The common carotid artery is crossed at the level of the 6th cervical vertebra by the omohyoid muscle (Fig. 13.4). Above this level the artery is superficial and its pulsation can easily be felt whereas below, the artery is covered by the infrahyoid muscles and the SCM. The artery is enclosed in the carotid sheath with the internal jugular vein lateral to it and the vagus nerve between the artery and the vein at a deeper plane. The internal carotid artery passes vertically upwards as a continuation of the common carotid without giving any branches in the neck. The artery which is also enclosed in the carotid sheath is separated from the external carotid by (Fig. 13.5):

• styloid process;

• stylopharyngeus muscle;

• glossopharyngeal nerve; and

• pharyngeal branch of the vagus.

Fig. 13.5 The cranial nerves related to the carotid arteries.

Source: Rogers op. cit.

It is accompanied by a plexus of sympathetic nerves. At the base of the skull the artery enters the carotid canal. The intracranial part supplies the eye and the brain.

The external carotid artery extends from the point of bifurcation of the common carotid to a point midway between the angle of mandible and the mastoid process. The upper part of the artery enters the parotid gland where it divides into its two terminal branches: the maxillary artery and the superficial temporal artery. At its commencement the artery is anteromedial to the internal and can be distinguished from the internal by the presence of branches (the internal carotid has no branches in the neck). The branches of the external carotid artery are (Fig. 13.6):

• superior thyroid artery;

• lingual artery;

• facial artery;

• occipital artery;

• posterior auricular artery;

• ascending pharyngeal artery;

• maxillary artery; and

• superficial temporal artery.

Fig. 13.6 The subclavian and carotid arteries.

Source: Rogers op. cit.

The superior thyroid artery arising at the commencement of the external carotid is closely related to the external laryngeal nerve. The nerve should be identified and separated before ligating the artery during thyroid surgery.

The external carotid artery may have to be ligated to control bleeding from one of its inaccessible branches. However, ligation will not eliminate blood flow through it because of the anastomoses of the branches of the arteries of the two sides.

Internal jugular vein

This is the largest vein in the neck and is formed in the jugular foramen as a continuation of the sigmoid sinus. At its commencement the vein lies behind the internal carotid artery. However, as it descends, the internal jugular vein occupies a position lateral to the internal carotid artery and the common carotid artery. The carotid sheath in which the artery andthe vein lie is not thick over the vein allowing thevein to distend. The deep cervical group of lymph nodes is found along the internal jugular vein within the carotid sheath. In a block dissection the internal jugular vein is removed to facilitate removal of the nodes.

In the root of the neck, the internal jugular vein lies behind the gap between the sternal and the clavicular heads of the SCM and ends by joining the subclavian vein to form the brachiocephalic vein. Just below the jugular foramen the inferior petrosal sinus joins the internal jugular vein. The pharyngeal veins, the common facial vein and the superior and middle thyroid veins also drain into the internal jugular vein. The middle thyroid vein or veins may vary in number. They are short and are thin walled. Undue traction during thyroid surgery can result in avulsion of these veins from the internal jugular. Gentle traction, double ligation and sectioning of these veins are important steps in mobilization of the thyroid lobe.

Internal jugular vein cannulation

This can be done by using a high or low approach. Catheterization is usually done on the right side as the right vein is in a straight line with the right brachiocephalic vein and the SVC. In the high approach the vein is palpated lateral to the common carotid artery pulsation deep to the anterior border of the sternocleidomastoid at the level of C6 vertebra, the vein is punctured and a cannula is introduced. In the low approach the needle is inserted near the apex of the triangular gap between the sternal and the clavicular heads of the sternocleidomastoid.

POSTERIOR TRIANGLE

The boundaries are

• anterior – sternocleidomastoid;

• posterior – trapezius;

• apex – meeting points of upper attachments of the trapezius and SCM;

• base – middle third of clavicle;

• roof – investing layer of fascia extending between trapezius and SCM;

• floor – splenius capitis, levator scapulae, scalenus medius, scalenus anterior; all covered by the prevertebral fascia.

The skin over the triangle has platysma only in its anterior part. Its absence and hence relatively lower vascularity makes development of skin flaps in the posterior part more difficult.

The posterior triangle contains the subclavian artery (3rd part), transverse cervical artery, suprascapular artery and the occipital artery.

