CHAPTER 33 Pharynx
The pharynx is a 12–14 cm long musculomembranous tube shaped like an inverted cone. It extends from the cranial base to the lower border of the cricoid cartilage (the level of the sixth cervical vertebra), where it becomes continuous with the oesophagus. The width of the pharynx varies constantly because it is dependent on muscle tone, especially of the constrictors: at rest the pharyngo-oesophageal junction is closed as a result of tonic closure of the upper oesophageal sphincter, and during sleep muscle tone is low and the dimensions of the pharynx are markedly decreased (which may give rise to snoring and sleep apnoea). The pharynx is limited above by the posterior part of the body of the sphenoid and the basilar part of the occipital bone, and it is continuous with the oesophagus below. Behind, it is separated from the cervical part of the vertebral column and the prevertebral fascia which covers longus colli and longus capitis by loose connective tissue in the retropharyngeal space above and the retrovisceral space below.
The pharynx lies behind, and communicates with, the nasal, oral and laryngeal cavities via the nasopharynx, oropharynx and laryngopharynx respectively (Figs 33.1, 33.2). Its lining mucosa is continuous with that lining the pharyngotympanic tubes, nasal cavity, mouth and larynx.
NASOPHARYNX
BOUNDARIES
The nasopharynx lies above the soft palate and behind the posterior nares, which allow free respiratory passage between the nasal cavities and the nasopharynx (Figs 33.1, 33.2). The nasal septum separates the two posterior nares, each of which measures approximately 25 mm vertically and 12 mm transversely. Just within these openings lie the posterior ends of the inferior and middle nasal conchae. The nasopharynx has a roof, a posterior wall, two lateral walls and a floor. These are rigid (except for the floor, which can be raised or lowered by the soft palate) and the cavity of the nasopharynx is therefore never obliterated by muscle action, unlike the cavities of the oro- and laryngopharynx. The nasal and oral parts of the pharynx communicate through the pharyngeal isthmus which lies between the posterior border of the soft palate and the posterior pharyngeal wall. Elevation of the soft palate and constriction of the palatopharyngeal sphincter close the isthmus during swallowing.
The lateral walls of the nasopharynx display a number of important surface features. On either side each receives the opening of the pharyngotympanic tube (also termed the auditory or Eustachian tube), situated 10–12 mm behind and a little below the level of the posterior end of the inferior nasal concha (Fig. 33.2). The tubal aperture is approximately triangular in shape, and is bounded above and behind by the tubal elevation which consists of mucosa overlying the protruding pharyngeal end of the cartilage of the pharyngotympanic tube. A vertical mucosal fold, the salpingopharyngeal fold, descends from the tubal elevation behind the aperture (Fig. 33.2) and covers salpingopharyngeus in the wall of the pharynx (see Fig. 33.4); a smaller salpingopalatine fold extends from the anterosuperior angle of the tubal elevation to the soft palate in front of the aperture. As levator veli palatini enters the soft palate it produces an elevation of the mucosa immediately around the tubal opening (see Fig. 33.4). A small variable mass of lymphoid tissue, the tubal tonsil, lies in the mucosa immediately behind the opening of the pharyngotympanic tube, and further behind the tubal elevation there is a variable depression in the lateral wall, the lateral pharyngeal recess (fossa of Rosenmüller). The floor of the nasopharynx is formed by the nasal, upper surface of the soft palate.
MICROSTRUCTURE OF NON-TONSILLAR NASOPHARYNX
The nasopharyngeal epithelium anteriorly is ciliated, pseudostratified respiratory in type with goblet cells (see Fig. 2.2). The ducts of mucosal and submucosal seromucous glands open onto its surface. Posteriorly, the respiratory epithelium changes to non-keratinized stratified squamous epithelium which continues into the oropharynx and laryngopharynx. The transitional zone between the two types of epithelium consists of columnar epithelium with short microvilli instead of cilia. Superiorly, this zone meets the nasal septum; laterally, it crosses the orifice of the pharyngotympanic tube; and it passes posteriorly at the union of the soft palate and the lateral wall. Numerous mucous glands surround the tubal orifices.
ADENOID OR PHARYNGEAL TONSIL
The adenoid, or pharyngeal tonsil, is a median mass of mucosa-associated lymphoid tissue (MALT) situated in the roof and posterior wall of the nasopharynx (Fig. 33.3). At its maximal size (during the early years of life) it is shaped like a truncated pyramid, often with a vertically oriented median cleft, so that its apex points towards the nasal septum and its base to the junction of the roof and posterior wall of the nasopharynx.
