Neck Anatomy
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.
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 inferiorly, 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.
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).
This
triangle is bounded anteriorly by sternocleidomastoid and posteriorly by
trapezius. Inferiorly, the upper border of the clavicle 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 subclavian 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.
Contents
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.
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.
Roof
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).
Floor
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.
Contents
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.
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.
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.
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).
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.
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.
