Pharynx

The pharynx is the superior expanded part of the alimentary system posterior to the nasal and oral cavities, extending inferiorly past the larynx. The pharynx extends from the cranial base to the inferior border of the cricoid cartilage anteriorly and the inferior border of the C₆ vertebra posteriorly. The pharynx is widest (approximately 5 cm) opposite the hyoid and narrowest (approximately 1.5 cm) at its inferior end, where it is continuous with the esophagus. The flat posterior wall of the pharynx lies against the prevertebral layer of deep cervical fascia.

Interior of Pharynx

The pharynx is divided into three parts:

• Nasopharynx: posterior to the nose and superior to the soft palate.

• Oropharynx: posterior to the mouth.

• Laryngopharynx: posterior to the larynx.

The nasopharynx has a respiratory function; it is the posterior extension of the nasal cavities. The nose opens into the nasopharynx through two choanae (paired openings between the nasal cavity and the nasopharynx). The roof and posterior wall of the nasopharynx form a continuous surface that lies inferior to the body of the sphenoid bone and the basilar part of the occipital bone.

The abundant lymphoid tissue in the pharynx forms an incomplete tonsillar ring around the superior part of the pharynx. The lymphoid tissue is aggregated in certain regions to form masses called tonsils. The pharyngeal tonsil (commonly called the adenoid when enlarged) is in the mucous membrane of the roof and posterior wall of the nasopharynx. Extending inferiorly from the medial end of the pharyngotympanic tube is a vertical fold of mucous membrane, the salpingopharyngeal fold. It covers the salpingopharyngeus muscle, which opens the pharyngeal orifice of the pharyngotympanic tube during swallowing. The collection of lymphoid tissue in the submucosa of the pharynx near the nasopharyngeal opening, or orifice of the pharyngotympanic tube, is the tubal tonsils. Posterior to the torus of the pharyngotympanic tube and the salpingopharyngeal fold is a slit-like lateral projection of the pharynx, the pharyngeal recess, which extends laterally and posteriorly.

The oropharynx has a digestive function. It is bounded by the soft palate superiorly, the base of the tongue inferiorly, and the palatoglossal and palatopharyngeal arches laterally. It extends from the soft palate to the superior border of the epiglottis.

Deglutition (swallowing) is the complex process that transfers a food bolus from the mouth through the pharynx and esophagus into the stomach. Solid food is masticated (chewed) and mixed with saliva to form a soft bolus (mass) that is easier to swallow. Deglutition occurs in three stages:

• Stage 1: voluntary; the bolus is compressed against the palate and pushed from the mouth into the oropharynx, mainly by movements of the muscles of the tongue and soft palate.

• Stage 2: involuntary and rapid; the soft palate is elevated, sealing off the nasopharynx from the oropharynx and laryngopharynx. The pharynx widens and shortens to receive the bolus of food as the suprahyoid muscles and longitudinal pharyngeal muscles contract, elevating the larynx.

• Stage 3: involuntary; sequential contraction of all three pharyngeal constrictor muscles creates a peristaltic ridge that forces the food bolus inferiorly into the esophagus.

The palatine tonsils are collections of lymphoid tissue on each side of the oropharynx in the interval between the palatine arches. The tonsil does not fill the tonsillar sinus (fossa) between the palatoglossal and palatopharyngeal arches in adults. The submucosal tonsillar bed, in which the palatine tonsil lies, is between these arches. The tonsillar bed is formed by the superior pharyngeal constrictor and the thin, fibrous sheet of pharyngobasilar fascia. This fascia blends with the periosteum of the cranial base and defines the limits of the pharyngeal wall in its superior part.

The laryngopharynx lies posterior to the larynx, extending from the superior border of the epiglottis and the pharyngo-epiglottic folds to the inferior border of the cricoid cartilage, where it narrows and becomes continuous with the esophagus. Posteriorly, the laryngopharynx is related to the bodies of the C₄-C₆ vertebrae. Its posterior and lateral walls are formed by the middle and inferior pharyngeal constrictor muscles. Internally, the wall is formed by the palatopharyngeus and stylopharyngeus muscles. The laryngopharynx communicates with the larynx through the laryngeal inlet on its anterior wall.

The piriform fossa (recess) is a small depression of the laryngopharyngeal cavity on either side of the laryngeal inlet. This mucosa-lined fossa is separated from the laryngeal inlet by the ary-epiglottic fold. Laterally, the piriform fossa is bounded by the medial surfaces of the thyroid cartilage and the thyrohyoid membrane. Branches of the internal laryngeal and recurrent laryngeal nerves lie deep to the mucous membrane of the piriform fossa and are vulnerable to injury when a foreign body lodges in the fossa.

