpation and provide invaluable assistance in surface anatomical and clinical examination.

Fig. 28.3 The anterior and posterior triangles of the neck, left lateral aspect. See note in caption for Fig. 28.1

(Adapted from Drake, Vogl and Mitchell 2005.)

ANTERIOR TRIANGLE OF THE NECK

The anterior triangle of the neck is bounded anteriorly by the median line of the neck and posteriorly by the anterior margin of sternocleidomastoid. Its base is the inferior border of the mandible and its projection to the mastoid process, and its apex is at the manubrium sterni. It can be subdivided into suprahyoid and infrahyoid areas above and below the hyoid bone, and into digastric, submental, muscular and carotid triangles by the passage of digastric and omohyoid across the anterior triangle (see Fig. 28.5).

Fig. 28.5 Muscles of the neck: note that the head is slightly extended in order to expose the suprahyoid muscles.

(From Sobotta 2006.)

Digastric triangle

The digastric triangle is bordered above by the base of the mandible and its projection to the mastoid process, posteroinferiorly by the posterior belly of digastric and by stylohyoid, and anteroinferiorly by the anterior belly of digastric. It is covered by the skin, superficial fascia, platysma and deep fascia, which contain branches of the facial and transverse cutaneous cervical nerves. Its floor is formed by mylohyoid and hyoglossus. The anterior region of the digastric triangle contains the submandibular gland, which has the facial vein superficial to it and the facial artery deep to it. The submental and mylohyoid arteries and nerves lie on mylohyoid. The submandibular lymph nodes are variably related to the submandibular gland. The posterior region of the digastric triangle contains the lower part of the parotid gland. The external carotid artery, passing deep to stylohyoid, curves above the muscle, and overlaps its superficial surface as it ascends deep to the parotid gland before entering it. The internal carotid artery, internal jugular vein and vagus nerve lie deeper and are separated from the external carotid artery by styloglossus, stylopharyngeus and the glossopharyngeal nerve.

Submental triangle

The single submental triangle is demarcated by the anterior bellies of both digastric muscles. Its apex is at the chin, its base is the body of the hyoid bone and its floor is formed by both mylohyoid muscles. It contains lymph nodes and small veins that unite to form the anterior jugular vein. The structures within the digastric and submental triangles are described in more detail with the floor of the mouth (on p. 501).

Muscular triangle

The muscular triangle is bounded anteriorly by the median line of the neck from the hyoid bone to the sternum, inferoposteriorly by the anterior margin of sternocleidomastoid and posterosuperiorly by the superior belly of omohyoid. The triangle contains omohyoid, sternohyoid, sternothyroid and thyrohyoid.

Carotid triangle

The carotid triangle is limited posteriorly by sternocleidomastoid, anteroinferiorly by the superior belly of omohyoid and superiorly by stylohyoid and the posterior belly of digastric. In the living (except the obese) the triangle is usually a small visible triangular depression, sometimes best seen with the head and cervical vertebral column slightly extended and the head contralaterally rotated. The carotid triangle is covered by the skin, superficial fascia, platysma and deep fascia containing branches of the facial and cutaneous cervical nerves. The hyoid bone forms its anterior angle and adjacent floor and can be located on simple inspection, verified by palpation. Parts of thyrohyoid, hyoglossus and inferior and middle pharyngeal constrictor muscles form its floor. The carotid triangle contains the upper part of the common carotid artery and its division into external and internal carotid arteries. Overlapped by the anterior margin of sternocleidomastoid, the external carotid artery is first anteromedial, then anterior to the internal carotid artery. Branches of the external carotid artery are encountered in the carotid triangle. Thus the superior thyroid artery runs anteroinferiorly, the lingual artery anteriorly with a characteristic upward loop, the facial artery anterosuperiorly, the occipital artery posterosuperiorly and the ascending pharyngeal artery medial to the internal carotid artery. Arterial pulsation greets the examining finger. The superior thyroid, lingual, facial, ascending pharyngeal and sometimes the occipital, veins, correspond to the branches of the external carotid artery, and all drain into the internal jugular vein. The hypoglossal nerve crosses the external and internal carotid arteries. It curves round the origin of the lower sternocleidomastoid branch of the occipital artery, and at this point the superior root of the ansa cervicalis leaves it to descend anteriorly in the carotid sheath. The internal laryngeal nerve and, below it, the external laryngeal nerve, lie medial to the external carotid artery below the hyoid bone. Many structures in this region, such as all or part of the internal jugular vein, associated deep cervical lymph nodes, and the vagus nerve, may be variably obscured by sternocleidomastoid, and, pedantically, are thus ‘outside the triangle’.

