13 Head and neck
A thorough knowledge of anatomy is required to treat surgical conditions affecting the neck. For descriptive purposes the neck is divided into various triangles (Fig. 13.1). The sternocleidomastoid (SCM) divides it into two large triangles, the anterior triangle between the SCM and the midline and the posterior triangle between it and the trapezius.
Fig. 13.2 Surface anatomy of the neck.
Source: Jacob S, Anatomy: a dissection manual and atlas; Churchill Livingstone, Edinburgh (1996).
The pulsation of the common carotid artery can be felt at the anterior border of SCM at the lower border of the cricoid cartilage (C6 level). The common carotid artery usually bifurcates at the upper border of the thyroid cartilage.
In the neck, skin incisions are made transversely following Langer’s lines or crease lines. The superficial fascia contains the platysma, a striated muscle, which extends from the region of the clavicle, pectoralis major, and the deltoid to the mandible above. To prevent retraction of the severed muscle contributing to a broad scar platysma is sutured with the skin when the neck wounds are sutured. The muscle has good vascularity. Hence when skin flaps are raised platysma is included to maintain good blood supply.
The cutaneous nerves and the superficial veins lie deep to the platysma between it and the deep fascia. The anterior jugular veins course beneath the platysma on either side of the midline. Just above the suprasternal notch, the veins unite and then pass laterally beneath the SCM to drain into the external jugular vein. The external jugular vein will be described later.
The neck has distinct fascial layers which facilitate block dissection in the treatment of metastatic tumours. The layers of the fascia form lines of cleavage during operative dissection and also to a certain extent limits the spread of pus during infection.
The investing layer is the outer of the three, arising from the ligamentum nuchae and the spines of the cervical vertebrae to completely surround the neck. It splits to enclose the trapezius and the SCM and between these two forms the roof of the posterior triangle and also contributes to the fascial capsules of the parotid gland and the submandibular glands. Above it is attached to the external occipital protuberance, mastoid process, and the zygomatic arch and the mandible. Below, it is attached to the manubrium sterni, the clavicle, the acromion and the spine of the scapula.
The prevertebral fascia is anterior to the vertebral column, the prevertebral muscles and the scalene muscles. It prolongs into the axilla as the axillary sheath enclosing the brachial plexus and the subclavian artery. In an axillary block of the brachial plexus the local anaesthetic is introduced into the axillary sheath. In an interscalene block the plane deep to the fascia is infiltrated as it contains the roots and trunks of the brachial plexus.
The pretracheal fascia splits into an anterior layer that encloses the infrahyoid (strap) muscles and a posterior layer which forms the fascial capsule of the thyroid gland. The fascia extends into the mediastinium and merges with the pericardium. Laterally it blends with the investing layer deep to the SCM. It also contributes to the carotid sheath.
The anterior triangle is bounded by the anterior border of the SCM, the midline and inferior margin of the mandible. It is subdivided into four smaller triangles: submental, submandibular, carotid, and muscular (Fig. 13.1). The anterior triangle contains among other structures the thyroid gland, the submandibular gland, the carotid sheath, the deep cervical group of lymph nodes, and the supra and infra hyoid groups of muscles.
The posterior belly of the digastric is closely related to the major blood vessels and nerves of the neck. The anterior and posterior bellies of the digastric bound the submandibular triangle which contains the submandibular gland. The mylohyoid muscles of both sides fuse to form the floor of the mouth. The mylohyoid separates the deep part of the submandibular gland from its superficial portion.
Deep to these lie the thyroid gland, the larynx, and the trachea. The infahyoids or the strap muscles are supplied by the ansa cervicalis (C1,C2,C3) which is a nerve loop on the internal jugular vein (Fig. 13.4). The branches to the muscles enter in their lower half. During exposure of a large goitre the strap muscles are cut in their upper half to preserve the nerve supply from the ansa cervicalis.