Pharyngeal Muscles

The wall of the pharynx is exceptional for the alimentary tract, having a muscular layer composed entirely of voluntary muscle, arranged with longitudinal muscles internal to a circular layer of muscles. Most of the alimentary tract is composed of smooth muscle, with a layer of longitudinal muscle external to a circular layer. The external circular layer of pharyngeal muscles consists of three pharyngeal constrictors: superior, middle, and inferior. The internal longitudinal muscles consists of the palatopharyngeus, stylopharyngeus, and salpingopharyngeus. These muscles elevate the larynx and shorten the pharynx during swallowing and speaking.

The pharyngeal constrictors have a strong internal fascial lining, the pharyngobasilar fascia, and a thin external fascial lining, the buccopharyngeal fascia. Inferiorly, the buccopharyngeal fascia blends with the pretracheal layer of the deep cervical fascia. The pharyngeal constrictors contract involuntarily so that contraction takes place sequentially from the superior to the inferior end of the pharynx, propelling food into the esophagus. All three pharyngeal constrictors are supplied by the pharyngeal plexus of nerves that is formed by pharyngeal branches of the vagus and glossopharyngeal nerves and by sympathetic branches from the superior cervical ganglion. The pharyngeal plexus lies on the lateral wall of the pharynx, mainly on the middle pharyngeal constrictor.

The overlapping of the pharyngeal constrictor muscles leaves four gaps in the musculature for structures to enter or leave the pharynx:

1. Superior to the superior pharyngeal constrictor, the levator veli palatini, pharyngotympanic tube, and ascending palatine artery pass through a gap between the superior pharyngeal constrictor and the cranium. It is here that the pharyngobasilar fascia blends with the buccopharyngeal fascia to form, with the mucous membrane, the thin wall of the pharyngeal recess.

2. A gap between the superior and middle pharyngeal constrictors forms a passageway that allows the stylopharyngeus, glossopharyngeal nerve, and stylohyoid ligament to pass to the internal aspect of the pharyngeal wall.

3. A gap between the middle and inferior pharyngeal constrictors allows the internal laryngeal nerve and superior laryngeal artery and vein to pass to the larynx.

4. A gap inferior to the inferior pharyngeal constrictor allows the recurrent laryngeal nerve and inferior laryngeal artery to pass superiorly into the larynx.

Vessels of Pharynx

A branch of the facial artery, the tonsillar artery passes through the superior pharyngeal constrictor muscle and enters the inferior pole of the palatine tonsil. The tonsil also receives arterial twigs from the ascending palatine, lingual, descending palatine, and ascending pharyngeal arteries. The large external palatine vein (paratonsillar vein) descends from the soft palate and passes close to the lateral surface of the tonsil before it enters the pharyngeal venous plexus.

The tonsillar lymphatic vessels pass laterally and inferiorly to the lymph nodes near the angle of the mandible and the jugulodigastric node, referred to as the tonsillar node because of its frequent enlargement when the tonsil is inflamed (tonsillitis). The palatine, lingual, and pharyngeal tonsils form the pharyngeal lymphatic (tonsillar) ring, an incomplete circular band of lymphoid tissue around the superior part of the pharynx. The antero-inferior part of the ring is formed by the lingual tonsil in the posterior part of the tongue. Lateral parts of the ring are formed by the palatine and tubal tonsils, and posterior and superior parts are formed by the pharyngeal tonsil.

Pharyngeal Nerves

The nerve supply to the pharynx (motor and most of sensory) derives from the pharyngeal plexus of nerves. Motor fibers in the plexus are derived from the vagus nerve (CN X) via its pharyngeal branch or branches. They supply all muscles of the pharynx and soft palate, except the stylopharyngeus (supplied by CN IX) and the tensor veli palatini (supplied by CN V₃). The inferior pharyngeal constrictor also receives some motor fibers from the external and recurrent laryngeal branches of the vagus. Sensory fibers in the plexus are derived from the glossopharyngeal nerve. They are distributed to all three parts of the pharynx. In addition, the mucous membrane of the anterior and superior nasopharynx receives innervation from the maxillary nerve (CN V₂). The tonsillar nerves are derived from the tonsillar plexus of nerves formed by branches of the glossopharyngeal and vagus nerves.