POSTERIOR TRIANGLE OF THE NECK

The posterior triangle is delimited anteriorly by the posterior edge of sternocleidomastoid, posteriorly by the anterior edge of trapezius, and inferiorly by the middle third of the clavicle (Fig. 28.3). Its apex is between the attachments of sternocleidomastoid and trapezius to the occiput and is often blunted, so that the ‘triangle’ becomes quadrilateral. The roof of the posterior triangle is formed by the investing layer of the deep cervical fascia. The floor of the triangle is formed by the prevertebral fascia overlying splenius capitis, levator scapulae and the scalene muscles. It is crossed, approximately 2.5 cm above the clavicle, by the inferior belly of omohyoid, which subdivides it into occipital and supraclavicular triangles. The contents of the posterior triangle include fat, lymph nodes (level V – see later), spinal accessory nerve, cutaneous branches of the cervical plexus, inferior belly of omohyoid, branches of the thyrocervical trunk (transverse cervical and suprascapular arteries), the third part of the subclavian artery, and the external jugular vein. The anterior and lateral groups of prevertebral muscles form the floor of the posterior triangle.

Occipital triangle

The occipital triangle constitutes the upper and larger part of the posterior triangle, with which it shares the same borders, except that inferiorly it is limited by the inferior belly of omohyoid. Its floor, from above down, is formed by splenius capitis, levator scapulae, and scaleni medius and posterior; semispinalis capitis occasionally appears at the apex. The triangle is covered by skin, superficial and deep fasciae and inferiorly by platysma. The spinal accessory nerve pierces sternocleidomastoid and crosses levator scapulae obliquely downwards and backwards to reach the deep surface of trapezius. The surface marking of its course is in a line from the junction of the superior one-third and inferior two-thirds of sternocleidomastoid, to the junction of the inferior one-third and superior two-thirds of trapezius. Cutaneous and muscular branches of the cervical plexus emerge at the posterior border of sternocleidomastoid. Inferiorly, supraclavicular nerves, transverse cervical vessels and the uppermost part of the brachial plexus cross the triangle. Lymph nodes lie along the posterior border of sternocleidomastoid from the mastoid process to the root of the neck.

Supraclavicular triangle

The supraclavicular triangle is the lower and smaller division of the posterior triangle, with which it shares the same boundaries, except that superiorly it is limited by omohyoid. It corresponds in the living neck with the lower part of a deep, prominent hollow, namely, the greater supraclavicular fossa. Its floor contains the first rib, scalenus medius and the first slip of serratus anterior. Its size varies with the extent of the clavicular attachments of sternocleidomastoid and trapezius and also the level of the inferior belly of omohyoid. The triangle is covered by skin, superficial and deep fasciae and platysma and crossed by the supraclavicular nerves. Just above the clavicle, the third part of the subclavian artery curves inferolaterally from the lateral margin of scalenus anterior across the first rib to the axilla. The subclavian vein is behind the clavicle and is not usually in the triangle; but it may rise as high as the artery and even accompany it behind scalenus anterior. The brachial plexus is partly superior, and partly posterior to the artery and is always closely related to it. The trunks of the brachial plexus may easily be palpated here if the neck is contralaterally flexed and the examining finger is drawn across the trunks at right angles to their length. With the musculature relaxed, pulsation of the subclavian artery may be felt and the arterial flow can be controlled by retroclavicular compression against the first rib. The suprascapular vessels pass transversely behind the clavicle, below the transverse cervical artery and vein. The external jugular vein descends behind the posterior border of sternocleidomastoid to end in the subclavian vein. It receives the transverse cervical and suprascapular veins, which form a plexus in front of the third part of the subclavian artery; occasionally it is joined by a small vein that crosses the clavicle anteriorly from the cephalic vein. Other structures within the triangle include the nerve to subclavius, which crosses the triangle, and lymph nodes.

CERVICAL FASCIA

SUPERFICIAL CERVICAL FASCIA

The superficial cervical fascia is usually a thin lamina covering platysma and is hardly demonstrable as a separate layer. It may, however, contain considerable amounts of adipose tissue, especially in females. Like all superficial fascia it is not a separate stratum, but merely a zone of loose connective tissue between dermis and deep fascia, and is joined to both. In the lower cervical region, aponeurotic fibres of platysma gradually fan out in the superficial fascia: they either become skin ligaments or continue into the fascia covering pectoralis major and deltoid. These aponeurotic fibres cover the anterior and lateral cervical regions and often mimic the investing layer of deep cervical fascia, forming a tissue barrier (Nash et al 2005).

DEEP CERVICAL FASCIA

Descriptions of the organization of the deep cervical fascia are largely based on the classic work of Grodinsky & Holyoke in 1938. However recent anatomical studies using techniques such as sheet plastination and confocal microscopy have indicated that the arrangement of the deep cervical fascia is more complicated than was previously thought (Zhang & Lee 2002, Nash et al 2005) (Fig. 28.4).

Fig. 28.4 Transverse sections through the neck at the levels of the second (A) and sixth (B) cervical vertebrae, showing the arrangement of the deep cervical fascia, much of which has been coloured blue. Diagrams are based on plastinated sections: note that many muscles have not been individually delineated.

