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Figure 2-16 Anatomic relations of the marginal mandibular branch of the facial nerve. mn, marginal nerve; fv, anterior facial vein; fa, facial artery; SG, submandibular gland.

Sternocleidomastoid Muscle

The sternocleidomastoid muscle is a broad straplike muscle that remains covered by the superficial layer of the deep fascia. The sternocleidomastoid muscle is a key reference in neck surgery. It runs superolaterally from the sternum and clavicle to the lateral surface of the mastoid process (Fig. 2-17), covering the great vessels of the neck and the deep branches of the cervical plexus. The superior end attaches to the lateral surface of the mastoid process, the temporal bone, and the lateral half of the superior nuchal line of the occipital bone. The inferior end has two different heads. The sternal head attaches to the anterior surface of the manubrium of the sternum, lateral to the jugular notch. The clavicular head attaches to the superior surface of the medial third of the clavicle. The sternocleidomastoid muscle is innervated by the spinal accessory nerve and branches of the second and third cervical nerves.

The posterior border of the sternocleidomastoid muscle, the anterior border of the trapezius muscle, and the clavicle define a triangular area known as the posterior triangle of the neck. The need to include this area in the resection is one of the main subjects of controversy in functional and selective neck dissection.

Posterior Triangle of the Neck

The posterior triangle of the neck is bounded by the sternocleidomastoid muscle, the anterior border of the trapezius, and the middle third of the clavicle (Fig. 2-18).

The deep muscular floor of the posterior triangle is formed (superior to inferior) by the splenius capitis, the levator scapulae muscle, and the scalene muscles, covered by the prevertebral part of the deep cervical fascia.

The splenius capitis muscle forms the upper portion of the floor of the posterior triangle. It has its origin in the inferior half of the ligamentum nuchae and spinous processes of the upper six thoracic vertebrae and goes to the lateral aspect of the mastoid and lateral third of the superior nuchal line. It is innervated by dorsal rami of the inferior cervical nerves.

Figure 2-17 Sternocleidomastoid muscle.

Figure 2-18 Contents and boundaries of the posterior triangle of the neck. sc, splenius capitis muscle; ls, levator scapulae muscle; sm, scalene muscles; sa, spinal accessory nerve; cp, deep branches of the cervical plexus; BP, brachial plexus; pn, phrenic nerve; TC, transverse cervical artery; Erb's point.

The levator scapulae muscle arises from the posterior tubercles of the transverse processes of C1 and C4, and goes to the superior part of the medial border of the scapula. It runs medial and inferior to the splenius capitis. Between both muscles there is a "step" that may be identified during the dissection of the posterior triangle of the neck. The levator scapulae muscle is innervated by the dorsal scapular (C5) and cervical spinal (C3 and C4) nerves.

The scalene muscles constitute a triangular block that extends between the first two ribs and the transverse processes of the cervical vertebrae (Fig. 2-19). The scalene group is formed by three different muscles: the anterior, medial, and posterior scalene muscles.

The anterior scalene muscle arises from the anterior tubercles of the transverse processes of the fourth, fifth, and sixth cervical vertebrae and inserts into the scalene tubercle on the upper surface of the body of the first rib.

The medial scalene muscle arises from the lateral edge of the costotransverse lamellae of the lower five cervical vertebrae and, like the anterior scalene, goes to the upper surface of the first rib behind the subclavian groove. The lower insertion usually extends to the second rib. It is innervated by the ventral rami of the forth, fifth, sixth, seventh, and eighth cervical nerves and lies posterior to the ventral roots of the brachial plexus and the third part of the subclavian artery.

The posterior scalene muscle is the smallest and most deeply situated of the three scalene muscles. It arises by short tendons from the posterior tubercles of the transverse processes of the fifth and sixth cervical vertebrae but may have its origin as high as the fourth vertebra or as low as the seventh. It is inserted by a short tendon into the lateral surface of the second rib or, occasionally, into the third rib.

The spinal accessory nerve, the internal jugular vein, and the occipital artery are the most important anatomical landmarks in the upper part of the posterior triangle. The deep branches of the cervical plexus run over the muscular floor of the posterior triangle, deep to the internal jugular vein and sternocleidomastoid muscle.

