Nerve supply. By the spinal part of the accessory nerve, from a branch which leaves the nerve proximal to its point of entry into the muscle. The pathway for innervation by the cerebral cortex of the anterior horn cells of the segments concerned (mostly C2 and 3) is disputed; projection to the muscle from either or both hemispheres has been described. Branches from the cervical plexus (C2, 3) carrying proprioceptive fibres enter the muscle directly or by joining the accessory nerve.
Action. Contraction of one muscle tilts the head towards the ipsilateral shoulder, and rotates the head and face to the opposite side. Both muscles acting together from below draw the head forwards. With the head fixed, the muscles can assist in raising the roof of the thorax in forced inspiration.
Test. The face is turned to the opposite side against resistance and the muscle palpated.
Posterior triangle
This is an area enclosed between the sternocleidomastoid and trapezius muscles. Its apex lies high up at the back of the skull on the superior nuchal line, where there is a small gap between the attachments of the two muscles. Its base is the middle third of the clavicle at the side of the root of the neck. Its roof is formed by the investing layer of deep cervical fascia. Its floor consists of the prevertebral fascia lying on, from above downwards, splenius, levator scapulae and scalenus medius. Depending on the size of the sternocleidomastoid and the degree of depression of the shoulder, scalenus anterior and the first digitation of serratus anterior may contribute to the floor, and at the apex of the triangle, splenius may be low enough to expose a little of semispinalis capitis.
Although the subclavian artery, the three trunks of the brachial plexus and branches of the cervical plexus are deep to the prevertebral fascia, they are listed as contents of the posterior triangle; in operations on the triangle all these structures are safe provided the prevertebral fascia is left intact.
The pulsation of the subclavian artery can be felt by pressing downwards behind the clavicle at the posterior border of sternocleidomastoid.
The cutaneous branches of the cervical plexus pierce the investing fascia at the posterior border of sternocleidomastoid. The cervical branches to trapezius pass across the floor of the triangle deep to the prevertebral fascia.
Lying between the roof and floor are the lymph nodes of the posterior triangle. Two or three occipital nodes lie in the subcutaneous tissue at the apex and several supraclavicular nodes lie above the clavicle; the latter are really outlying members of the lower group of deep cervical nodes (see p.
410).
The accessory nerve emerges from sternocleidomastoid, about a third of the way or a little lower down its posterior border. It passes downwards and backwards, with a characteristic wavy course adherent to the inner surface of the fascia of the roof of the triangle, to disappear beneath the anterior border of trapezius, about a third of the way from its lower end and 3–5 cm above the clavicle. These points of reference to the borders of sternocleidomastoid and trapezius enable the surface marking of the accessory nerve in the posterior triangle, where it is particularly liable to injury in operations involving the removal of lymph nodes, one or two of which may lie in contact with the nerve. More proximally, the nerve lies in front of the transverse process of the atlas (palpable between the mastoid process and mandibular ramus) and it enters the substance of sternocleidomastoid between the upper
two quarters of the muscle.
The inferior belly of omohyoid crosses the lower medial part of the triangle and is kept in place by its sling of investing fascia. Deep to the omohyoid are the transverse cervical and suprascapular vessels, just above the clavicle. The external jugular vein pierces both split layers of the lower part of investing fascia to enter the posterior triangle on its way to the subclavian vein, which itself is too low to be a content of the triangle; the wall of the vein is adherent to the fascia as it passes through.
Cervical plexus
The cervical plexus (Fig. 6.3) is formed by loops between the anterior rami of the upper four cervical nerves, after each has received a grey ramus communicans from the superior cervical ganglion. It lies in series with the brachial plexus, on the scalenus medius, behind the prevertebral fascia. It is covered by the upper part of sternocleidomastoid, and does not lie in the posterior triangle. The upper three cervical nerves have meningeal branches for the posterior cranial fossa. C1 fibres ascend with the hypoglossal nerve, C1 and 2 fibres ascend with the vagus nerve and C2 and 3 fibres ascend through the foramen magnum.
Figure 6.3 Right cervical plexus.