(By courtesy of Dr M Zhang and Mr Robbie McPhee.)

Deep fascia in the neck is conventionally divided into an investing layer, pretracheal fascia, and prevertebral fascia.

Investing layer

The investing layer of the deep cervical fascia is continuous posteriorly with the corresponding fascia from the opposite side. It forms a thin covering for trapezius and encloses sternocleidomastoid. The portions between trapezius and sternocleidomastoid and in the anterior triangle of the neck are formed of areolar tissue, indistinguishable from that in the superficial cervical fascia and deep potential tissue spaces. Superiorly, the deep fascia fuses with periosteum along the superior nuchal line of the occipital bone, over the mastoid process and along the entire base of the mandible. Between the angle of the mandible and the anterior edge of sternocleidomastoid it is particularly strong. Between the mandible and the mastoid process it is related to the parotid gland, extending beneath it to become attached to the zygomatic arch. From this region the strong stylomandibular ligament ascends to the styloid process. Inferiorly, along trapezius and sternocleidomastoid, the investing layer of the deep cervical fascia is attached to the acromion, clavicle and manubrium sterni, fusing with their periostea. A short distance above the manubrium, the investing layer interweaves with aponeurotic fibres of platysma and the fascia investing the strap muscles. It is organized into superficial and deep layers which are attached to the anterior border of the manubrium, and to the posterior border and the interclavicular ligament respectively. Between these two layers a slit-like interval, the suprasternal space, contains a small amount of areolar tissue, the lower parts of the anterior jugular veins and the jugular venous arch, the sternal heads of the sternocleidomastoid muscles and sometimes a lymph node.

Pretracheal fascia

The pretracheal layer of the deep cervical fascia is very thin. It provides fascial sheaths for the thyroid gland, larynx, pharynx, trachea, oesophagus and the infrahyoid strap muscles. Superiorly, it is attached to the hyoid bone; inferiorly, it continues into the superior mediastinum along the great vessels and merges with the fibrous pericardium; laterally, it merges with the investing layer of deep cervical fascia and with the carotid sheath.

Prevertebral fascia

The prevertebral layer of the deep cervical fascia covers the anterior vertebral muscles and extends laterally on scalenus anterior, scalenus medius and levator scapulae, forming a fascial floor for the posterior triangle of the neck. As the subclavian artery and the brachial plexus emerge from behind scalenus anterior they carry the prevertebral fascia downwards and laterally behind the clavicle as the axillary sheath.

The prevertebral fascia is particularly prominent in front of the vertebral column, where there may be two distinct layers of fascia. The space created by the splitting of the anterior prevertebral fascia, the danger space, is a part of the prevertebral space. Traced laterally, the prevertebral fascia becomes thin and areolar. Superiorly it is attached to the base of the skull. Inferiorly it descends in front of longus colli into the superior mediastinum, where it blends with the anterior longitudinal ligament. Anteriorly the prevertebral fascia is separated from the pharynx and its covering buccopharyngeal fascia by a loose areolar zone, the retropharyngeal space. Laterally this loose tissue connects the prevertebral fascia to the carotid sheath and the fascia on the deep surface of sternocleidomastoid. All the ventral rami of the cervical nerves are initially behind the prevertebral fascia. The nerves to the rhomboids and serratus anterior and the phrenic nerve retain this position throughout their course in the neck, but the accessory nerve lies superficial to the prevertebral fascia.

Carotid sheath

The carotid sheath is a condensation of deep cervical fascia around the common and internal carotid arteries, the internal jugular vein, the vagus nerve and the constituents of the ansa cervicalis. It is thicker around the arteries than the vein, an arrangement that allows the vein to expand. Peripherally the carotid sheath is connected to adjacent fascial layers by loose areolar tissue.

Tissue spaces and the spread of infection and injectate

The fascial layers of the neck define a number of potential tissue ‘spaces’ above and below the hyoid bone. In health, the tissues within these spaces are closely applied to each other or are filled with relatively loose connective tissue. However, infections arising superiorly, such as dental, tonsillar, vertebral or intervertebral disc-related infections, can alter these relationships. The organisms responsible are often betahaemolytic streptococci or a variety of anaerobes. Streptococci produce proteolytic enzymes which digest the loose connective tissue and so open up the tissue spaces. Since there are no tissue barriers running horizontally in the neck, infections which are not treated promptly can rapidly spread from the infratemporal fossa down to the mediastinum below (see Ch. 55), cross the midline through the sublingual and submental spaces and even track into the axilla.

Understanding the configuration of the cervical fasciae and spaces is essential for the placement of local anaesthetic cervical plexus blocks in the neck to facilitate operations such as thyroidectomy, parathyroidectomy and carotid endarterectomy (Pandit et al 2000). For example, injection of local anaesthetic either in the superficial fascial plane or under the investing fascia of the posterior triangle can provide a similarly effective cervical plexus block (Pandit et al 2003), presumably because the investing fascia between sternocleidomastoid and trapezius is not a well-defined fascial sheet and is indistinguishable from the surrounding loose connective tissue (Zhang & Lee 2002).