Figure 2-19 Anterior view of the deep muscles of the neck. as, anterior scalene muscle; ms, middle scalene muscle; ps, posterior scalene muscle; lca, longus capitis muscle; lco, longus colli muscle.

Spinal Accessory Nerve

The eleventh cranial nerve is called the spinal accessory nerve because of if its dual origin, for it has a cranial root and a spinal root. It is exclusively motor.

The spinal or inferior root emerges from the lateral aspect of the spinal cord dorsal to the denticulate ligament. As the fibers emerge, they unite to form an ascending strand, which enters the posterior cranial fossa through the foramen magnum. The strand turns laterally and unites with the cranial part to exit the cranial cavity through the jugular foramen. The fibers in the cranial or superior root unite, and pass laterally in the posterior cranial fossa to form a part of the nerve that pierces the dura mater and enters the jugular foramen.

The superior branch, which contains the fibers of the cranial root, joins the vagus nerve, and the fibers are distributed with the branches of that nerve. The inferior branch, which contains the fibers of the spinal root, runs dorsally and distally covered by the posterior belly of the digastric muscle and the sternocleidomastoid muscle. It crosses to the lateral side of the internal jugular vein and then anterior or posterior to the occipital artery. The nerve has been found to cross anterior to the jugular vein in approximately two thirds of the cases. The point where the nerve crosses the jugular vein can be identified by locating the transverse process of the atlas. After crossing the internal jugular vein, the accessory nerve descends obliquely downward and backward to the upper part of the sternocleidomastoid muscle. It gives off a branch into the deep surface of this muscle and passes downward and backward, either deep to the sternocleidomastoid or through it, to course across the posterior triangle. The nerve leaves the sternocleidomastoid muscle above Erb's point, where the superficial branches of the cervical plexus turn around the posterior border of the muscle (Fig. 2-18). In the posterior triangle the nerve runs a superficial course reaching the anterior border of the trapezius 2 cm above the clavicle.

Phrenic Nerve

The phrenic nerve is an important muscular — deep — branch of the cervical plexus (Fig. 2-20) and constitutes the sole motor nerve supply to the diaphragm. It arises mainly from the ventral primary rami of C4, but it has some fibers from C3 and C5. The nerve curves around the lateral border of the anterior scalene muscle and descends obliquely across the anterior surface of the muscle (Fig. 218), deep to the transverse cervical and supraclavicular arteries. At the root of the neck the phrenic nerve passes off the anterior border of the anterior scalene muscle and descends anterior to the first part of the subclavian artery and the pleura immediately below that artery.

Brachial Plexus

The brachial plexus is formed by the ventral primary rami of C5 to T1 and provides neural supply to the upper limb.

In the posterior triangle of the neck, the brachial plexus runs anterior to the medial scalene and first digitation of the anterior serratus muscles (Fig. 2-18). It is covered by the skin and superficial fascia, the platysma, the supraclavicular nerves, the fibrofatty tissue of the supraclavicular fossa, and the deep cervical fascia. The nerve branches of the brachial plexus are crossed by the lower part of the external jugular vein, the nerve to the subclavius muscle, the transverse cervical vein, the suprascapular vein, the posterior belly of the omohyoid muscle, and the transverse cervical artery. At the root of the neck, the brachial plexus lies posterior to the clavicle, whereas the subclavius muscle and the suprascapular artery cross anterior to the plexus.

Figure 2-20 Schematic representation of the deep branches of the left cervical plexus. HN, hypoglossal nerve; acs, superior root of the ansa cervicalis; aci, inferior root of the ansa cervicalis; pn, phrenic nerve; SA, spinal accessory nerve.

Submandibular Triangle

The submandibular triangle is located at the upper boundary of the surgical field, with the submandibular gland almost entirely filling its space. The floor of this triangle is formed by the suprahyoid muscles. The hypoglossal and lingual nerves, as well as the lingual vessels, traverse the submandibular triangle and must be identified at surgery.