Muscular branches. Muscular branches are given off segmentally to the prevertebral muscles (longus capitis, longus colli and the scalenes). Other muscular branches are:
• A branch from C1 to the hypoglossal nerve, by which the fibres are carried to the superior root of the ansa cervicalis and the nerves to thyrohyoid and geniohyoid.
• Branches from C2 and 3 to sternocleidomastoid, and from C3 and 4 to trapezius. These fibres are mainly proprioceptive, but occasionally the whole of trapezius is not paralysed when the accessory nerve is damaged, as some of the cervical fibres may be motor.
• The inferior root of the ansa cervicalis is formed by union of a branch each from C2 and C3.
The nerve spirals around the lateral side of the internal jugular vein and descends to join the superior root (C1) at the ansa (see p. 344).
• The phrenic nerve is formed mainly from C4 with contributions from C3 and C5 and runs down vertically over the obliquity of the scalenus anterior muscle, passing from lateral to medial borders, beneath the prevertebral fascia, lateral to the ascending cervical branch of the inferior thyroid artery. It passes behind the subclavian vein into the mediastinum (see p. 195). It may be
joined below the vein by a branch (the accessory phrenic nerve) from the nerve to subclavius; this branch may descend in front of the subclavian vein. The phrenic nerve is one of the most important in the body, being the sole motor supply to its own half of the diaphragm (see p. 187), and it also has an extensive afferent distribution, not only to the diaphragm but to the pericardium, pleura and peritoneum (see pp. 197, 212 and 238).
Cutaneous branches. Cutaneous branches of the plexus (Fig. 6.4) supply the front and sides of the neck and contribute to the supply of the scalp, face and chest.
Figure 6.4 Right lower face and posterior triangle of neck. The fat in the lower part of the triangle overlies deeper structures. The accessory nerve has been depicted taking a lower than usual course through the posterior triangle. The transverse cervical and suprascapular arteries are usually deep to the inferior belly of omohyoid.
The lesser occipital nerve (C2) is a slender branch that hooks around the accessory nerve and runs up along the posterior border of sternocleidomastoid to supply the posterior part of the upper neck and adjacent scalp behind the auricle. It may contribute to the supply of the auricle.
The great auricular nerve (C2, 3, mostly 2) is a large trunk passing almost vertically upwards over sternocleidomastoid; it is distributed to an area of skin on the face over the angle of the mandible and the parotid gland and to the parotid fascia. It also supplies the skin of the auricle over the whole of its cranial surface and on the lower part of its lateral surface below the external acoustic meatus, and skin over the mastoid region. Branches passing deep to the parotid gland supply the deep layer of the parotid fascia.
The transverse cervical nerve (C2, 3) curves round the posterior border of sternocleidomastoid, perforates the investing fascia and divides into ascending and descending branches that innervate the skin of the front of the neck from chin to sternum. The ascending branch communicates with the cervical branch of the facial nerve.
The supraclavicular nerve (C3, 4, but mostly 4) emerges with the other three nerves at the posterior border of sternomastoid and soon divides into several branches. They are distributed in three main
groups (see Fig. 1.8, p. 12). The medial group supply the skin as far down as the sternal angle. The intermediate group proper pass anterior to the clavicle and supply skin as far down as the second rib.
The lateral group cross the acromion to supply skin halfway down the deltoid muscle, and pass posteriorly to supply skin as far down as the spine of the scapula.
Dermatomes of the neck
In addition to the cutaneous branches of the cervical plexus described above, which supply the anterior and lateral skin of the neck, the greater occipital and third occipital nerves from posterior rami of C2 and C3 respectively provide sensory fibres for the back of the neck (Fig. 6.15). The first cervical nerve does not supply any skin. C2 supplies most of the superior part of the neck, extending into the occipital region of the scalp and forwards to the auricle and the face over the parotid gland.
C3 supplies the cylindrical part of the neck, C4 extends over the clavicle to the sternal angle, across the top of the shoulder and down to the scapular spine at the back. There is much overlap across dermatome boundary lines.
Anterior triangle
Beneath the investing layer of deep cervical fascia, between the mandible and the manubrium of the sternum, are longitudinal muscles supplied by the anterior rami of the upper three cervical nerves.