The tissue spaces above the hyoid bone are the submandibular and submental spaces beneath the inferior border of the mandible; the pharyngeal spaces; and the prevertebral space near the base of the skull. These spaces are described on pages 524 and 568. Tissue spaces around the larynx are described on page 584.

Tissue spaces below the hyoid bone are the pretracheal and retrovisceral tissue spaces in the visceral compartment of the neck; the prevertebral space in front of the vertebral column; and a space associated with the carotid sheath.

Pretracheal space

The pretracheal tissue space lies behind the pretracheal fascia and the strap muscles, and in front of the anterior wall of the oesophagus, and therefore immediately surrounds the trachea. It is bounded superiorly by the attachments of the strap muscles to the thyroid cartilage of the larynx. Inferiorly, it extends down into the anterior portion of the superior mediastinum. Infection usually spreads into the pretracheal space either by perforating the anterior wall of the oesophagus or from the retrovisceral space.

Radiologically, the portion of the pretracheal space between the strap fascia and the fascia of the thyroid gland is referred as the anterior cervical space. The posterolateral border of the space is the carotid sheath or the fascia of sternocleidomastoid. The anterior cervical space often provides a symmetric landmark on transverse imaging (Smoker & Harnsberger 1991).

Retrovisceral space

The retrovisceral space is continuous superiorly with the retropharyngeal space. It is situated between the posterior wall of the oesophagus and the prevertebral fascia. Inferiorly, the retrovisceral space extends into the superior mediastinum. Should the prevertebral fascia merge with the connective tissue on the posterior surface of the oesophagus – usually at the level of the fourth thoracic vertebra – the retrovisceral space then has a distinct inferior boundary.

Prevertebral space

The prevertebral tissue space has been variously described as the potential space lying between the prevertebral fascia and the vertebral column, and as the space between the two layers of the prevertebral fascia, the so-called danger space. Infection usually spreads into the space via its fascial walls from the retrovisceral area because it is closed superiorly and laterally. Inferiorly, it extends into the posterior mediastinum. Clinically, however, the danger space and the retropharyngeal or retrovisceral space are often considered together because they cannot be differentiated radiologically.

Carotid space

The carotid sheath is a layer of loose connective tissue demarcated by adjacent portions of the investing layer of deep cervical fascia, the pretracheal fascia, and the prevertebral fascia. It delineates a potential space into which infections from the visceral spaces may track. Infections around the carotid sheath may be restricted because superiorly (near the hyoid bone) and inferiorly (near the root of the neck) the connective tissues adhere to the vessels.

Cellulitis in the neck

The main cause of cellulitis of the neck is infection arising from the region of the mandibular molar teeth. Several fascial spaces are accessible from this area, and several anatomical factors contribute to the spread of infection. Thus, the apices of the second and, more especially, the third, mandibular molar teeth are often close to the lingual surface of the mandible. The apices of the roots of the third mandibular molars are usually, and the second molars are often below the attachment of mylohyoid on the inner aspect of the mandible and so drain directly into the submandibular tissue space. The posterior free border of mylohyoid is close to the sockets of the third mandibular molars, and at this point, the floor of the mouth consists only of mucous membrane covering part of the submandibular salivary gland. Any virulent periapical infection of the mandibular third molar teeth may therefore penetrate the lingual plate of the mandible and is then at the entrance to the submandibular and sublingual spaces anteriorly, and the parapharyngeal and pterygoid spaces posteriorly. Infection in this area may also spread from an acute pericoronitis, particularly when the deeper tissues are opened to infection by extraction of the tooth during the acute phase.

In general, cellulitis around the jaw is only likely to develop when the tissues are infected by virulent and invasive organisms at a point where there is access to the fascial spaces: the predisposing causes do not often coincide, and cellulitis is therefore uncommon. Cellulitis in the region of the maxilla is even more uncommon, but fascial space infections may develop in various sites as the result of infected local anaesthetic needles. Since there are no barriers running horizontally with respect to the tissue spaces in the neck, infection entering in this site can rapidly spread more or less unhindered down the neck and may enter the mediastinum (see Ch. 55).

All forms of cellulitides of the neck or deep neck space infections are potentially serious. Narrowing of the upper airway can occur as a result of inflammation and oedema, leading to dyspnoea or obstruction of the upper airway (with stridor) and reduced oxygenation of the lungs. This situation can be extremely difficult to manage by conventional techniques. The increased rigidity and reduced compliance of the tissues mean that manoeuvres such as manual anterior jaw thrust or laryngoscopy may fail to re-open the airway. Specialized techniques, e.g. flexible fibreoptic-assisted tracheal intubation or surgical tracheostomy under local anaesthesia, may be required to provide general anaesthesia to facilitate the surgical drainage and treatment of the cellulitis or deep space abscess.