The free surface of the pharyngeal tonsil is marked by folds that radiate forwards and laterally from a median blind recess, the pharyngeal bursa (bursa of Luschka), which extends backwards and up. The recess is present in the fetus and the young, but only occasionally present in the adult, and marks the rostral end of the embryological notochord. The number and position of the folds and of the deep fissures which separate them vary. A median fold may pass forwards from the pharyngeal bursa towards the nasal septum, or instead a fissure may extend forwards from the bursa, dividing the nasopharyngeal tonsil into two distinct halves which reflect its paired developmental origins (Fig. 33.3).
The prenatal origins of the nasopharyngeal tonsil are described on page 604. After birth it initially grows rapidly, but usually undergoes a degree of involution and atrophy from the age of 8–10 years (although hypoplasia may still occur in adults up to the seventh decade). Relative to the volume of the nasopharynx, the size of the tonsil is largest at 5 years, which may account for the frequency of nasal breathing problems in preschool children, and the incidence of adenoidectomy in this age group.
Vascular supply and lymphatic drainage
The arterial supply of the pharyngeal tonsil is derived from the ascending pharyngeal and ascending palatine arteries, the tonsillar branches of the facial artery, the pharyngeal branch of the maxillary artery and the artery of the pterygoid canal (see Figs 33.5, 33.6). In addition, a nutrient or emissary vessel to the neighbouring bone, the basisphenoid artery, which is a branch of the inferior hypophysial arteries, supplies the bed of the pharyngeal tonsil and is a possible cause of persistent postadenoidectomy haemorrhage in some patients.
Numerous communicating veins drain the pharyngeal tonsil into the internal submucous and external pharyngeal venous plexuses. They emerge from the deep lateral surface of the tonsil and join the external palatine (paratonsillar) veins, and pierce the superior constrictor either to join the pharyngeal venous plexus, or to unite to form a single vessel that enters the facial or internal jugular vein; they may also connect with the pterygoid venous plexus.
PHARYNGOTYMPANIC TUBE
The pharyngotympanic tube (see Fig. 36.7) connects the tympanic cavity to the nasopharynx and allows the passage of air between these spaces in order to equalize the air pressure on both aspects of the tympanic membrane. It is about 36 mm long and descends anteromedially from the tympanic cavity to the nasopharynx at an angle of approximately 45° with the sagittal plane and 30° with the horizontal (these angles increase with age and elongation of the skull base). It is formed partly by cartilage and fibrous tissue and partly by bone.
The cartilaginous part, which is approximately 24 mm long, is formed by a triangular plate of cartilage, the greater part of which is in the posteromedial wall of the tube. Its apex is attached by fibrous tissue to the circumference of the jagged rim of the bony part of the tube, and its base is directly under the mucosa of the lateral nasopharyngeal wall, forming a tubal elevation (torus tubarius; Eustachian cushion) behind the pharyngeal orifice of the tube (Fig. 33.2; see Fig. 30.1). The upper part of the cartilage is bent laterally and downwards, producing a broad medial lamina and narrow lateral lamina. In transverse section it is hook-like and incomplete below and laterally, where the canal is composed of fibrous tissue. The cartilage is fixed to the cranial base in the groove between the petrous part of the temporal bone and the greater wing of the sphenoid, and ends near the root of the medial pterygoid plate. The cartilaginous and bony parts of the tube are not in the same plane, the former descending a little more steeply than the latter. The diameter of the tube is greatest at the pharyngeal orifice, least at the junction of the two parts (the isthmus), and increases again towards the tympanic cavity.
The pharyngotympanic tube is innervated by filaments from the tympanic plexus and from the pharyngeal branch of the pterygopalatine ganglion. The tympanic plexus ramifies on the promontory in the middle ear cavity and is formed by the tympanic branch of the glossopharyngeal nerve and caroticotympanic nerves of sympathetic origin (see also Ch. 36).
OROPHARYNX
BOUNDARIES
The oropharynx extends from below the soft palate to the upper border of the epiglottis (Figs 33.1, 33.2). It opens into the mouth through the oropharyngeal isthmus, demarcated by the palatoglossal arch, and faces the pharyngeal aspect of the tongue. Its lateral wall consists of the palatopharyngeal arch and palatine tonsil (see Fig. 30.3). Posteriorly, it is level with the bodies of the second, and upper part of the third, cervical vertebrae (Fig. 33.2).