They lie above or below the hyoid bone and there are four muscles in each group. The suprahyoid muscles comprise digastric, stylohyoid, mylohyoid and geniohyoid; the mylohyoids of each side unite to form the floor of the mouth, with the digastrics and stylohyoids superficial (below) and geniohyoids deep (above) to them. The infrahyoid muscles are sternohyoid and omohyoid, lying side by side in the same plane, and more deeply a wider sheet of muscle attached to the thyroid cartilage, namely thyrohyoid and sternothyroid. The last four are called the ‘strap muscles’ from their flat shape.
Suprahyoid muscles
Digastric
This arises as the posterior belly, from the digastric groove on the medial surface of the base of the mastoid process (Fig. 6.35). The triangular fleshy belly tapers down to the intermediate tendon, which is held beneath a fibrous sling attached to the junction of the body and the greater horn of the hyoid bone. The tendon is lubricated by a synovial sheath within the fibrous sling. The bifurcated tendon of insertion of stylohyoid which embraces the tendon plays no part in holding it down. The anterior belly lies on the inferior surface of mylohyoid, and connects the intermediate tendon to the digastric fossa on the inner surface of the mandible near the midline.
Figure 6.35 External surface of the central and left part of the base of the skull.
Nerve supply. The posterior belly is supplied by the facial nerve, by a branch arising between the stylomastoid foramen and the parotid gland, and the anterior belly by the nerve to mylohyoid.
Action. To depress and retract the chin, and to assist the lateral pterygoid in opening the mouth.
Stylohyoid
This arises from the back of the styloid process, high up near the base of the skull, and slopes down along the upper border of digastric. Its lower end divides to embrace the digastric tendon and is inserted by two slips into the junction of the greater horn and body of the hyoid bone.
Nerve supply. By the facial nerve, by a branch from that to the posterior belly of digastric.
Action. To retract and elevate the hyoid bone when swallowing.
Mylohyoid
The muscles of each side unite to make a thin sheet forming the ‘diaphragm’ of the floor of the mouth (Fig. 6.6). Each arises from the whole length of the mylohyoid line of its own side on the inner aspect of the mandible from as far back as medial to the third molar tooth to below the mental spines (see Fig. 8.5B, p. 510). The two muscles slope downwards towards each other, and the posterior quarter of each is inserted into the anterior surface of the body of the hyoid bone. In front of this the anterior three-quarters of each interdigitate in a midline raphe which extends from the chin to the hyoid bone.
Nerve supply. By its own nerve, a branch of the inferior alveolar (from the mandibular division of the trigeminal nerve), which arises just before the parent nerve enters the mandibular foramen, pierces the sphenomandibular ligament and runs forward on the inferior surface of the mylohyoid, supplying it and the anterior belly of the digastric.
Action. It forms a mobile but stable floor of the mouth. The two muscles together form a gutter;
contraction makes the gutter more shallow, thus elevating the tongue and the hyoid bone as when swallowing or protruding the tongue.
Geniohyoid
This slender muscle extends from the inferior mental spine (genial tubercle) of the mandible (see Fig.
8.5B, p. 510) to the upper border of the body of the hyoid bone (see Fig. 8.6, p. 512). The two muscles lie side by side between the mylohyoids and the base of the tongue (genioglossus), on the floor of the mouth.
Nerve supply. By a branch from the hypoglossal nerve, consisting of fibres from the C1 nerve and not from the hypoglossal nucleus.
Action. To protract and elevate the hyoid bone in swallowing, or if the hyoid is fixed, to depress the mandible.
Infrahyoid muscles
Sternohyoid
This flat strap of muscle is attached to the back of the upper part of the manubrium and the adjoining sternoclavicular joint and clavicle. Its upper attachment is to the lower border of the body of the hyoid bone. The two muscles lie edge to edge at the hyoid bone, but diverge from each other below.
Nerve supply. By a branch from the ansa cervicalis which enters the lower part of the muscle.