MUSCLES

The superficial muscles of the neck are platysma, which lies in the subcutaneous tissue of the neck, and sternocleidomastoid and trapezius. Sternocleidomastoid is a key landmark because it divides the neck into anterior and lateral regions (anterior and posterior triangles respectively); the anterior region may be further subdivided into several smaller named triangles (see above). Muscles in the anterior region are organized into supra- and infra-hyoid groups, and with one exception are all attached to the hyoid bone. The supra-hyoid muscles, which connect the hyoid bone to the mandible and the base of the skull, include mylohyoid, geniohyoid, stylohyoid and digastric. The infrahyoid (strap) muscles, which connect the hyoid, sternum, clavicle and scapula, are arranged in two planes, a superficial plane consisting of sternohyoid and omohyoid and a deep plane consisting of sternothyroid and thyrohyoid.

The muscles that form part of the musculoskeletal column in the neck are described in Chapter 42. They can be considered in three groups, anterior, lateral and posterior; very broadly speaking, the muscles in these groups lie anterior, lateral or posterior to the cervical vertebrae. The anterior and lateral groups include longi colli and capitis; recti capitis anterior and lateralis; scaleni anterior, medius, posterior and minimi (when present). The posterior muscle group is composed of the cervical components of the intrinsic muscles of the back, overlaid by some of the extrinsic ‘immigrant’ muscles of the back that run between the upper limb and the axial skeleton (trapezius, levator scapulae, see Ch. 42). The intrinsic muscles are arranged in superficial and deep layers. The superficial layer contains splenius capitis and cervicis. The deeper layers include the transversospinal group (semispinales cervicis and capitis, multifidus and rotatores cervicis), interspinales and intertransversarii, and the suboccipital group (recti capitis posterior major and minor and obliquus capitis superior and inferior).

The muscles associated with the pharynx and larynx are described in Chapters 33 and 34 respectively.

Platysma

Platysma is a broad sheet of muscle of varying prominence which arises from the fascia covering the upper parts of pectoralis major and deltoid. Its fibres cross the clavicle and ascend medially in the side of the neck. Anterior fibres interlace across the midline with the fibres of the contralateral muscle, below and behind the symphysis menti. Other fibres attach to the lower border of the mandible or to the lower lip or cross the mandible to attach to skin and subcutaneous tissue of the lower face. Careful elevation of a myocutaneous flap in the subplatysmal plane as part of a neck dissection will include this muscle and its associated blood supply, thereby minimizing the risk of skin necrosis and wound breakdown.

Sternocleidomastoid

Sternocleidomastoid (Fig. 28.5) descends obliquely across the side of the neck and forms a prominent surface landmark, especially when contracted. It is thick and narrow centrally, and broader and thinner at each end. The muscle is attached inferiorly by two heads. The medial or sternal head is rounded and tendinous, arises from the upper part of the anterior surface of the manubrium and sterni and ascends posterolaterally. The lateral or clavicular head, which is variable in width and contains muscular and fibrous elements, ascends almost vertically from the superior surface of the medial third of the clavicle. The two heads are separated near their attachments by a triangular interval which corresponds to a surface depression, the lesser supraclavicular fossa. As they ascend, the clavicular head spirals behind the sternal head and blends with its deep surface below the middle of the neck, forming a thick, rounded belly. Sternocleidomastoid inserts superiorly by a strong tendon into the lateral surface of the mastoid process from its apex to its superior border, and by a thin aponeurosis into the lateral half of the superior nuchal line. The clavicular fibres are directed mainly to the mastoid process; the sternal fibres are more oblique and superficial, and extend to the occiput. The direction of pull of the two heads is therefore different, and the muscle may be classed as ‘cruciate’ and slightly ‘spiralized’.

Relations

The superficial surface of sternocleidomastoid is covered by skin and platysma, between which lie the external jugular vein, the great auricular and transverse cervical nerves and the superficial lamina of the deep cervical fascia. Near its insertion the muscle is overlapped by a small part of the parotid gland. The deep surface of the muscle near its origin is related to the sternoclavicular joint and sternohyoid, sternothyroid and omohyoid. The anterior jugular vein crosses deep to it, but superficial to the infrahyoid muscles, immediately above the clavicle. The carotid sheath and the subclavian artery are deep to these muscles. Between omohyoid and the posterior belly of digastric, the anterior part of sternocleidomastoid lies superficial to the common, internal and external carotid arteries, the internal jugular, facial and lingual veins, the deep cervical lymph nodes, the vagus nerve and the rami of the ansa cervicalis. The sternocleidomastoid branch of the superior thyroid artery crosses deep to the muscle at the upper border of omohyoid. The posterior part of sternocleidomastoid is related on its internal surface to splenius capitis, levator scapulae and the scalene muscles, the cervical plexus, the upper part of the brachial plexus, the phrenic nerve and the transverse cervical and suprascapular arteries. The occipital artery crosses deep to the muscle at, or under cover of, the lower border of the posterior belly of digastric. At this point the accessory nerve passes deep to sternocleidomastoid, then pierces and supplies the muscle, before it reappears just above the middle of the posterior border. At its insertion the muscle lies superficial to the mastoid process, splenius capitis, longissimus capitis and the posterior belly of digastric.