The external jugular vein courses in the superficial fascia obliquely, pierces the deep fascia just above the clavicle and drains into the subclavian vein. Dissection of the lower part of the triangle may cause troublesome bleeding from this vein. The spinal accessory nerve is the most important structure in the posterior triangle. It exits from the jugular foramen, passes through the deep part of the sternocleidomastoid and enters the posterior triangle where it lies fairly superficially embedded in the deep fascia along the roof. It then enters the under surface of the trapezius. The nerve supplies the sternocleidomastoid and the trapezius. The accessory nerve can be damaged during biopsy of lymph nodes in the posterior triangle. This will paralyse the trapezius resulting in inability to raise the arm above the level of the shoulder as well as inability to shrug the shoulder.

Surface marking of the accessory nerve

Draw a line connecting the junction between the upper third and the lower two-thirds of the posterior border of the stenocleidomastoid to a point joining the upper two-thirds and lower one-third of the anterior border of the trapezius. The nerve can be identified as it enters the deep surface of the SCM about 4 cms below the mastoid. It can also be found at Erb’s point, just above where great auricular, transverse cervical and lesser occipital nerves (all branches of the cervical plexus) emerges from behind the SCM.

MANDIBLE

The mandible, or the lower jaw, consists of a horizontal body bearing the alveolar process and the lower teeth, and a vertically orientated ramus. The junction between the body and the ramus is the angle of the mandible. The upper part of the ramus divides into an anterior coronoid process and a posterior condyloid process which bears the head and neck of the mandible (Fig. 13.7). The head articulates with the mandibular fossa at the base of the skull to formthe temporo-mandibular joint. The neck has a depression, the pterygoid fovea, in its upper part for the insertion of the lateral pterygoid muscle. The coronoid process receives the attachment of the temporalismuscle.

Fig. 13.7 The mandible: external surface.

Source: Rogers A W, Textbook of anatomy; Churchill Livingstone, Edinburgh (1992).

Medial surface

On the medial aspect of the ramus is the mandibular foramen (Fig. 13.8). This is guarded anteriorly by a projecting process called the lingula to which the sphenomandibular ligament is attached. The inferior alveolar (dental) nerve enters the mandibular foramen and traverses the body within the mandibular canal. It divides into the mental nerve and the incisive nerve. The incisive nerve which supplies of the incisors and canine teeth runs beyond the mental foramen within the body in the incisive canal. The trunk of the inferior alveolar nerve supplies the premolars and the molars.

Fig. 13.8 The mandible: internal surface.

Source: Rogers op. cit.

A small groove runs inferiorly and forward from the mandibular foramen. This is the mylohyoid groove and is produced by the nerve to mylohyoid which supplies the mylohyoid and the anterior belly of the digastric muscles. Above the groove is a prominent ridge, the mylohyoid line for the attachment of the mylohyoid muscle. The muscle extends from the level of the last molar tooth to the midline. The two mylohyoids which form the floor of the mouth separate the oral cavity from the neck. The slight depression on the bone below the mylohyoid line is the submandibular fossa where the superficial part of the submandibular gland is located. The deep part of the gland and the sublingual gland lie above the mylohyoid line in the oral cavity. This part of the mandible is lined by the mucous membrane of the mouth.

The rough area on the medial surface of the angle of the mandible is for the attachment of the medial pterygoid muscle.

Anteriorly in the midline are two pairs of irregular elevations, the genial tubercles or mental tubercles. The superior pair give attachments to the genioglossi and the inferior to the geniohyoids.

Lateral surface

The anterior border of the ramus extends forward on the body as the external oblique ridge. The buccinator muscle is attached to this ridge. The mental foramen lies halfway between the upper and lower border of the body of the mandible in the region of the apices of the premolar teeth. The mental nerve emerges through the mental foramen to supply the lower lip and the buccal and labial gingiva.

The lateral surface of the ramus gives attachment to the masseter which extends from the angle forward as far as the external oblique line and the second molar tooth.

TEMPOROMANDIBULAR JOINT

This is a synovial joint where the head of the mandible articulates with the mandibular fossa (glenoid fossa) and the articular eminence of the temporal bone. The articular surfaces of this joint are covered by fibrocartilage (not hyaline) and there is also a fibro-cartilaginous articular disc dividing the joint cavity into upper and lower compartments.

The capsule of the joint is attached to the neck of the mandible around the head. Above it is attached just anterior to the articular eminence in front and to the squamo-tympanic fissure posteriorly.

The articular disc is attached around its periphery to the joint capsule. Anteriorly it is attached to the lateral pterygoid muscle and posteriorly to the temporal bone. The posterior attachment is elastic allowing forward movement of the disc with the mandible by the contraction of the lateral pterygoid during opening of the mouth.