Muscles Mylohyoid muscle (Fig. 2-21): This thin triangular muscle originates from the mylohyoid ridge of the mandible and inserts into a median raphe extending from the middle of the anterior surface of the hyoid bone to the posterior aspect of the inferior margin of the mandible. It is covered partially by the submandibular gland, anterior belly of the digastric muscle, and superficial layer of the deep cervical fascia. The submental artery crosses the muscle. It is innervated by the mylohyoid nerve, a branch of the inferior alveolar nerve. The mylohyoid muscle elevates the hyoid bone, the floor of the mouth, and the tongue during swallowing and speaking.

Geniohyoid muscle: This short and narrow muscle is located superior to the mylohyoid. It has its origin at the mental spine of the mandible and inserts into the anterior surface of the body of the hyoid bone, where it contacts with the contralateral muscle. It is innervated by the first cervical nerve and pulls the hyoid anterosuperiorly, shortening the floor of the mouth and widening the pharynx.

Stylohyoid muscle (Fig. 2-21): This muscle takes its origin from the styloid process of the temporal bone and divides into two slips, which pass on either side of the digastric tendon to attach to the body of the hyoid. It is innervated by a branch of the facial nerve leaving the main trunk as it emerges from the stylomastoid foramen. The stylohyoid muscle elevates and retracts the hyoid bone, elongating the floor of the mouth.

Digastric muscle (Fig. 2-21): This muscle has two bellies united by an intermediate tendon, which is connected to the body and greater horn of the hyoid bone by a strong loop of fibrous connective tissue. The posterior belly arises by a tendinous process from the mastoid notch of the temporal bone. The fiber bundles form a ribbonlike belly that converges on the intermediate tendon a short distance above the hyoid bone. The posterior belly lies medial to the mastoid and sternocleido-mastoid muscle, and lateral to the internal jugular vein, internal carotid artery, and the last three

Figure 2-21 Muscles of the submandibular triangle. mh, mylohyoid muscle; sh, stylohyoid muscle; ad, anterior belly of the digastric muscle; pd, posterior belly of the digastric muscle.

cranial nerves. It is innervated by a branch of the facial nerve given off at the stylomastoid foramen. The intermediate tendon lies deep to the inferior lobe of the submandibular gland and superficial to the hyoglossus and mylohyoid muscles. The anterior belly arises by a short tendinous process from the digastric fossa at the mandible. The fibers converge on both surfaces of the flattened anterior end of the intermediate tendon. The anterior belly lies on the mylohyoid muscles and is covered by the superficial fascia and the platysma muscle. It is innervated by a branch of the mandibular nerve.

Nerves The hypoglossal nerve crosses the submandibular triangle to provide motor innervation for all the muscles of the tongue except the palatoglossus. After leaving the cranial cavity through the hypoglossal canal, the trunk of the nerve emerges between the internal carotid artery and the internal jugular vein, medial to the vagus nerve (Fig. 2-22). Before reaching the muscles of the tongue, the nerve is usually crossed by one or more lingual veins that may be a source of troublesome bleeding at surgery. On its way to the tongue, the hypoglossal nerve disappears between the genioglossus and mylohyoid muscles.

The lingual nerve is the smallest terminal branch of the posterior division of the V3. It provides general sensory fibers to the anterior two thirds of the tongue, the floor of the mouth, and the gingiva of the mandibular teeth. At first it descends on the medial side of the lateral pterygoid muscle, to pass between the medial pterygoid muscle and the ramus of the mandible toward the posterior part of the mylohyoid line. At this point it is situated a short distance posterior to the last molar tooth and is covered by the mucous membrane of the oral cavity. After leaving the medial pterygoid muscle, it crosses the lateral superior constrictor muscle of the pharynx and turns toward the tip of the tongue, crossing the lateral surface of the styloglossus, hyoglossus, and genioglossus muscles. As it crosses the hyoglossus muscle, it first lies superior to, then to the lateral side of, and finally inferior to the duct of the submandibular gland (Fig. 2-23). As it ascends on the genioglossus muscle it lies on the medial side of the duct.

IJ

Figure 2-22 Anatomic relations of the hypoglossal nerve in the submandibular triangle. hn, hypoglossal nerve; vn, vagus nerve; IJ, internal jugular vein; fv, facial vein; lv, lingual vein.

Figure 2-22 Anatomic relations of the hypoglossal nerve in the submandibular triangle. hn, hypoglossal nerve; vn, vagus nerve; IJ, internal jugular vein; fv, facial vein; lv, lingual vein.