Vascular supply

Sternocleidomastoid receives its blood supply from branches of the occipital and posterior auricular arteries (upper part of muscle), the superior thyroid artery (middle part of muscle), and the suprascapular artery (lower part of muscle). A superiorly based flap can be raised on sternocleidomastoid to include a paddle of skin supplied by perforator vessels. This flap has been used to reconstruct the lips, floor of mouth and inner aspect of the cheeks, however its use has been superseded by microvascular free transfer flaps or by conventional myocutaneous flaps such as the pectoralis major flap.

Innervation

Sternocleidomastoid is supplied by the spinal part of the accessory nerve. Branches from the ventral rami of the second, third, and sometimes fourth, cervical spinal nerves also enter the muscle. Although these cervical rami were believed to be solely proprioceptive, clinical evidence suggests that some of their fibres are motor.

Actions

Acting alone, each sternocleidomastoid will tilt the head towards the ipsilateral shoulder, simultaneously rotating the head so as to turn the face towards the opposite side. This movement occurs in an upward, sideways glance. A more common visual movement is a level rotation from side to side, and this probably represents the most frequent use of the sternocleidomastoids. Acting together from below, the muscles draw the head forwards and so help longi colli to flex the cervical part of the vertebral column, which is a common movement in feeding. The two sternocleidomastoids are also used to raise the head when the body is supine, and when the head is fixed, they help to elevate the thorax in forced inspiration.

Branchial cysts and fistulae

Branchial cysts usually present in the upper neck in early adulthood as fluctuant swellings at the junction of the upper and middle thirds of the anterior border of sternocleidomastoid. The cyst typically passes backwards and upwards through the carotid bifurcation and ends at the pharyngeal constrictor muscles, a course which brings it into intimate association with the hypoglossal, glossopharyngeal and spinal accessory nerves. Great care must be taken to avoid damage to these nerves during surgical removal of a branchial cyst.

Branchial fistulae represent a persistent connection between the second branchial pouch and the cervical sinus. The fistula typically presents as a small pit adjacent to the anterior border of the lower third of sternocleidomastoid, which may weep saliva and become intermittently infected. Excision involves following the tract of the fistula up the neck – often through the carotid bifurcation – and into the distal tonsillar fossa where it opens into the pharynx.

Branchial cysts, sinuses and fistulae are thought to arise from inclusions of salivary gland tissue in lymph nodes: they may also occur around the parotid gland.

MUSCLES OF THE ANTERIOR TRIANGLE OF THE NECK

Apart from the superficial neck muscles already described, the anterior triangle contains two of the suprahyoid muscles, namely digastric and stylohyoid, and the four infrahyoid strap muscles (Fig. 28.5). The other suprahyoid muscles, namely mylohyoid and geniohyoid, are described with the floor of the mouth on page 501.

Digastric

Digastric has two bellies and lies below the mandible, extending from the mastoid process to the chin (Fig. 28.5). The posterior belly, which is longer than the anterior, is attached in the mastoid notch of the temporal bone, and passes downwards and forwards. The anterior belly is attached to the digastric fossa on the base of the mandible near the midline, and slopes downwards and backwards. The two bellies meet in an intermediate tendon which perforates stylohyoid and runs in a fibrous sling attached to the body and greater cornu of the hyoid bone and is sometimes lined by a synovial sheath. The two bellies of digastric mark out the borders of the submandibular triangle.

Variations

Digastric may lack the intermediate tendon and is then attached midway along the body of the mandible. The posterior belly may be augmented by a slip from the styloid process or arise wholly from it. The anterior belly may cross the midline and it is not uncommon for it to fuse with mylohyoid.

Relations

Superficial to digastric are platysma, sternocleidomastoid, splenius capitis, longissimus capitis and stylohyoid, the mastoid process, the retromandibular vein and the parotid and submandibular salivary glands. Mylohyoid is medial to the anterior belly, and hyoglossus, superior oblique and rectus capitis lateralis, the transverse process of the atlas vertebra, the accessory nerve, internal jugular vein, occipital artery, hypoglossal nerve, internal and external carotid, facial and lingual arteries are all medial to the posterior belly.

Vascular supply

The posterior belly is supplied by the posterior auricular and occipital arteries. The anterior belly of digastric receives its blood supply chiefly from the submental branch of the facial artery.