Omohyoid
This flat strap of muscle lies edge to edge with sternohyoid at its attachment to the lateral part of the inferior border of the hyoid bone (Fig. 6.5). As it descends it diverges somewhat from the sternohyoid and, passing deep to sternocleidomastoid, it comes to lie over the carotid sheath. Where it lies over the internal jugular vein, the muscle fibres are replaced by a flat tendon, a useful guide at operation to the underlying vein. A change of direction now occurs, and the inferior belly runs almost horizontally just above the level of the clavicle to pass back to its attachment to the upper border of the scapula and the transverse scapular ligament. The intermediate tendon and supraclavicular portion of the muscle are bound down close to the clavicle in a fascial sling derived from the deep layer of the investing layer of deep cervical fascia (see Fig. 2.2, p. 39), which results in the angulated course of the muscle.
Figure 6.5 Thyroid gland and the front of the neck.
Nerve supply. The superior root of the ansa cervicalis supplies the superior belly and the ansa supplies the inferior belly.
Thyrohyoid
This is a broader and shorter muscle that lies under cover of the upper ends of sternohyoid and omohyoid. It arises from the greater horn of the hyoid bone, and is inserted into the oblique line of the thyroid cartilage alongside sternothyroid.
Nerve supply. By a branch of the hypoglossal nerve, but the fibres are all ‘hitch-hiking’ from C1.
Sternothyroid
Broader than sternohyoid and lying deep to it, this muscle is attached lower down than sternohyoid to the posterior surface of the manubrium and the adjacent first costal cartilage. Its upper attachment is to the oblique line of the thyroid cartilage.
Nerve supply. By the ansa cervicalis, which gives a branch to the lower part of the muscle.
Actions of the infrahyoid muscles
They are all depressors of the larynx. Sternothyroid acts directly on the thyroid cartilage, the others act indirectly via the hyoid bone. Depression of the larynx increases the volume of the resonating chambers during phonation and thus affects the quality of the voice. The infrahyoid muscles also oppose the elevators of the larynx (mylohyoid, palatopharyngeus, stylopharyngeus, salpingopharyngeus), enabling them to act progressively and gradually. The infrahyoid muscles prevent ascent of the hyoid bone when the digastric and geniohyoid lower the mandible.
Submandibular gland
The submandibular gland, mixed mucous and serous in type, consists of a large superficial part and a
small deep part which are continuous with one another round the free posterior margin of mylohyoid (Fig 6.24).
The superficial part (Fig. 6.6) has three surfaces: lateral, inferior and medial. The lateral surface lies against the submandibular fossa of the mandible (see Fig. 8.5B, p. 510), overlapping the front of the medial pterygoid insertion and being deeply grooved posteriorly by the facial artery which hooks under the mandible to reach the face at the front of the masseter muscle. The inferior or superficial surface is covered by skin, platysma and the investing fascia and is crossed by the facial vein and the cervical branch of the facial nerve, and sometimes by the marginal mandibular branch of the facial nerve (see p. 353), the nerves lying outside the investing fascia. Submandibular lymph nodes lie in contact with the surface of the gland and within its substance, hence the need to remove the gland as well as nodes in the operation of radical neck dissection. The medial surface lies against the mylohyoid, and behind it on the hyoglossus, lingual nerve, hypoglossal nerve and its accompanying veins. The facial artery is at first deep to the gland, and then grooves the posterosuperior part as it hooks over the top of the gland on to its lateral surface.
Figure 6.6 Left submandibular region.
T he deep part of the gland extends forwards for a variable distance, between mylohyoid and hyoglossus, below the lingual nerve and above the hypoglossal nerve.
The submandibular duct (of Wharton) is 5 cm long (the same length as the parotid duct) and emerges from the medial surface of the superficial part of the gland near the posterior border of mylohyoid. It runs with the deep part, forwards and slightly upwards, first between mylohyoid and hyoglossus, and then between the sublingual gland and genioglossus, to open into the floor of the mouth on the sublingual papilla beside the frenulum of the tongue. As it lies on hyoglossus, the duct is crossed laterally by the lingual nerve which then turns under the duct to pass medially to the tongue (Fig. 6.7).