The right common carotid artery is a branch of the branchiocephalic trunk; the left common carotid is a branch of the arch of the aorta. The common carotid artery divides into the external and the internal carotid arteries at the upper border of the thyroid cartilage. The bifurcation can be at a higher level, a point worth remembering to avoid ligation of the common carotid instead of the external carotid.
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):
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):
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.
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.
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.
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.
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.
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).
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.
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 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.
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 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 masseter extends from the zygomatic arch to the ramus of the mandible. It has a superficial and a deep part. The superficial fibres run downwards and back wards whereas the deep fibres are vertical. The superficial part elevates the mandible as well as assists in protrusion. When the jaw is protruded the superficial fibres become more vertical and the deep slightly oblique. The two sets of fibres thus allow the muscle to elevate the mandible in all positions of the mandible.
The temporalis takes origin from the temporal fossa and the temporal fascia covering the muscle and is inserted into the coronoid process. Its insertion extends into the retromolar fossa behind the last molar tooth. When lower dentures are fitted they should not extend into the retromolar fossa to avoid soreness of the mucosa due to contraction of the temporalis muscle. The temporalis elevates the mandible. Its posterior fibres retract the mandible after protrusion.
The lateral pterygoid, which originates from the lateral surface of the lateral pterygoid plate and from the infratemporal surface of the skull, is inserted into the capsule of the temporomandibular joint, the articular disc, and also into the upper part of the neck of the mandible. Its contraction pulls the head of the mandible and the articular disc forward during protraction and during the act of opening the mouth. Unilateral contraction of the lateral pterygoid allows the mandible to move to the opposite side. The forward movement of the disc may help to pack the space between the incongruent articular surfaces of the condyle and the articular eminence thus stabilising the joint.
The medial pterygoid extends from the medial surface of the lateral pterygoid plate to the medial surface of the ramus of the mandible. It has a small superficial head of origin from the maxillary tuberosity. It is an elevator of the mandible. Unilateral contraction of the medial pterygoid is important in the side-to-side movement of the mandible as it deviates the jaw to the opposite side.
The four muscles of mastication are supplied by the mandibular division of the trigeminal nerve. The actions of the muscles of mastication and movements of the mandible at the temporomandibular joint are:
Fractures of the mandible happen more often than those of the upper facial skeleton. In many cases they are bilateral. The condyle of the mandible can fracture due to a blow to the chin and this may result in dislocation of the temporo-mandibular joint. Fractures of the angle can run downwards and forwards, or downwards and backwards. In the former case impaction of the two fragments prevents displacement. However, if the fracture line runs downwards and backwards, muscular contraction tends to displace the posterior fragment upwards.
Fractures of the body of the mandible are most common in the canine region as the length of the root of the canine tooth weakens the bone in this position. A blow on the side of the face may fracture the body of the mandible on the side of impact and fracture the condylar process on the opposite side. Fractures of the body are always compound fractures lacerating the mucosa of the oral cavity.
This most commonly occurs in a forward direction when the condyloid process of the mandible slides forward on to the articular eminence and then into the infratemporal fossa. This can be reduced by pressing down the mandible on the molar teeth to stretch the masseter and the temporalis which are in spasm and then pulling up the chin to lever the condyle back into the mandibular fossa.
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.
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).
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.
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.
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.
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.
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:
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 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.
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 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 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:
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.
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).
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 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:
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.
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.
This part of the gland lying superficial to the mylohyoid muscle has a superficial surface facing inferiorly in the submandibular triangle. The upper part of this surface lies deep to the body of the mandible. Its deep surface is related to the digastric below and above this to the mylohyoid anteriorly, and to the hyoglossus muscle posteriorly. The facial artery grooves the deep surface and emerges on to the face by passing between the gland and the mandible (Fig. 13.15). Several submandibular lymph nodes lie on the superficial surface.
This lies in the floor of the mouth, superior (deep) to the mylohyoid and is covered by the mucosa of the oral cavity. Medially it lies on the hyoglossus and is related to the lingual nerve, the submandibular ganglion and the hypoglossal nerve.
