Pharynx

The pharynx (Latin: pharynx), also known as the throat, is a funnel-shaped muscular organ located in the head and neck. It connects the oral and nasal cavities with the esophagus and larynx. The pharynx is located posterior to the mentioned cavities and above the esophagus and larynx. Within the pharynx the respiratory and digestive tracts intersect. Therefore, it functions as a passageway for air, as well as for ingested food and liquids that move further from the upper to the lower respiratory and digestive system organs.

Pharynx anatomy

The pharynx is approximately 5 to 5.5 inches (12 - 14 cm) long, depending on body size. Its superior part extends to the external cranial base but the inferior aspect ending at the level of the sixth cervical vertebra (C6). The upper part is broader, while the lower portion of the pharynx appears narrower.

• Superior and anterior to the pharynx is the nasal cavity, but inferior it continues with the esophagus.
• Anterior is the oral cavity and larynx.
• Posterior to the pharynx is the cervical part of the spine, deep muscles of the neck, cervical fascia and retropharyngeal space.
• Along the lateral walls go blood vessels and nerves.

The pharynx consists of three parts, and it mainly contains muscles. Therefore, it is also called the muscular pharynx.

Pharynx parts

The pharynx is composed of three anatomical parts. Superior to inferior, they are the following:

• Nasopharynx
• Oropharynx
• Laryngopharynx

Nasopharynx

The nasopharynx is the nasal part of the pharynx. It is the most superior portion of the pharynx that is located at the first and second cervical vertebrae (C1 - C2) level, posterior and inferior to the nasal cavity. The nasopharynx extends from the external cranial base to the uvula of the soft palate.

The upper aspect (roof) of the nasopharynx is firmly connected with the body of the sphenoid bone, and this connection is called the vault of the pharynx (pharyngeal fornix). The nasopharynx is mainly involved in respiratory processes, and therefore it is considered a part of the respiratory system.

Superior attachment line

The superior attachment line of the pharynx to the external cranial base of the skull goes in the posterior and lateral directions from the pharyngeal tubercle of the occipital bone to the petrous part of the temporal bone. The line crosses the basilar part of the occipital bone and reaches the inferior surface of the petrosal part. It continues to go along the inferior surface forward.

The line has a sharp turn at the junction site of the sphenoid and temporal bones. The superior attachment line continues forward and medially to reach the base of the medial plate of the pterygoid process (sphenoid bone). It firmly fuses with the medial plate and reaches the pterygoid hamulus.

Walls and anatomical landmarks

The anterior wall of the nasopharynx connects the pharynx to the nasal cavity through two openings called the choanae. In the middle of the lateral wall on either side is located one more opening - the pharyngeal orifice of the auditory tube that connects the nasopharynx with the tympanic cavity of the middle ear.

The tympanic cavity receives air through the auditory tube. This connection between the middle ear and the nasopharynx helps to equalize the pressure on both sides of the tympanic membrane as it is positioned between the external ear and middle ear. Also, the pharyngeal opening provides drainage of the middle ear if needed. Along the superior aspect and sides of the pharyngeal opening is positioned the torus tubarius - a mucosal elevation that marks the pharyngeal end of the cartilaginous part of the auditory tube.

In the posterior wall, behind the torus tubarius, is a deep fossa called the pharyngeal recess. It is also known as the fossa of Rosenmüller or lateral pharyngeal recess, and it is the most common site for nasopharyngeal carcinoma (epithelial cancer). More below the pharyngeal opening is another mucosal elevation called the torus levatorius. It is a mucosal fold overlying the levator veli palatini muscle.

Inferior to the torus levatorius one more fold can be found - it is called the salpingopharyngeal fold. It covers the salpingopharyngeal muscle. The junction between the superior and posterior walls is marked by the pharyngeal tonsil. It is unpaired lymphoid tissue aggregation situated above the torus tubarius. The pharyngeal tonsil is also called the adenoid tonsil. Paired tubal tonsil, or the Gerlach's tonsil, is located close to the pharyngeal orifice.

Oropharynx

The oropharynx is the middle part of the pharynx that is located posterior to the oral cavity and is its continuation. The oropharynx is situated approximately at the level of the third and fourth cervical vertebrae (C3 - C4). It extends from the uvula of the soft palate to the superior border of the epiglottis.

This part has only one opening called the oropharyngeal isthmus (oropharyngeal opening). Sometimes it is also referred to as the isthmus of fauces. It is situated anteriorly, connecting the oropharynx with the oral cavity. Overall, it is formed by the uvula, palatine and lingual tonsils, root of the tongue, and palatopharyngeal arch.

