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Neck Anatomy

Neck Anatomy
Fascial layers
These are complex and are divided into four components: investing fascia, prevertebral fascia, pretracheal fascia and carotid sheaths (Fig. 7.5). The investing fascia is analogous to deep fascia in the limbs. Deep to the skin and superficial tissues, it surrounds the neck, extending from the pectoral girdle below to the base of the skull and mandible above, and splits to enclose trapezius and sternocleidomastoid. Superiorly, the investing fascia is attached to the superior nuchal lines and to the mastoid processes. Between the mastoid process and the angle of the mandible the fascia encloses the parotid gland. Its superficial layer passes superiorly over the surface of the gland to attach to the zygomatic arch; on its deep surface the fascia is thickened to form the stylomandibular ligament. Inferiorly, the investing fascia is attached to the spine of the scapula, the acromion, the superior border of the clavicle, and the manubrium.

Transverse section at the level of the fourth cervical vertebra showing the layers of cervical fascia (green lines).

The prevertebral fascia encloses the vertebral column, pre-and postvertebral muscles and origins of the cervical and brachial plexuses. Superiorly, it attaches to the base of the skull and infe- riorly, it extends into the superior mediastinum.
The pretracheal fascia covers the anterior and lateral aspects of the trachea and larynx, limited superiorly by the hyoid bone and by the oblique lines on the thyroid cartilage. It splits to enclose the thyroid gland and inferiorly fuses with the adventitia of the aortic arch in the superior mediastinum. Posterolaterally on each side, the pretracheal fascia blends with the carotid sheath.
Each of the two carotid sheaths contains a common, an internal and part of an external carotid artery, a vagus nerve and an internal jugular vein. The sheaths are attached to the base of the skull around the jugular and carotid foramina and pass inferiorly to the aortic arch and brachiocephalic veins to fuse with the adventitia covering these vessels.
Infection can track superiorly and inferiorly between these fascial layers.
For purposes of anatomical description, the superficial part of each side of the neck is divided into anterior and posterior triangles separated by sternocleidomastoid.

Oblique view of the neck showing sternocleidomastoid after removal of the investing fascia covering sternocleidomastoid and trapezius.

Sternocleidomastoid (sternomastoid)
This muscle passes obliquely upwards and backwards from the manubrium and the medial end of the clavicle to the mastoid process and superior nuchal line of the skull (Fig. 7.6). One sternocleidomastoid acting alone turns the head towards the opposite shoulder, whereas acting together both muscles protrude the head forwards. Sternocleidomastoid is innervated by the spinal part of the accessory nerve (XI).

Posterior triangle of neck
This triangle is bounded anteriorly by sternocleidomastoid and posteriorly by trapezius. Inferiorly, the upper border of the clavi- cle forms the base, while superiorly, the attachments of sternoclei-domastoid and trapezius converge onto the superior nuchal line to form the apex of the triangle. The posterior triangle does not lie in a flat plane but spirals so that the inferior portion is directed anteriorly, while the apex faces posterolaterally.
The roof of the triangle is formed by the investing fascia, which spans the interval between trapezius and sternocleidomastoid. The external jugular vein initially lies vertically over the sternocleidomastoid just beneath the skin and then passes onto the roof of the lower part of the triangle. The vein pierces the roof just above the clavicle to enter the triangle and drain into the subcla- vian vein. Cutaneous branches of the cervical plexus also lie superficial to the roof of the triangle (Fig. 7.7).
The floor of the posterior triangle is formed by the prevertebral fascia covering the paravertebral muscles, which are, from above downwards, splenius capitis, levator scapulae and scalenus posterior, medius and anterior (Fig. 7.8). Deep to the prevertebral fascia are the subclavian artery, the three trunks of the brachial plexus and the cervical plexus. Continuing laterally to reach the axilla, the brachial plexus and the subclavian artery are enclosed in a prolongation of the prevertebral fascia, the axillary sheath. Injection of local anaesthetic inside the axillary sheath blocks sensation from the upper limb. Deep to the scalene muscles, subclavian vessels and brachial plexus are the pleura and apex of the lung. These are vulnerable to accidental penetration during cannulation of the subclavian vessels.
Boundaries and roof of the posterior triangle of the neck. The external jugular vein and cutaneous branches of the cervical plexus, lying superficial to the roof, are also present.

