The pharynx is a muscular tube, which is continuous inferiorly with the oesophagus and into which the nasal, oral and laryngeal cavities open (Fig. 7.60). For descriptive purposes, the pharynx is divided into nasopharynx, oropharynx and laryngopharynx (hypopharynx Fig. 7.55). The nasopharynx is attached to the base of the skull and is bounded anteriorly by the choanae. Inferiorly, it is continuous with the oropharynx at the level of the soft palate. The oropharynx begins anteriorly at the palatoglossal ridge and extends inferiorly to the level of the upper border of the epiglottis, where it is in continuity with the laryngopharynx. The laryngopharynx lies behind the laryngeal inlet, the arytenoids and the cricoid lamina, and on either side of the inlet forms recesses, the piriform fossae (Fig. 7.61). In normal deglutition, these fossae are traversed by fluid and food which pass behind the cricoid cartilage in the terminal part of the laryngopharynx. Foreign bodies such as fish bones may lodge in these recesses during swallowing. At the inferior border of the cricoid cartilage, it is continuous with the oesophagus. The musculature of the pharynx consists of incomplete outer circular and inner longitudinal layers. The pharyngobasilar fascia lies internal to the muscle coat and is lined by mucous membrane.
The circular layer of muscles comprises three overlapping constrictors, attaching posteriorly to the midline raphe (Figs 7.58 & 7.59), which is suspended from the pharyngeal tubercle of the occipital bone.
The superior constrictor attaches anteriorly to the lower part of the medial pterygoid plate, the pterygomandibular raphe (in company with buccinator; p. 336) and the posterior end of the mylohyoid line on the mandible. Posteriorly, its fibres attach to the pharyngeal tubercle and the pharyngeal raphe.
The middle constrictor attaches to the lower part of the stylohyoid ligament and to the angle between the greater and lesser horns of the hyoid bone. Posteriorly, it attaches to the raphe, the upper fibres overlapping those of the superior constrictor.
The inferior constrictor has an anterior attachment to the oblique line on the thyroid cartilage, the lateral surface of the cricoid cartilage and the intervening fascia covering the cricothyroid muscle. Its superior fibres curve upwards to the pharyngeal raphe, overlapping those of the middle constrictor. The lower fibres, known as cricopharyngeus, pass horizontally around the lumen of the pharynx below the raphe.
The inner longitudinal layer consists of the stylopharyngeus, palatopharyngeus and salpingopharyngeus muscles. Stylopharyngeus (p. 348) attaches to the styloid process and lies lateral to the superior constrictor. Its fibres descend between the superior and middle constrictors (Fig. 7.59) to blend with the other longitudinal muscles on the medial surface of the two lower constric- tors. Palatopharyngeus inclines posteroinferiorly from the soft palate, while salpingopharyngeus descends from the auditory tube, both muscles lying on the inner surface of the constrictors (Fig. 7.56). The longitudinal muscles attach inferiorly to the posterior border of the lamina of the thyroid cartilage.
During swallowing, the pharyngeal constrictors contract sequentially from above downwards to propel the bolus of food into the oesophagus. Simultaneously, the longitudinal muscles shorten the pharynx and elevate the larynx, thus closing its inlet against the base of the tongue. Due to the shape of the epiglottis, the bolus tends to traverse the piriform fossae. At the same time, the soft palate is raised to prevent food entering the nasopharynx and the cricopharyngeus relaxes to allow the bolus to enter the oesophagus.
The pharyngeal plexus (Fig. 7.58) consists of nerve fibres lying on the outer surface of the pharynx and receives the pharyngeal branches of the glossopharyngeal (IX) and vagus (X) nerves. The motor component of the plexus is derived from the cranial part of the accessory (XI) nerve via the vagus nerve. It supplies the muscles of the pharynx, except for stylopharyngeus (glossopharyngeal nerve; p. 352).
Sensory fibres from the glossopharyngeal nerve traverse the plexus to supply the oropharynx and mediate the gag reflex. The vagal fibres of the plexus, assisted by those travelling in the laryngeal nerves (p. 370), innervate the laryngopharynx. The sensory innervation of the nasopharynx is supplied by the maxillary (V2) division of the trigeminal nerve via its pharyngeal branch from the pterygopalatine ganglion.
Pharyngobasilar fascia and auditory tube
The inner surface of the pharyngeal musculature is lined by the pharyngobasilar fascia. This fascial sheet completes the wall of the pharynx superiorly and forms a continuous attachment to the base of the skull. Inferiorly, the fascia becomes gradually thinner, blending with the epimysium of the pharyngeal muscles.
The auditory (Eustachian) tube (Figs 7.55–7.57) opens into the nasopharynx above the superior constrictor by piercing the pharyngobasilar fascia. Laterally, the auditory tube leads into the cavity of the middle ear.
An incomplete ring of lymphoid tissue, the pharyngeal lymphoid ring (Waldeyer’s ring) (Fig. 7.55), lies in the wall of the pharynx between the mucosa and the muscles. This tissue is often subject to infection, particularly in children. On each side, a palatine tonsil lies between the palatoglossal and palatopharyngeal ridges and has a rich blood supply, including its own tonsillar branch from the facial artery. The adenoids, or pharyngeal tonsils, are situated near the roof of the nasopharynx, close to the auditory tubes with their tubal tonsils. When enlarged, the adenoids and tubal tonsils may obstruct one or both tubes, giving rise to middle ear disease. The lingual tonsil lies under the mucosa of the posterior third of the tongue and comprises a diffuse collection of small lymphoid follicles.
The arterial supply to the pharynx is derived from branches of the facial, lingual and maxillary arteries (p. 333). The ascending pharyngeal artery (Fig. 7.58) is a direct branch of the external carotid artery and passes upwards medial to the styloid muscles on the lateral surface of the pharynx. Venous blood drains into the pterygoid plexus and the internal jugular vein.