SOFT PALATE
The soft palate is a mobile flap suspended from the posterior border of the hard palate, sloping down and back between the oral and nasal parts of the pharynx (Figs 33.2, 33.4). The boundary between the hard and soft palate is readily palpable and may be distinguished by a change in colour, the soft palate being a darker red with a yellowish tint. The soft palate is a thick fold of mucosa enclosing an aponeurosis, muscular tissue, vessels, nerves, lymphoid tissue and mucous glands; almost half its thickness is represented by numerous mucous glands which lie between the muscles and the oral surface of the soft palate. The latter is covered by a stratified squamous epithelium, while the nasal surface is covered with a ciliated columnar epithelium. In most individuals, two small pits, the fovea palatini, may be seen, one on each side of the midline: they represent the orifices of ducts from some of the minor mucous glands of the palate. In its usual relaxed and pendant position, the anterior (oral) surface of the soft palate is concave, and has a median raphe. The posterior aspect is convex and continuous with the nasal floor, the anterosuperior border is attached to the posterior margin of the hard palate, and the sides blend with the pharyngeal wall. The inferior border is free, and hangs between the mouth and pharynx. A median conical process, the uvula, projects downwards from its posterior border (see Fig. 30.3). Taste buds are found on the oral aspect of the soft palate.
The anterior third of the soft palate contains little muscle and consists mainly of the palatine aponeurosis. This region is less mobile and more horizontal than the rest of the soft palate and is the chief area acted upon by tensor veli palatini.
Palatoglossal and palatopharyngeal arches
The lateral wall of the oropharynx presents two prominent folds, the palatoglossal and palatopharyngeal folds (anterior and posterior pillars of the fauces respectively) (see Fig. 30.3). The palatoglossal arch, the anterior fold, runs from the soft palate to the side of the tongue and contains palatoglossus. The palatopharyngeal arch, the posterior fold, projects more medially and passes from the soft palate to merge with the lateral wall of the pharynx; it contains palatopharyngeus. A triangular tonsillar fossa (tonsillar sinus), lies on each side of the oropharynx between the diverging palatopharyngeal and palatoglossal arches, and contains the palatine tonsil.
Uvulopalatopharyngoplasty
The pharyngeal airway is kept patent in the patient who is awake by the combined dilating action of genioglossus, tensor veli palatini, geniohyoid and stylohyoid, which act to counter the negative pressure generated in the lumen of the pharynx during inspiration. The tone in the muscles is reduced during sleep, and is also affected by alcohol and other sedatives, hypothyroidism and a variety of neurological disorders. If the dilator muscle tone is insufficient, the walls of the pharynx may become apposed. Intermittent pharyngeal obstruction may cause snoring, and complete obstruction may cause apnoea, hypoxia and hypercarbia which lead to arousal and sleep disturbance.
PALATINE TONSIL
The right and left palatine tonsils form part of the circumpharyngeal lymphoid ring. Each tonsil is an ovoid mass of lymphoid tissue situated in the lateral wall of the oropharynx (Fig. 33.5; see Figs 30.3, 30.5). Size varies according to age, individuality and pathological status (tonsils may be hypertrophied and/or inflamed). It is therefore difficult to define the normal appearance of the palatine tonsil. For the first 5 or 6 years of life the tonsils increase rapidly in size. They usually reach a maximum at puberty, when they average 20–25 mm in vertical, and 10–15 mm in transverse, diameters, and they project conspicuously into the oropharynx. Tonsillar involution begins at puberty, when the reactive lymphoid tissue begins to atrophy, and by old age only a little tonsillar lymphoid tissue remains.
The lateral or deep surface of the tonsil spreads downwards, upwards and forwards. Inferiorly, it invades the dorsum of the tongue, superiorly, the soft palate, and, anteriorly, it may extend for some distance under the palatoglossal arch. This deep, lateral aspect is covered by a layer of fibrous tissue, the tonsillar hemicapsule. The latter is separable with ease for most of its extent from the underlying muscular wall of the pharynx, which is formed here by the superior constrictor, and sometimes by the anterior fibres of palatopharyngeus, with styloglossus on its lateral side (Fig. 33.5). Anteroinferiorly, the hemicapsule adheres to the side of the tongue and to palatoglossus and palatopharyngeus. In this region, the tonsillar artery, a branch of the facial artery, pierces the superior constrictor to enter the tonsil, accompanied by venae comitantes (Fig. 33.5). An important and sometimes large vein, the external palatine or paratonsillar vein, descends from the soft palate lateral to the tonsillar hemicapsule before piercing the pharyngeal wall. Haemorrhage from this vessel from the upper angle of the tonsillar fossa may complicate tonsillectomy. The muscular wall of the tonsillar fossa separates the tonsil from the ascending palatine artery, and, occasionally, from the tortuous facial artery itself, which may lie near the pharyngeal wall at the lower tonsillar level. The glossopharyngeal nerve lies immediately lateral to the muscular wall of the tonsillar fossa (Figs 33.4A, 33.5): it is at risk if the wall is pierced, and is commonly temporarily affected by oedema following tonsillectomy. The internal carotid artery lies approximately 25 mm behind and lateral to the tonsil. In some individuals the styloid process may be elongated and deviate towards the tonsillar bed.