The capsule of the joint is reinforced by a lateral temporomandibular ligament extending downwards and backwards from the tubercle of the zygoma to the posterior border of the neck of the mandible. The sphenomandibular ligament and the stylomandibular ligament act as accessory ligaments of the joint.

The temporomandibular joints allow depression, elevation, protrusion, retraction, and side to side movements of the mandible.

MUSCLES OF MASTICATION

There are four pairs of muscles in this group attaching the mandible to the base of the skull:

• masseter;

• temporalis;

• medial pterygoid; and

• lateral pterygoid.

TONGUE

The tongue lies on the floor of the mouth and extends into the anterior wall of the oropharynx. It is a mass of striated muscles covered by mucous membrane. Its mobility is essential for mastication, swallowing and speech. It is derived from a variety of embyonic sources. The anterior two-thirds of the mucosa is developed from the first branchial arch and the posterior third from the third. Both intrinsic and extrinsic muscles are from the occipital myotomes.

Mucosal surface

The dorsum of the tongue is divided into an anterior two-third and a posterior third by a V-shaped groove, the sulcus terminalis, the apex of which has the foramen caecum from which the thyroglossal duct giving rise to the thyroid gland develops. The mucosa of the anterior twothird carries the filiform papillae, which gives the tongue its rough feel. Slightly larger and reddish fungiform papillae are also present scattered in between these papillae. Just in front of the sulcus terminalis and parallel to it is a row of even larger papillae, the vallate papillae, about 8–12 in number. The vallate papillae carry taste buds (Fig. 13.9).

Fig. 13.9 The buccal cavity: the mouth and oropharynx.

Source: Rogers op. cit.

The inferior surface of the tongue is smooth and shiny and in the midline has the frenulum of the tongue. On either side of the frenulum the deep vein of the tongue can be seen (Fig. 13.10).

Fig. 13.10 The buccal cavity: the inferior surface of the tongue and the floor of the mouth.

Source: Rogers op. cit.

The posterior third of the tongue faces the oropharynx and the laryngeal part of the pharynx. There are a number of elevations seen here which form the lingual tonsil, a lymphoid aggregation embedded in the musculature.

Nerve supply

This is based on the development. The lingual nerve which is a branch of the mandibular division of the trigeminal (nerve of the first branchial arch) carries common sensation from the anterior two-thirds. Taste is carried by the chorda tympani fibres with the lingual nerve. The sensory supply of the posterior third, including the vallate papillae, is by the glossopharyngeal nerve which is the nerve of the third branchial arch. The intrinsic and extrinsic muscles are supplied by the hypoglossal nerve.

Blood supply

Arteries

The tongue is supplied by the lingual artery, a branch of the external carotid artery the course of which is illustrated in Fig. 13.12. The dorsal lingual arteries are branches which supply the mucous membrane as well as the palatine tonsil and the soft palate. The artery is accompanied by the deep lingual vein. At its commencement, the hypoglossal nerve and its companion vein crosses superficial to the artery. At the posterior third, branches from the tonsillar artery (branch of the facial) and ascending pharyngeal artery anastomose with those of the lingual artery. There is only a poor communication between the two lingual arteries across the median septum.

Fig. 13.12 The course of the lingual artery.

Source: Rogers op. cit.

Veins

The tongue has two main veins:

• the lingual vein accompanying the lingual artery; and

• the deep lingual vein which is visible on the inferior surface drains the sublingual gland as well.

Lymphatic drainage

Lymphatic spread in cancer of the tongue is by tumour emboli. The drainage is essentially to the deep cervical nodes. In the anterior two-thirds there is only minimal communication of lymphatics across the midline septum so that metastases from this portion tend to be ipsilateral. Posterior third lymphatics form extrinsic networks and facilitate early bilateral metastases.

Lymphatics from the tip of the tongue pass to the submental nodes and from there to the lower deep cervical nodes. From the mid portion lymphatics pass to the submandibular nodes and then to the deep cervical from the margin of the tongue ipsilaterally and the rest bilateral. From the posterior third the drainage is to the upper deep cervical of both sides.

FLOOR OF THE MOUTH

The floor of the mouth separating the oral cavity from the neck is formed by the mylohyoid diaphragm formed by the fusion of the mylohyoid muscles of both sides along the midline raphe Above the mylohyoid is the mouth and below is the neck. The mylohyoids are reinforced superiorly by the two geniohyoids. The anterior part of the tongue rests on the mucosa covering the floor of the mouth. In the midline, the frenulum of the tongue is seen on the floor connecting the tongue to the mandible. On either side of the frenulum is the sublingual papilla on which the submandibular gland duct opens (Fig. 13.10). Lateral to this is the sublingual fold produced by the sublingual gland.