The duct of the submandibular gland (Wharton’s duct) starts in the superficial part, running posteriorly and superiorly to reach the deep part. Here it turns forward and medially and emerges on to the surface of the hyoglossus muscle. It runs forward deep to the mucosa of the floor of the mouth between the mucosa and the sublingual gland and the geniohyoid muscle to open into the floor of the mouth on either side of the frenulum of the tongue. The duct, on the floor of the mouth is closely related to the lingual nerve. As it goes forward it crosses medial to the nerve to lie above the nerve and then crosses back, this time lateral to it to reach a position once again below the nerve (Fig. 13.16).
Fig. 13.16 Right styloid process and submandibular region. The right half of the mandible and part of the submandibular and sublingual glands have been removed. The glossopharyngeal nerve, stylohyoid ligament and lingual artery pass deep to the posterior border of hyoglossus; the lingual nerve, submandibular duct and hypoglossal nerve are superficial to hyoglossus.
Four nerves are closely related to the submandibular glands and hence are vulnerable during its removal. marginal mandibular branch of the facial nerve may be bruised during skin incision. The nerve to mylohyoid is closely related to the superficial part of the gland. The lingual nerve can be damaged during ligation of the submandibular duct. The hypoglossal nerve is related to the deep part of the gland.
The sublingual gland lies in the floor of the mouth and raises the sublingual fold of the oral mucosa. The gland is medially related to the genioglossus muscle and laterally to the sublingual fossa of the mandible. Posteriorly it extends as far as the deep part of the submandibular gland. The submandibular duct runs along the medial side of the sublingual gland. Several small ducts emerge from the gland. The posterior ducts open directly into the mouth on the sublingual fold. The anterior part has a duct which drains into the submandibular duct.
The secretomotor supply to the parotid gland is from the glossopharyngeal nerve, the parasympathetic fibres synapsing in the otic ganglion. Postganglionic fibres reach the gland via the auriculotemporal nerve.
The parasympathetic supply of the submandibular and sublingual glands is from the facial nerve through its chorda tympani branch. The chorda tympani joins the lingual nerve and the preganglionic fibres synapse in the submandibular ganglion. The postganglionic fibres rejoin the lingual nerve to be distributed to the glands.
The Eustachian tube (auditory or pharyngotympanic tube) which connects the pharynx to the middle ear opens into the nasopharynx. The cartilaginous end of the tube has a prominence at the postero-superior part of the opening. This is the tubal elevation and is a guide to the opening during catheterisation. The area posterolateral to the tubal elevation is the pharyngeal recess. The roof and the posterior wall has lymphoid accumulation in the mucosa forming the adenoids. There is also lymphoid accumulation around the opening of the Eustachian tube.
The nasopharyngeal tonsils are prominent in children but like all lymphoid tissues undergo atrophy after puberty. Infection from the nasopharynx can easily spreadinto the middle ear through the Eustachian tube.
The most important structure in the oropharynx is the palatine tonsil or the tonsil. It lies in the tonsillar fossa bounded by the anterior and posterior pillars of the fauces. The anterior pillar is the palatoglossal arch produced by the palatoglossus muscle and the posterior pillar is the palatopharyngeal arch by the palatopharyngeus muscle. The superior constrictor forms the floor of the tonsillar fossa. Pharyngobasilar fascia lining the inner surface of the constrictor forms the capsule of the tonsil and lies between the tonsil and the muscle. The capsule is normally separated from the muscle by loose areolar tissue.
The tonsil is an accumulation of lymphoid tissue. Its oral surface is lined by mucous membrane having stratified squamous epithelium. Tonsillar crypts are clefts on the inner surface and these too are lined by the mucosa (these crypts are not present in adenoids which are lined by ciliated columnar epithelium).
The main arterial supply is derived from the tonsillar branch of the facial artery which pierces the superior constrictor to enter the lower pole of the tonsil. There are additional branches from the lingual, ascending palatine and ascending pharyngeal arteries as well.