Figure 2-23 Anatomic relations of the lingual nerve in the submandibular triangle. ln, lingual nerve; SG, submandibular gland; wd, Wharton s duct.

As the nerve lies on the medial side of the lateral pterygoid muscle, it is joined at an acute angle by the chorda tympani nerve. Between the ramus of the mandible and the medial pterygoid muscle, the lingual nerve gives off two small branches to the palatine tonsil and to the adjacent mucous membrane of the mouth. Superior to the submandibular duct the nerve gives off branches to the submandibular ganglion and a branch to the sublingual gland.

Vessels The lingual artery is the second branch of the external carotid artery, arising at the level of the greater horn of the hyoid bone, below or covered by the posterior belly of the digastric muscle and the angle of the mandible. From here it runs forward or curves upward, giving off branches to the base of the tongue. It enters the tongue above the hyoid bone, deep to the hyoglossus muscle and hypoglossal nerve. At the tip of the tongue the terminal part of the lingual artery, called the deep lingual artery, forms an anastomotic loop with the contralateral artery. The sublingual artery arises from the lingual artery at the anterior border of the hyoglossus muscle. It runs anterosuper-iorly to supply the sublingual gland and the adjacent muscles.

The lingual vein begins near the tip of the tongue, where it accompanies the deep lingual artery. It first lies beneath the mucous membrane covering the lower surface of the tongue. Then, it courses with the lingual artery deep to the hyoglossus muscle. In the vicinity of the posterior border of this muscle it receives the dorsal lingual veins coming from the dorsum of the tongue, pharyngeal wall, and palatine tonsils. At the posterior border of the hyoglossus muscle the lingual vein is joined by the accompanying veins of the hypoglossal nerve.

Submandibular Gland

The submandibular gland has a superficial part and a small deep lobe. The superficial portion is variable in size and may be palpated at the floor of the mouth by applying pressure from the outside. The deep lobe is located internal to the mylohyoid muscle.

vein; fa, facial artery; SG, submandibular gland; *, lymph nodes.

The superficial layer of the cervical fascia surrounds the gland, serving as a capsule. This capsule is crossed by the facial vein and the mandibular branch of the facial nerve (Fig. 2-16). Several lymph nodes draining the anterior facial region lie upon or are embedded in this capsule (Fig. 2-24). The lateral surface of the gland is in contact with the submandibular fovea of the medial surface of the mandible and with the caudal part of the medial pterygoid muscle. The dorsal part of the gland is deeply grooved by the facial artery, and it is separated from the parotid gland by the stylomandibular ligament. The deep lobe is in contact anteriorly with the superficial surface of the mylohyoid muscle and, posterior to this, with the hyoglossus, stylohyoid, and posterior belly of the digastric muscle. The mylohyoid nerve and artery as well as the submental artery lie between the gland and the mylohyoid muscle. The hypoglossal nerve, the lingual vein, and the first part of the lingual artery are closely related to the submandibular gland.

The deep portion of the gland is a tonguelike extension that passes around the posterior border of the mylohyoid muscle and extends anteriorly along with the submandibular duct. This glandular prolongation is located internal to the mylohyoid muscle and related medially with the hyoglossus and genioglossus muscles. At first, the deep process lies just caudal to the lingual nerve and submandibular ganglion, and it often extends as far as the sublingual gland.

Inferior and posterior to the submandibular region, the superficial layer of the deep cervical fascia fuses with the fascia of the posterior belly of the digastric and stylohyoid muscles and attaches to the hyoid bone. As it bridges the submandibular triangle and passes to the mandible, it splits into two laminae to enclose the submandibular gland, forming its sheath. These laminae attach to the mandible at the margins of the submandibular fovea. Posteriorly, the submandibular space is adjacent to that of the parotid gland, the fascial thickening between them being the stylomandibular ligament. Lymph nodes can be found on and around the gland. Their involvement by metastatic cancer depends on the location of the primary tumor. As a general rule, the submandibular triangle should be included in the dissection when the primary lesion is located on the anterior portion of the tongue, the floor of the mouth, the lower lip, the tonsil, and the lower anterior portion of the gingiva.