Between the floor and the roof of the triangle lie the contents (Fig. 7.9), which include a number of vascular structures, the spinal part of the accessory (XI) nerve, components of the cervical plexus and supraclavicular and occipital lymph nodes. The spinal part of the accessory nerve passes obliquely across the triangle from beneath the posterior border of sternocleidomastoid to leave deep to the anterior border of trapezius. It supplies both of these muscles. In the lower part of the triangle the inferior belly of omohyoid passes towards its scapular attachment. Two branches of the thyrocervical trunk, namely the transverse cervical and suprascapular arteries, also pass laterally across the triangle to the scapula. At the apex of the triangle the occipital artery emerges to supply part of the scalp. The subclavian vein is sometimes visible just above the clavicle.

Cervical plexus
The cervical plexus is formed from the anterior rami of the first four cervical spinal nerves and supplies the paravertebral muscles with segmental branches. It provides a branch from C1 to the hypoglossal nerve and branches from C2 and C3, which all contribute to the ansa cervicalis. The phrenic nerve, the principal innervation of the diaphragm, is formed from C3, C4 and C5 and runs vertically downwards on the anterior surface of scalenus anterior, behind the prevertebral fascia.
Sensory branches from the cervical plexus (Fig. 7.7) pass through the triangle and emerge by piercing the roof near the midpoint of the posterior border of sternocleidomastoid. These convey sensation from the neck, the lower part of the face and pinna, the side of the scalp and the upper part of the thoracic wall. The lesser occipital nerve (C2) ascends along the posterior border of sternocleidomastoid and supplies the side of the occipital region of the scalp. The great auricular nerve (C2 & C3) runs vertically upwards across sternocleidomastoid and conveys sensation from the lower part of the pinna and the skin over the parotid gland. The transverse cervical nerve (C2 & C3) passes horizontally, supplying the skin over sternocleidomastoid and the anterior triangle. Finally, the supraclavicular nerves (C3 & C4) radiate downwards to convey sensation from skin over the upper part of the anterior thoracic wall and the shoulder region.

Floor of the posterior triangle from which the prevertebral fascia has been removed. The inferior belly of the omohyoid muscle, one of the contents of the triangle, is still present.

Anterior triangle of neck
By convention, the two anterior triangles of the neck extend medially to the midline. Posterolaterally, each triangle is bounded by the anterior border of sternocleidomastoid and superiorly by the inferior border of the mandible. That part of the triangle above the hyoid bone will be described with the mylohyoid and related structures.
The roof of the anterior triangle (Fig. 7.10) is formed by the investing fascia of the neck. Superficial to the fascia are platysma (p. 336) and the anterior jugular vein. This vessel pierces the roof and passes deep to sternocleidomastoid to drain into the external jugular vein just before its termination in the subclavian vein. The cutaneous innervation of the skin over the triangle has already been described (p. 326).
The floor of the anterior triangle is composed of the pretracheal fascia and posterolaterally, the carotid sheath. The thyroid gland (Fig. 7.13) is enclosed by the pretracheal fascia, while the larynx and trachea lie deep to it. Laterally, the carotid arteries, internal jugular vein and vagus (X) nerve all lie within the carotid sheath.
The contents of the anterior triangle (Fig. 7.11) comprise infrahyoid or strap muscles (sternohyoid, sternothyroid, thyrohyoid and omohyoid) and their immediate nerve supply. The most superficial muscle, sternohyoid, is attached inferiorly to the deep surface of the manubrium and superiorly to the lower border of the body of the hyoid bone. Deep to sternohyoid are both ster- nothyroid and thyrohyoid. Sternothyroid extends from the manubrium to the oblique line on the lamina of the thyroid cartilage. In the same plane, thyrohyoid runs from the thyroid cartilage to the inferior edge of the body of the hyoid bone. Omohyoid consists of two bellies linked by an intermediate tendon. The inferior belly is attached to the suprascapular ligament and the adjacent part of the scapula. It crosses the posterior triangle and ends deep to sternocleidomastoid in the intermediate tendon, which is anchored to the clavicle by a loop of investing fascia. The superior belly continues upwards to its attachment on the lower border of the hyoid bone lateral to the other muscles.
All four muscles are supplied segmentally by branches from the first three cervical spinal nerves. Thyrohyoid is supplied by fibres from C1 that have travelled with the hypoglossal (XII) nerve; the remaining muscles are supplied via the ansa cervicalis. The infrahyoid muscles depress the hyoid bone and the larynx.