The palatine and lingual tonsils, the root of the tongue and the superior constrictor muscle are located within the oropharynx. The oropharynx is involved in respiratory and digestion processes, and it is considered a part of both - the respiratory and digestive systems.

Waldeyer's tonsillar ring

The Waldeyer's tonsillar ring is a lymphoid tissue aggregation composed of several unpaired and paired tonsils. The tonsils are located at the borders of the oral and nasal cavities and pharynx, and their layout resembles a ring. The Waldeyer's tonsilar ring is responsible for immune functions such as protection and defense against various antigens, which enter the human body via the nasal cavity or oral cavity. Therefore, the Waldeyer's ring protects both organ systems - respiratory and gastrointestinal. It is composed of six tonsils, of which two are unpaired and two paired:

• Pharyngeal tonsil - unpaired;
• Lingual tonsil - unpaired;
• Palatine tonsils - paired;
• Tubal tonsils - paired.

The upper part of the Waldeyer's ring is formed by the single pharyngeal tonsil, but the inferior part is marked by the lingual tonsil. The lateral aspects of the ring include the tubal tonsils being more superior and palatine tonsils - inferior.

Laryngopharynx

The laryngopharynx, also known as the hypopharynx, is the most distal and final part of the pharynx. The laryngopharynx is situated at the level of the fifth and sixth cervical vertebrae (C5 - C6). It extends from the superior edge of the epiglottis to the beginning of the esophagus. It ends at the inferior border of the cricoid cartilage of the larynx. Anteriorly to the laryngopharynx is the larynx, but in the inferior direction, it continues as the esophagus. The laryngopharynx is involved in respiratory and digestion processes. Therefore, it is considered a part of the respiratory and digestive systems.

The laryngopharynx has two openings - the laryngeal inlet (also known as the laryngeal aditus) and the esophageal opening. The first one connects the pharynx with the lower airways, but the second leads into the esophagus. On the sides of the laryngeal inlet is located the piriform fossa or sinus - a slight depression in the lateral walls of the pharynx that extends from the pharyngoepiglottic fold to the esophageal opening.

Within the submucosa of the piriform sinus pass the internal and recurrent laryngeal nerves. The piriform fossa is a common site for entrapment of foreign bodies, and mentioned nerves can get injured during removal procedures. The piriform fossa medially borders with the aryepiglottic fold and posterolateral surfaces of the arytenoid and cricoid cartilages of the larynx. Laterally, it ends with the thyroid membrane and thyroid cartilage.

Muscular pharynx

The pharynx contains two groups of very well-developed skeletal muscles. All muscles have pairs, and the following groups are:

• Constrictor muscles - circular muscles that constrict the pharynx;
• Levator muscles - longitudinal muscles that lift the pharynx.

Together with the tongue, soft palate and larynx, the muscles of the pharynx are involved in digestion and provide swallowing.

Constrictor muscles

The constrictor muscles are circular pharyngeal muscles, which extend from the anterior aspect of the pharynx to its sides and end within the midline of the neck posteriorly. Therefore, the constrictor muscle fibers form an incomplete circle. All constrictors share the same function - they constrict and narrow the pharynx.

All muscles produce wave-like consecutive contractions that help to move food bolus and swallow it. Muscle fibers from both sides connect in the middle of the posterior wall forming the median pharyngeal raphe that extends downward from the external cranial base. The pharyngeal constrictors include the following muscles:

• Superior pharyngeal constrictor - the most superior constrictor muscle, located in the oropharynx. It originates from the medial plate and pterygoid hamulus of the pterygoid process of the sphenoid bone, pterygomandibular raphe of buccopharyngeal fascia, posterior end of the mylohyoid line and lingula of the mandible. Fibers insert into the pharyngeal tubercle of the occipital bone and the median pharyngeal raphe;
• Middle pharyngeal constrictor - originates from both horns of the hyoid bone and the stylohyoid ligament. Muscle fibers insert at the median pharyngeal raphe and blend with the fibers of inferior and superior constrictors. The upper aspect of the middle constrictor partially covers the lower part of the superior pharyngeal constrictor. The middle pharyngeal constrictor is primarily located in the laryngopharynx;
• Inferior pharyngeal constrictor - extends from the thyroid and cricoid cartilages to the median pharyngeal raphe. The upper aspect of the muscle partially covers the lower portion of the middle pharyngeal constrictor. It is primarily located in the laryngopharynx.