Carotid Sheath

The structures surrounded by the carotid sheath constitute important anatomical landmarks for functional and selective neck dissection. Precise knowledge of the anatomy of the internal jugular vein, carotid artery and its branches, and vagus nerve is crucial for a successful surgery. The sympathetic trunk, which is closely related to the carotid sheath, may also appear in the surgical field.

Internal Jugular Vein The internal jugular vein is usually the largest vein in the neck and drains the brain and the superficial parts of the face and neck (Fig. 2-25). It begins at the jugular fossa as the continuation of the sigmoid sinus. The internal jugular vein on the right side of the neck is usually larger because of the greater volume of blood entering from the superior sagittal sinus through the sigmoid sinus.

At first, the internal jugular vein lies in front of the rectus capitis muscle and posterolateral to the internal carotid artery, from which it is separated by the carotid plexus of the sympathetic trunk as well as by the hypoglossal, glossopharyngeal, and vagus nerves. As it descends, it passes gradually to the lateral side of the internal carotid artery and retains this relation as far as the superior border of the thyroid cartilage. Then, it runs to its termination along the lateral side of the common carotid artery, in the same sheath as the artery and vagus nerve, but separated from these structures by a distinct septum. On its way to the base of the neck, the vein gradually overlaps the artery anteriorly.

At the upper part, the internal jugular vein receives the inferior petrosal sinus and a meningeal vein. At the level of the angle of the mandible it receives some veins from the pharyngeal plexus as well as a communicating branch from the external jugular vein. The facial vein enters the internal jugular vein at the level of the carotid bifurcation. Further inferiorly, the lingual, sternocleidomas-toid, and superior thyroid veins join the main trunk of the internal jugular vein. Sometimes these veins enter the internal jugular vein through a common trunk, the thyrolinguofacial trunk, that crosses over the hypoglossal nerve. Along the lateral surface of the thyroid gland the internal jugular vein is joined by the middle thyroid vein.

Figure 2-25 Internal jugular vein.

Figure 2-26 Detail of the lower portion of the internal jugular vein on the right side of the neck. IJ, internal jugular vein; S, subclavian vein; BT, brachiocephalic trunk; as, anterior scalene muscle; ms, middle scalene muscle; bp, brachial plexus.

The upper portion of the internal jugular vein is covered by the digastric muscle. At the lower part of the neck, the vein is crossed by the omohyoid muscle. The internal jugular vein courses inferiorly through the neck along with the carotid artery, toward the inferior border of the sternoclavicular articulation, where it joins the subclavian vein to form the brachiocephalic trunk (Fig. 2-26).

A large number of lymph nodes lie along the internal jugular vein, in the interstices of the fascial laminae of the carotid sheath (Fig. 2-5). Thus, careful dissection of this structure is one of the characteristic surgical steps of functional and selective neck dissection. Longitudinal incision of the carotid sheath allows the removal of the lymph nodes located along the vascular axis of the neck as well as preservation of the important neurovascular structures surrounded by this fascial sheath.

Carotid Artery The right common carotid artery arises at the bifurcation of the brachiocephalic trunk, whereas the left common carotid artery comes from the aortic arch. The common carotid artery has no branches until its termination, keeping the same diameter throughout its full course (Fig. 2-27). The cranial portion of the common carotid artery has a dilatation, known as the carotid sinus, which is characterized by more elastic walls and a special innervation through the carotid sinus branch of the glossopharyngeal nerve. The carotid sinus collaborates in the regulation of blood pressure. The common carotid artery lies medial and posterior to the internal jugular vein at the level of the sternoclavicular joint, running more anterior and lateral as it ascends. The vagus nerve is located between the internal jugular vein and the common carotid artery. The common carotid artery ascends in the vascular sheath up to the level of the superior cornu of the thyroid cartilage, where it divides into internal and external branches. After its division, the internal and external carotid arteries ascend in the neck, diverging from each other in the form of a V and running in an anterior posterior direction.

The internal carotid artery is the continuation of the common carotid artery. It has no branches in the neck, and ascends medial and posterior to the internal jugular vein toward the skull base (Fig. 2-27). At its origin it runs lateral and posterior to the external carotid artery, lying on the longus capitis muscle. As it ascends, it passes internal and posterior to the external branch. The internal carotid artery enters the middle cranial fossa through the carotid canal in the petrous portion of the temporal bone.