The venous drainage is to the pharyngeal plexus of veins. The troublesome paratonsillar vein which often bleeds during tonsillectomy extends from the soft palate to lie on the lateral surface of the tonsil before piercing the superior constrictor.
This part of the pharynx which is also known as the hypopharynx has the inlet of the larynx and the piriform fossa. The inlet of the larynx which is vertical is bounded by the epiglottis, aryepiglottic fold and the arytenoids. Anterolateral to the inlet is a recess known as the pyriform fossa. It is a common site for lodging foreign bodies such as fish bones and also notorious for malignant tumours which may be silent in the early stages. The internal laryngeal nerve which supplies the laryngopharynx and most of the larynx is also found in the piriform fossa. As this part has a rich lymphatic drainage the tumour rapidly spreads into the deep cervical nodes.
The pharyngobasilar fascia lies deep to the mucosa and lines the muscles of the pharynx. It is thick in the upper part and its attachment to the base of the skull gives firm anchorage to the pharynx. In the oropharynx the fascia contributes to the capsule of the tonsil.
Each constrictor muscle starts from a limited origin anteriorly and broadens out laterally and posteriorly to insert into a posterior midline raphe – the pharyngeal raphe. Each constrictor overlaps the one above posteriorly. There are gaps laterally. The gap between the inferior and middle are occupied by the thyrohyoid ligament and associated structures. The stylopharyngeus muscle accompanied by the glossopharyngeal nerve enters the pharynx through the gap between the middle and superior constrictors. The gap between the upper border of the superior constrictor and the base of the skull is bridged by the thick pharyngobasilar fascia. The Eustachian tube enters the pharynx through this gap.
Anteriorly the superior constrictor is attached to the pterygomandibular raphe and the middle constrictor to the greater horn of the hyoid bone. The inferior constrictor has two parts. The thyropharyngeus part of the inferior constrictor is fan-shaped like the other constrictors and is attached to the lamina of the thyroid cartilage. The cricopharyngeus part of the inferior constrictor is circular and acts like a sphincter. The weakest area of the pharyngeal wall is the gap between the thyropharyngeus and the cricopharyngeus posteriorly in the midline. This is the Killian’s dehiscence, a common site for pharyngeal diverticulum (pouch).
All the muscles of the pharynx except stylopharyngeus are supplied by the pharyngeal branch of the vagus, fibres coming from the nucleus ambiguus through the cranial part of the accessory nerve. Stylopharyngeus is supplied by the glossopharyngeal nerve.
The larynx is an integral part of the respiratory tract and is the organ of voice production. It also plays an essential role in the swallowing mechanism. It is held open by a series of cartilages on its wall.
Shaped like a signet ring, the cricoid cartilage has a narrow arch anteriorly and a broad lamina at the back. The arch can be felt in the neck below the thyroid cartilage. The cricotracheal ligament connects the cricoid to the first tracheal ring.
The thyroid cartilage is the largest of the laryngeal cartilages and has two laminae meeting in the midline anteriorly. The oblique line on the lamina recieves attachment of the infrahyoid muscles. This cartilage articulates inferiorly with the cricoid at the cricothyroid joints and is connected to the hyoid bone by the thyrohyoid ligament.
This is a leaf-shaped cartilage forming the anterior wall of the inlet of the larynx. Its narrow lower end is attached to the thyroid cartilage by the thyroepiglottic ligament. The thyroepiglottic ligament tethers its anterior surface to the back of the hyoid bone in the midline.
These are paired cartilages which are pyramidal in shape articulating with the lamina of the cricoid. In its broader lower part the arytenoid has the vocal process projecting anteriorly and the muscular process laterally. The former receives attachment of the vocal ligament and the latter the abductors and adductors of the vocal cord.
There are two pairs of minor cartilages. The corniculate cartilage articulates with the apex of the arytenoid. The cuneiform cartilage is a nodule in the aryepiglottic fold. Though small, these are essential for complete approximation of the inlet of the larynx.