Innervation

The anterior belly of digastric is supplied by the mylohyoid branch of the inferior alveolar nerve, and the posterior belly is supplied by the facial nerve. The different innervation of the two parts reflects their separate derivations from the mesenchyme of the first and second branchial arches.

Actions

Digastric depresses the mandible and can elevate the hyoid bone. The posterior bellies are especially active during swallowing and chewing.

Stylohyoid

Stylohyoid arises by a small tendon from the posterior surface of the styloid process, near its base. Passing downwards and forwards, it inserts into the body of the hyoid bone at its junction with the greater cornu (and just above the attachment of the superior belly of omohyoid). It is perforated near its insertion by the intermediate tendon of digastric (Fig. 28.5). The muscle may be absent or double. It may lie medial to the external carotid artery and may end in the suprahyoid or infrahyoid muscles.

Stylohyoid ligament

The stylohyoid ligament is a fibrous cord extending from the tip of the styloid process to the lesser cornu of the hyoid bone. It gives attachment to the highest fibres of the middle pharyngeal constrictor and is intimately related to the lateral wall of the oropharynx. Below, it is overlapped by hyoglossus. The ligament is derived from the cartilage of the second branchial arch, and may be partially calcified.

INFRAHYOID MUSCLES

The infrahyoid muscles are organized so that sternohyoid and omohyoid lie superficially and sternothyroid and thyrohyoid lie more deeply (Fig. 28.5). The muscles are involved in movements of the hyoid bone and thyroid cartilage during vocalization, swallowing and mastication and are mainly innervated from the ansa cervicalis.

Sternohyoid

Sternohyoid is a thin, narrow strap muscle that arises from the posterior surface of the medial end of the clavicle, the posterior sternoclavicular ligament and the upper posterior aspect of the manubrium (Fig. 28.5). It ascends medially and is attached to the inferior border of the body of the hyoid bone. Inferiorly, there is a considerable gap between the muscle and its contralateral fellow, but the two usually come together in the middle of their course, and are contiguous above this. Sternohyoid may be absent or double, augmented by a clavicular slip (cleidohyoid), or interrupted by a tendinous intersection.

Omohyoid

Omohyoid consists of two bellies united at an angle by an intermediate tendon (Fig. 28.5). The inferior belly is a flat, narrow band, which inclines forwards and slightly upwards across the lower part of the neck. It arises from the upper border of the scapula, near the scapular notch, and occasionally from the superior transverse scapular ligament. It then passes behind sternocleidomastoid and ends there in the intermediate tendon. The superior belly begins at the intermediate tendon, passes almost vertically upwards near the lateral border of sternohyoid and is attached to the lower border of the body of the hyoid bone lateral to the insertion of sternohyoid. The length and form of the intermediate tendon varies, although it usually lies adjacent to the internal jugular vein at the level of the arch of the cricoid cartilage. The angulated course of the muscle is maintained by a band of deep cervical fascia, attached below to the clavicle and the first rib, which ensheathes the tendon. A variable amount of skeletal muscle may be present in the fascial band; either belly may be absent or double; the inferior belly may be attached directly to the clavicle and the superior is sometimes fused with sternohyoid.

Sternothyroid

Sternothyroid is shorter and wider than sternohyoid, and lies deep and partly medial to it (Fig. 28.5). It arises from the posterior surface of the manubrium sterni inferior to the origin of sternohyoid and from the posterior edge of the cartilage of the first rib. It is attached above to the oblique line on the lamina of the thyroid cartilage, where it delineates the upward extent of the thyroid gland. In the lower part of the neck the muscle is in contact with its contralateral fellow, but the two diverge as they ascend.

Thyrohyoid

Thyrohyoid is a small, quadrilateral muscle that may be regarded as an upward continuation of sternothyroid (Fig. 28.5). It arises from the oblique line on the lamina of the thyroid cartilage, and passes upwards to attach to the lower border of the greater cornu and adjacent part of the body of the hyoid bone.

ANTERIOR VERTEBRAL MUSCLES

The anterior vertebral group of muscles are longi colli and capitis, and recti capitis anterior and lateralis (Fig. 28.6), all of which are flexors of the head and neck to varying degrees. Together with the lateral vertebral muscles they form the prevertebral muscle group.

Fig. 28.6 The anterior and lateral vertebral muscles. Scalenus anterior and longus capitis have been removed on the right side.

Longus capitis

Longus capitis (Fig. 28.6) has a narrow origin from tendinous slips from the anterior tubercles of the transverse processes of the third, fourth, fifth and sixth cervical vertebrae and becomes broad and thick above, where it is inserted into the inferior surface of the basilar part of the occipital bone.