Figure 2-27 Right carotid artery. CC, common carotid artery; IC, internal carotid artery; EC, external carotid artery; bt, brachiocephalic trunk; sa, subclavian artery; as, anterior scalene muscle; ms, middle scalene muscle.

The external carotid artery arises from the carotid sinus at the level of the fourth cervical vertebra. It runs vertical from the superior cornu of the thyroid cartilage to the anterior border of the tragus, anterior and medial to the internal carotid artery. It is crossed by the hypoglossal nerve and passes deep to the posterior border of the digastric and stylohyoid muscles. It is separated from the internal carotid artery by the stylopharyngeus and styloglossus muscles, styloid process, glosso-pharyngeal nerve, and pharyngeal branches of the vagus nerve. The superior laryngeal nerve lies

Figure 2-28 Branches of the right external carotid artery. st, superficial temporal artery; ma, maxillary artery; sth, superior thyroid artery; ap, ascending pharyngeal artery; l, lingual artery; o, occipital artery; f, facial artery; pa, posterior auricular artery.

medial to the artery in the carotid triangle. On its final portion the external carotid artery ascends posterior to the angle of the mandible and deep to the parotid gland, diverging laterally to become more superficial. It then perforates the parotid gland and accompanies the retromandibular vein through the gland toward the neck of the mandible, where it terminates by dividing into the superficial temporal and maxillary arteries (Fig. 2-28). Most of the branches of the external carotid artery arise in the carotid triangle. The branches of the external carotid artery are the superior thyroid, ascending pharyngeal, lingual, occipital, facial, and posterior auricular arteries.

The superior thyroid artery arises from the anterior border of the external carotid artery, just inferior to the great cornu of the hyoid bone. The artery arches anteriorly and then descends obliquely toward the superior pole of the thyroid gland, deep to the strap muscles. The main branches of the superior thyroid artery are the infrahyoid, sternocleidomastoid, superior laryngeal, cricothyroid, and glandular arteries. The infrahyoid artery runs inferior to the hyoid bone lying on the thyrohyoid membrane. The sternocleidomastoid artery runs posteriorly to enter the deep surface of the muscle. The superior laryngeal artery arises from the arching part of the superior thyroid artery. It passes forward toward the posterior border of the thyrohyoid muscle, along with the superior laryngeal vein and the internal branch of the superior laryngeal nerve. The neurovascular bundle pierces the thyrohyoid membrane and supplies the laryngeal muscles, the inferior pharyngeal constrictor muscle, and the endolaryngeal mucosa. The cricothyroid artery runs medially, supplying the cricothyroid muscle and membrane. It crosses the midline creating an intralaryngeal anastomotic arch with the branches from the opposite side. The glandular arteries are the direct continuation of the superior thyroid artery and constitute the final and largest branches of the superior thyroid artery. They divide at the superior pole of the thyroid gland into anterior and posterior branches.

The ascending pharyngeal artery is usually the second branch of the external carotid artery. It is a long, small vessel that arises from the posterior wall of the artery and runs on the pharynx, deep to the internal carotid artery, sending branches to the pharynx, prevertebral muscles, middle ear, and meninges.

The lingual artery arises from the anterior wall of the external carotid artery at the level of the greater cornu of the hyoid bone, between the superior thyroid and facial arteries. In its first portion it lies on the middle constrictor muscle, covered only by the superficial layer of the deep cervical fascia and the platysma muscle. It then arches upward, passing deep to the hypoglossal nerve, stylohyoid muscle, and posterior belly of the digastric muscle, to disappear into the depth of the hyoglossus muscle.

The facial artery arises from the anterior border of the external carotid artery, just above the lingual artery and sometimes from a common trunk. In the neck, the facial artery lies on the middle and superior constrictor muscles, deep to the stylohyoid and posterior belly of the digastric muscles. It enters the submandibular triangle and, close to the angle of the mandible, it arches laterally across the stylohyoid and posterior belly of the digastric muscles. It then descends toward the inferior border of the mandible, lying in a groove between the submandibular gland medially and the medial pterigoid muscle laterally. Turning around the inferior border of the mandible, the artery grooves the bone, pierces the superficial cervical fascia, and enters the face at the anterior edge of the masseter muscle.