Boundaries and roofs of both anterior triangles of the neck. The midline is the division between the two triangles.

Ansa cervicalis
Under cover of sternocleidomastoid two nerves, the roots of the ansa, unite to form a loop, the ansa cervicalis (Fig. 7.12), which provides the motor supply to the strap muscles. The superior root (descending limb) from the hypoglossal nerve consists solely of C1 fibres and descends to join the inferior root, C2 and C3 fibres from the cervical plexus, to form the ansa.

Thyroid gland
The thyroid gland (Fig. 7.13) is a vascular endocrine gland enclosed by the pretracheal fascia and closely applied to the anterior and lateral surfaces of the trachea. The fascia links the gland to the larynx, so that during swallowing both structures are elevated simultaneously. The two lateral lobes of the gland are joined across the midline by a narrow isthmus at the level of the third tracheal ring. A single pyramidal lobe is often present and projects upwards from the isthmus. Each lateral lobe is pear-shaped with its superior extremity reaching the oblique line on the thyroid cartilage, while its lower pole lies at the level of the fifth tracheal ring.
Lying anterior to the isthmus of the gland are the sternothyroid muscles and the anterior jugular veins. The lateral lobes are covered anterolaterally by the other infrahyoid muscles and the anterior borders of the sternocleidomastoid muscles. Posterolaterally lie the carotid sheaths, while posteromedially are the trachea, larynx and oesophagus. In the interval between the oesophagus and trachea the recurrent laryngeal nerves course upwards towards the larynx where they are vulnerable during thyroid or parathyroid surgery. A superior and an inferior parathyroid gland are embedded in the posterior surface of each lateral lobe. The thyroid gland is a highly vascular organ and is supplied on each side by superior and inferior thyroid arteries. The superior thyroid artery, from the external carotid artery, descends to the upper pole of the gland. The inferior thyroid artery, from the thyrocervical trunk of the subclavian artery, ascends to enter the posterolateral aspect of the gland from behind the carotid sheath. A venous plexus on the surface of the gland drains via superior and middle thyroid veins into the internal jugular veins and via inferior thyroid veins to the left brachiocephalic vein. Lymph drains from the gland into the jugular chain of nodes.

Ansa cervicalis, lying on the internal jugular vein, and its branches to the strap muscles.

Root of neck
The root of the neck is the region immediately above the superior thoracic aperture (p. 322). In the midline are the trachea and oesophagus, descending into the superior mediastinum (Fig. 7.14) between the apices of the lungs, which are each covered with pleura and a suprapleural membrane (Fig. 7.15). The other major structures in the root of the neck are vessels and nerves, which will be described in relation to scalenus anterior and its attachment to the scalene tubercle of the first rib (p. 28).
Root of the neck after removal of the clavicles, pretracheal fascia, thyroid gland, carotid sheaths and left internal jugular vein.