Longus colli

Longus colli (Fig. 28.6) is applied to the anterior surface of the vertebral column, between the atlas and the third thoracic vertebra. It can be divided into three parts which all arise by tendinous slips. The inferior oblique part is the smallest, running upwards and laterally from the fronts of the bodies of the first two or three thoracic vertebrae to the anterior tubercles of the transverse processes of the fifth and sixth cervical vertebrae. The superior oblique part passes upwards and medially from the anterior tubercles of the transverse processes of the third, fourth and fifth cervical vertebrae to be attached by a narrow tendon to the anterolateral surface of the tubercle on the anterior arch of the atlas. The vertical intermediate part ascends from the fronts of the bodies of the upper three thoracic and lower three cervical vertebrae to the fronts of the bodies of the second, third and fourth cervical vertebrae.

LATERAL VERTEBRAL MUSCLES

Scaleni anterior, medius and posterior extend obliquely between the upper two ribs and the cervical transverse processes. Scalenus minimus (pleuralis) is associated with the suprapleural membrane and cervical pleura, and is described in Chapter 57.

Scalenus anterior

Scalenus anterior lies at the side of the neck deep (posteromedial) to sternocleidomastoid (Fig. 28.5; see also Fig. 28.18). Above, it is attached by musculotendinous fascicles to the anterior tubercles of the transverse processes of the third, fourth, fifth and sixth cervical vertebrae. These converge, blend and descend almost vertically, to be attached by a narrow, flat tendon to the scalene tubercle on the inner border of the first rib, and to a ridge on the upper surface of the rib anterior to the groove for the subclavian artery.

Fig. 28.18 The root of the neck. Note the sympathetic chain, the middle and inferior cervical sympathetic ganglia, the trunks of the brachial plexus, the phrenic nerve and the vertebral artery.

(From Sobotta 2006.)

Relations

Scalenus anterior forms an important landmark in the root of the neck, because the phrenic nerve passes anterior to it, the subclavian artery posterior to it, and the brachial plexus lies at its lateral border. The clavicle, subclavius, sternocleidomastoid and omohyoid, lateral part of the carotid sheath, transverse cervical, suprascapular and ascending cervical arteries, subclavian vein, prevertebral fascia and phrenic nerve are all anterior to scalenus anterior. Posteriorly are the suprapleural membrane, pleura, roots of the brachial plexus and the subclavian artery: the latter two separate scalenus anterior from scalenus medius. The proximity of the muscle to the brachial plexus, subclavian artery and vein can give rise to compression syndromes. Below its attachment to the sixth cervical vertebra, the medial border of the muscle is separated from longus colli by an angular interval in which the vertebral artery and vein pass to and from the foramen transversarium of the sixth cervical vertebra. The inferior thyroid artery crosses this interval from the lateral to the medial side near its apex. The sympathetic trunk and its cervicothoracic ganglion are closely related to the posteromedial side of this part of the vertebral artery. On the left side the thoracic duct crosses the triangular interval at the level of the seventh cervical vertebra and usually comes into contact with the medial edge of scalenus anterior. The musculotendinous attachments of scalenus anterior to anterior tubercles are separated from those of longus capitis by the ascending cervical branch of the inferior thyroid artery.

Innervation

Scalenus anterior is innervated by branches from the ventral rami of the fourth, fifth and sixth cervical spinal nerves.

Actions

Acting from below, scalenus anterior bends the cervical portion of the vertebral column forwards and laterally and rotates it towards the opposite side. Acting from above, the muscle helps to elevate the first rib.

Scalenus medius

Scalenus medius, the largest and longest of the scaleni, is attached above to the transverse process of the axis and the front of the posterior tubercles of the transverse processes of the lower five cervical vertebrae (Fig. 28.6). It frequently extends upwards to the transverse process of the atlas. Below it is attached to the upper surface of the first rib between the tubercle of the rib and the groove for the subclavian artery.

Relations

The anterolateral surface of the muscle is related to sternocleidomastoid (Fig. 28.5). It is crossed anteriorly by the clavicle and omohyoid, and it is separated from scalenus anterior by the subclavian artery and ventral rami of the cervical spinal nerves. Levator scapulae and scalenus posterior lie posterolateral to it. The upper two roots of the nerve to serratus anterior and the dorsal scapular nerve (to the rhomboids) pierce the muscle and appear on its lateral surface.

Innervation

Scalenus medius is supplied by branches from the ventral rami of the third to eighth cervical spinal nerves.

Actions

Acting from below, scalenus medius bends the cervical part of the vertebral column to the same side. Acting from above, it helps to raise the first rib. The scalene muscles, particularly scalenus medius, are active during inspiration, even during quiet breathing in the erect attitude.

Scalenus posterior

Scalenus posterior is the smallest and most deeply situated of the scalene muscles (Fig. 28.6). It passes from the posterior tubercles of the transverse processes of the fourth, fifth, and sixth cervical vertebrae to the outer surface of the second rib, behind the tubercle for serratus anterior, where it is attached by a thin tendon.

Scalenus posterior is occasionally blended with scalenus medius. The scalene muscles vary a little in the number of vertebrae to which they are attached, in their degree of separation, and their segmental innervation.