The occipital artery arises from the posterior surface of the external carotid artery, near the level of the facial artery. It passes posteriorly along the inferior border of the posterior belly of the digastric muscle, ending in the posterior part of the scalp.

The posterior auricular artery is the third branch arising from the posterior wall of the external carotid artery, usually at the superior margin of the posterior belly of the digastric and stylohyoid muscles. It may arise as a common trunk with the occipital artery or as an independent branch. It arches laterally across the stylohyoid muscle, turns posterior, and enters the interval between the posterior margin of the external auditory canal and the mastoid process, where it divides into two terminal branches.

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Figure 2-29 Anatomic relations of the vagus nerve. vn, vagus nerve; CA, carotid artery; IJ, internal jugular vein; s, superior root of the ansa cervicalis; i, inferior root of the ansa cervicalis.

Figure 2-30 Anatomic relations of the cervical sympathetic trunk. st, sympathetic trunk; CA, carotid artery; IJ, internal jugular vein.

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Vagus Nerve The tenth cranial nerve receives its name from the Latin word vagus, which means "wandering." This nerve has the most extensive distribution of all cranial nerves. The vagus nerve leaves the skull through the jugular foramen along with the internal jugular vein and cranial nerves IX and XI. It enters the neck anterior and lateral to the superior cervical ganglion and runs posterior within the carotid sheath, between the internal carotid artery and the internal jugular vein (Fig. 2-29). On the lower part of the right side of the neck, the vagus nerve enters the mediastinum after crossing the origin of the subclavian artery, posterior to the brachiocephalic trunk and the sternoclavicular joint. The right recurrent laryngeal nerve leaves the main trunk of the vagus nerve to loop around the subclavian artery. On the left side of the neck, the vagus nerve descends between the common carotid and subclavian arteries, anterior to the thoracic duct. In the upper part of the superior mediastinum the vagus nerve is crossed by the phrenic nerve. In the lower part of the same region it crosses anterior to the root of the subclavian artery and the arch of the aorta. Below the arch of the aorta it passes dorsal to the left main bronchus and divides into branches. The left recurrent nerve loops around the arch of the aorta. Both right and left recurrent nerves pass superiorly to ascend in the tracheoesophageal groove toward the posteromedial aspect of the thyroid gland before entering the larynx.

Sympathetic Trunk The sympathetic trunk runs from the skull base to the subclavian artery lying anterolateral to the vertebral column, posterior to the great vessels, and anterior to the longus colli and longus capitis muscles (Fig. 2-30). It does not receive white rami communicans in the neck, but contains three cervical sympathetic ganglia (superior, medial, and inferior). These ganglia receive their preganglionic fibers from the superior thoracic spinal nerves through white rami communicans, whose fibers leave the spinal cord in the ventral roots of the thoracic spinal nerves. From the sympathetic trunk the fibers pass to cervical structures as postganglionic fibers in cervical spinal nerves, or leave as direct visceral branches.

The superior cervical ganglion is the largest of the three cervical sympathetic ganglia. It is located at the level of the atlas and axis, between the internal jugular vein and the internal carotid artery. It constitutes a good anatomical landmark to identify the sympathetic trunk in the neck. The carotid sheath lies anterior to the ganglion, and the longus colli muscle is located posterior to it. Postganglionic fibers pass along with the internal carotid artery and enter the cranial cavity. It also sends branches to the external carotid artery and into the four superior cranial nerves.

The middle cervical ganglion lies at the level of the cricoid cartilage and the transverse process of C6, anterior to the bend of the inferior thyroid artery. This ganglion may be double or missing entirely. Its postganglionic fibers pass to the thyroid gland and heart.

The inferior cervical ganglion lies at the level of the first rib, posterior to the vertebral artery or to the first part of the subclavian artery. It usually fuses with the first thoracic ganglion to form the stellate ganglion or cervicothoracic ganglion. Fibers from this ganglion pass into the vertebral plexus and to the heart.

Interruption of the sympathetic trunk in the neck causes Horner's syndrome (miosis, ptosis, enophthalmos, and anhidrosis of the ipsilateral eye).

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