More posteriorly between the mylohyoid and the tongue lies the hyoglossus muscle which in fact is the side wall of the tongue. A number of important structures in the floor of the mouth lie on the hyoglossus. These from above downwards are:

• lingual nerve;

• deep part of the submandibular gland and the submandibular duct; and

• hypoglossal nerve.

The deep part of the submandibular gland and the submandibular duct are described on page 72.

The lingual nerve, a branch of the mandibular division of the trigeminal nerve, runs forward above the mylohyoid. It gives off a gingival branch which supplies the whole of the lingual gingiva and the mucous membrane of the floor of the mouth. The lingual nerve winds round the submandibular duct (page 72) before getting distributed to the mucosa of the anterior two-thirds of the tongue. The submandibular ganglion is suspended from the lingual nerve as it lies on the hyoglossus. The preganglionic fibres in the chorda tympani synapse in this ganglion. Before reaching the floor of the mouth the lingual nerve lies against the periosteum of the alveolar process closely related to the 3rd molar tooth. The nerve can be damaged here during dental extraction.

The hypoglossal nerve descends between the internal jugular vein and the internal carotid artery, giving branches to thyrohyoid and geniohyoid muscles. It supplies the superior limb of the ansa cervicalis (C1) to innervate the infrahyoid muscles. It reaches the surface of the hyoglossus by passing deep to the posterior belly of the digastric. On the hyoglossus it breaks up into branches to supply all the muscles (both extrinsic and intrinsic) of the tongue except the palatoglossus. Paralysis of the hypoglossal nerve is manifested as fibrillation of the tongue as well as wasting of the muscles. The latter will show the mucosa loose on the paralysed side.

THE PALATE

The roof of the mouth is the palate. The anterior two-thirds is bony, forming the hard palate and the posterior third, the soft palate, is muscular. The midline projection of the soft palate backwards is the uvula. If the subject says ‘aah’ the soft palate will move upwards. The palatine process of the maxilla and the horizontal plate of the palatine bones form the hard palate. The tensor palatini, the levator palatini, the musculus uvuli, the palatoglossus and the palato-pharyngeus form the muscular core of the soft palate. The tensor palatini winds round the pterygoid hamulus of the medial pterygoid plate to enter the cavity of the pharynx and its tendon spreads out to become the palatine aponeurosis to be attached to the posterior aspect of the hard palate. The levator palatini takes origin from the base of the skull inside the pharynx and is inserted to the palatine aponeurosis. The other palatine muscles merge with the aponeurosis. Both the tensor and the levator palatini in their upper part are attached to the cartilaginous part of the Eustachian (auditory) tube. Their contraction opens the tube to transmit air from the pharynx to the middle ear. Children with cleft palate may develop deafness as this mechanism is often affected.

The mucosa of the palate has stratified squamous epithelium on the oral surface and ciliated columnar epithelium on the surface facing the nasal cavity. The sensory nerve supply of the palate is by branches from the maxillary nerve and the motor supply is by the cranial part of the accessory nerve transmitted through the vagus as its pharyngeal branch.

SALIVARY GLANDS

Parotid gland

This serous salivary gland has a complex shape, irregular surfaces and important relations. An anatomy teacher told his students that during the Creation of Man the Creator poured ‘liquid parotid tissue’ into the area between the mastoid process and the ramus of the mandible, the liquid trickled into all the crevices in this region and solidified around a number of important structures. The story emphasises the complex configuration and relations of the gland which will no doubt be appreciated by a surgeon doing a total parotidectomy.

The parotid gland lies between the mastoid process and the sternocleidomastoid posteriorly, and the ramus of the mandible, which it clasps anteriorly (Fig. 13.13).

Fig. 13.13 The parotid, submandibular and sublingual glands.

Source: Rogers op. cit.

The upper pole of the gland is a small concave surface and it adheres to the cartilaginous part of the auditory tube and it is wedged between the latter and the capsule of the temporomandibular joint.

The lower pole extends below and behind the angle of the mandible into the neck on to the SCM and the posterior belly of the digastric.

The parotid gland is enclosed in a tough capsule derived from the investing layer of the deep fascia. Inflammation of this gland produces pain as the gland swells within the unyielding capsule.

The parotid duct or Stensen’s duct emerges from the anterior border of the gland, lies over the masseter, turns medially to pierce the buccinator to enter the oral cavity at the level of the upper second molar tooth. It lies between the muscle and mucous membrane for a short distance before piercing it and the valvular flap thus produced prevents inflation of the gland when the intra-oral pressure is raised. The duct is palpable over the masseter when the jaw is clenched. It lies along a line joining the tragus of the ear and the philtrum of the upper lip. Classical descriptions attribute three surfaces to the gland:

• lateral or superficial surface;

• anteromedial or anterior surface; and

• posteromedial or deep surface.