Each subclavian vein (Fig. 7.14) begins at the outer border of the first rib as the continuation of the axillary vein (p. 80). The vessel passes over the rib in front of the attachment of scalenus anterior and receives the external jugular vein from above. The subclavian and internal jugular veins unite at the medial border of scalenus anterior to form the brachiocephalic vein, which enters the thorax anteriorly alongside the trachea. On each side of the neck a major lymphatic trunk terminates by drainage into the angle where the subclavian and internal jugular veins unite. On the left, this lymphatic vessel is the thoracic duct, which arches laterally over the apex of the lung from its position alongside the oesophagus. The duct passes between the carotid sheath and the vertebral vessels, crossing in front of the phrenic nerve and the subclavian artery. The thoracic duct is the ultimate drainage channel for lymph from the lower limbs, pelvis, abdomen, left upper limb and the left side of the thorax, head and neck. On the right side of the neck, the smaller right lymphatic trunk terminates similarly, draining lymph only from the right upper limb and the right side of the thorax, head and neck. Cannulating the thoracic duct allows collection of lymphocytes for immunological investigation and treatment.
The left common carotid and left subclavian arteries emerge from the thorax on the left of the trachea and oesophagus (Fig. 7.14).
On the right, the brachiocephalic trunk divides at the level of the superior thoracic aperture to form the right common carotid and right subclavian arteries (Fig. 7.14). Each common carotid artery ascends into the neck within its sheath and gives no branches before its termination. Each subclavian artery passes laterally over the upper surface of the first rib posterior to scalenus anterior and continues into the axilla as the axillary artery (p. 79).
Three branches of the subclavian artery, internal thoracic, thy- rocervical and vertebral arteries (Figs 7.14 & 7.15), arise medial to scalenus anterior. The internal thoracic artery (p. 34) descends into the thorax to supply the anterior thoracic and abdominal walls. The thyrocervical trunk is short and divides into three branches, the inferior thyroid (p. 329), suprascapular and transverse cervical arteries. The latter two vessels cross the posterior triangle of the neck. The suprascapular artery supplies the scapula and related structures and the transverse cervical artery supplies superficial structures in the posterior part of the neck. The vertebral artery (Fig. 7.15) inclines upwards and backwards medial to scalenus anterior and crosses in front of the transverse process of the seventh cervical vertebra, before continuing superiorly through the foramina transversaria of the upper six cervical vertebrae, to enter the skull through the foramen magnum (p. 376). The costocervical trunk (Fig. 7.15) arises from the subclavian artery behind scalenus anterior and arches backwards over the suprapleural membrane as far as the neck of the first rib, where it divides to form the superior intercostal artery supplying the upper two intercostal spaces (p. 35) and the deep cervical artery, which supplies the muscles of the back of the neck.
Deeper structures of the root of neck, revealed by removal of most of the left vagus and phrenic nerves, the left scalenus anterior, both common carotid arteries and the large veins.

The vagus (X) and phrenic nerves, both sympathetic chains and parts of both brachial plexuses all traverse the root of the neck. Each vagus nerve (Fig. 7.14) descends within the carotid sheath and enters the superior mediastinum between the main arterial and venous structures medial to the phrenic nerve. On the right side of the neck, the recurrent laryngeal nerve arises from the vagus, hooking under the subclavian artery to ascend in the groove formed by the lateral surfaces of the trachea and oesophagus. On the left, the recurrent laryngeal nerve follows a similar course but arises from the vagus in the thorax (p. 62).
The phrenic nerve (Fig. 7.15), formed from the anterior rami of the third, fourth and fifth cervical spinal nerves, passes inferiorly on the anterior surface of scalenus anterior beneath the prevertebral fascia. It leaves the medial side of the muscle near its lower end and enters the thorax between the main arterial and venous structures lateral to the vagus nerve.
The sympathetic trunks (Fig. 7.15), covered by the prevertebral fascia, lie alongside the bodies of the cervical vertebrae. In the neck each trunk bears only three sympathetic ganglia, the superior, middle and inferior. The lowest ganglion fuses frequently with the first thoracic ganglion to form the stellate (cervicothoracic) ganglion. The trunk continues into the thorax in front of the neck of the first rib. The middle and inferior cervical sympathetic ganglia are often linked by a nerve, the ansa subclavia, which curves around the subclavian artery.
The brachial plexus (Fig. 7.14) originates from the anterior rami of the lowest four cervical and first thoracic spinal nerves which, partly covered by scalenus anterior, constitute the roots of the plexus. They combine to form the trunks of the plexus, which emerge from behind the lateral border of the muscle. The plexus continues into the upper limb enclosed with the axillary artery in a prolongation of the prevertebral fascia called the axillary sheath. (A detailed account of the brachial plexus is given on p. 180.)
Scalene muscles
Scalenus anterior (Fig. 7.15) is attached superiorly to the transverse processes of the third, fourth, fifth and sixth cervical verte- brae. Inferiorly, it attaches to the scalene tubercle on the first rib (p. 28). Behind it lie scalenus medius (Fig. 7.15) and scalenus posterior, which arise from the transverse processes of the lower six cervical vertebrae and attach inferiorly to the upper surfaces of the first and second ribs, respectively. These muscles are supplied segmentally by cervical spinal nerves. They elevate the first and second ribs and laterally flex the neck.

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