Superficial surface

There is natural plane of separation between the skin and the superficial surface. Platysma may be present between these two.

Anteromedial surface

This, in fact, is U-shaped extending from the lateral surface of the masseter to the medial surface of the medial pterygoid muscle winding round the posterior border of the mandibular ramus. Where this surface meets the superficial surface is the convex anterior border from which emerges the parotid duct and the five branches of the facial nerve. The stylomandibular ligament separates the deep aspect of this surface from the submandibular gland.

Posteromedial surface

The posteromedial surface (also known as the deep surface) is very irregular and more complex. Part of it wraps around the mastoid process and the attached muscles, SCM laterally and the posterior belly of digastric medially). This part is also indented by these structures. The gland extends deep to the posterior belly of the digastric to be related to the styloid process and the stylohyoid muscle. The latter two separate the gland from the carotid sheath and its contents (internal carotid artery, internal jugular vein, and the last four cranial nerves).

Structures passing through the parotid gland

The external carotid artery, the retromandibular vein and the facial nerve pass through the parotid gland (Fig. 13.14).

Fig. 13.14 Structures in the parotid region and upper part of the neck.

Source: Rogers op. cit.

The external carotid artery enters the posteromedial surface inferiorly and divides within the gland into its terminal branches, the maxillary and the superficial temporal arteries. The terminal branches leave the anteromedial surface. The retromandibular vein, which emerges from the posteromedial surface, is formed within the gland by the union of the maxillary and the superficial temporal veins which enter the gland on its anteromedial surface.

The facial nerve leaves the base of the skull through the stylomastoid foramen. The main trunk of the nerve is located in the triangle formed by the mastoid, the angle of the mandible and the cartilaginous part of the external auditory meatus. During parotidectomy, the trunk of the nerve is approached along a plane in front of the anterior margin of the cartilage. The cartilage in this region has a small projection pointing towards the nerve.

The stylomastoid branch of the posterior auricular artery is superficial to the nerve and is also a guide to its proximity.

Before entering the gland the following three branches are given off from the nerve:

• the posterior auricular branch;

• a branch to the posterior belly of the digastric muscle; and

• a branch to the stylohyoid muscle.

The facial nerve enters the posteromedial surface of the parotid gland about 1 cm after emerging from the skull. It then passes forward in the gland as the most superficial of the three embedded structures. (The external carotid artery being the deepest.) Inside the gland the nerve usually divides into an upper temporofacial division having a vertical course and a cervicofacial division which is more horizontal. These two further divide to form the five terminal branches:

• temporal for the frontalis and the orbicularis oculi muscles;

• zygomatic for the orbicularis oculi;

• buccal supplying the buccinator and the upper lip muscles;

• marginal mandibular supplying the lower lip muscles; and

• cervical for the supply of platysma.

There is considerable variation in the pattern of the branching inside the gland. There are also a number of communicating rami between the branches.

The concept of a superficial and deep lobes for the parotid separated by the facial nerve is controversial as these lobes are not well defined or separated. The parotid is a common site for salivary gland tumours. Parotidectomy requires precise identification and dissection of the facial nerve and hence a precise knowledge of anatomy of the gland is essential to avoid injury to the nerve.

Submandibular gland

The submandibular gland (Fig. 13.14) and the submandibular group of lymph nodes fill the submandibular triangle which is bounded by the anterior and posterior bellies of the digastric muscle and the lower border of the mandible. The gland also extends upward deep to the mandible. Differentiating an enlargement of the gland from that of the lymph nodes can be difficult.

The superficial surface of the gland is covered by the skin, platysma and the investing layer of deep fascia and is crossed by the facial vein, the cervical branch of the facial nerve and also often by the marginal mandibular branch of the facial nerve. The marginal mandibular branch lies deep to the platysma and is one of the most important relations of the gland. This branch which supplies the depressor anguli oris and the depressor labii inferioris is liable to injury during surgery of the submandibular region. Injury of the nerve can result in facial asymmetry and occasional dribling. Skin incisions in the submandibular region are made about 4 cms below the mandible to avoid injury to the marginal mandibular branch.

Each submandibular gland has a larger, superficial part and a smaller, deep part. The two are separated by the mylohyoid muscle. The two parts, however, are continuous with each other posteriorly and the concavity thus formed is occupied by the free posterior border of the mylohyoid muscle.