Levator muscles

The levator muscles are longitudinal pharyngeal muscles. All muscles elevate the pharynx during swallowing by shortening and widening it, and they include the following:

• Stylopharyngeaus muscle - extends from the styloid process of the temporal bone to the posterior border of the thyroid cartilage and the lateral glossoepiglottic fold. Also, it blends with pharyngeal constrictors;
• Palatopharyngeus muscle - this is not only a muscle of the pharynx, but it is also classified as one the muscles of the soft palate. The palatopharyngeal muscle elevates the pharynx and pulls it superiorly, anteriorly and medially. It shortens the pharynx during swallowing and helps to separate the nasopharynx from the oropharynx. The muscle draws the larynx and esophagus towards the root of the tongue. Also, it tenses the soft palate and brings the palatopharyngeal arches closer together. The palatopharyngeus muscle fibers originate from the palatine aponeurosis and posterior border of the hard palate. They form the palatopharyngeal arch and insert in the superior and posterior border of the thyroid cartilage of the larynx and lateral wall of the pharynx;
• Salpingopharyngeus muscle - helps in pressure-equalizing between the pharynx and middle ear, as it elevates the pharynx and helps to open the auditory tube during swallowing. It originates from the inferior cartilaginous part of the auditory tube and ends blending with the palatopharyngeus muscle fibers.

Pharynx histology

The wall of the pharynx consists of four layers:

• Mucosa
• Submucosa (with pharyngobasilar fascia)
• Muscular layer (containing pharyngeal muscles)
• Adventitia

Mucosa

The mucosa covers all parts of the pharynx and, in some areas, forms mucosal folds. The pharynx is lined with two different types of epithelium. The nasopharynx contains ciliated respiratory epithelium, and it is a continuation of the mucosa of the nasal cavity. The respiratory epithelium is the ciliated pseudostratified columnar epithelium that contains mucus-producing goblet cells. Other parts of the pharynx are lined by the squamous stratified non-keratinized epithelium.

The connective tissue of the submucosa has many blood vessels and is very vascular. The pharynx contains many mucus-producing pharyngeal glands and mucosa-associated lymphoid tissue (MALT) aggregates that include B and T lymphocytes and macrophages, protecting the pharynx against various antigens.

The upper two-thirds of the pharynx has a very well-developed and thick connective tissue layer that becomes thinner towards the esophagus. The muscular layer is composed of two distinct layers - the inner layer contains longitudinal fibers, but the outer has circular skeletal muscle fibers. The adventitia is the outer loose connective tissue layer covering the pharynx externally.

Neurovascular supply of pharynx

Arterial blood supply

The arterial blood supply of the pharynx is provided by the ascending pharyngeal artery, which is a branch of the external carotid artery. The pharynx is also supplied by the dorsal lingual branch of the lingual artery, and the ascending palatine artery and tonsillar branches arising from the facial artery. The lingual and facial arteries also originate from the external carotid artery.

Small pharyngeal branches of the inferior thyroid artery also supply the pharynx. The inferior thyroid artery arises from the thyrocervical trunk. Additionally, the pharynx also receives arterial blood supply from the terminal branch of the external carotid artery - the maxillary artery and its branches, including the greater palatine, pharyngeal and pterygoid arteries.

Venous drainage

The venous drainage of the pharynx is provided by the pharyngeal plexus carrying blood next to facial and pharyngeal veins. These veins drain into the internal jugular vein.

Lymphatic drainage

The pharyngeal lymphatic vessels drain mostly directly into the deep cervical nodes or indirectly via the retropharyngeal nodes, which are located behind the pharynx, the paratracheal nodes - along the sides of the trachea, and via the infrahyoid lymph nodes.

Innervation

The innervation of the pharynx is mainly provided by the pharyngeal plexus. It is formed by the pharyngeal branches of the glossopharyngeal nerve (CN IX), vagus nerve (CN X), and fibers coming from the superior cervical ganglion of the sympathetic trunk.

The motor innervation of the pharynx is provided by two cranial nerves. All pharyngeal muscles, except the stylopharyngeus muscle, are innervated by fibers of the vagus nerve (CN X). The stylopharyngeus muscle is innervated by branches arising from the glossopharyngeal nerve (CN IX).

Most of the pharynx receives sensory innervation from the glossopharyngeal nerve (CN IX). However, the innervation of the nasopharynx and laryngopharynx involves two more nerves. The upper part of the nasopharynx is innervated by branches from the maxillary nerve (CN V2). The vagus nerve (CN X) innervates only the inferior part of the laryngopharynx via the internal laryngeal branch of the superior laryngeal nerve.

Pharynx functions

The primary function of the pharynx is to provide passage for the air and food from the oral and nasal cavities to other digestive and respiratory system parts. Primarly, the pharynx is involved in digestion and breathing processes. Other functions of the pharynx are described below.

• The pharynx is also a part of the vocal system, as its form and shape allow to make various speech sounds and help in voice production and resonance.
• As the pharyngeal mucosa contains a small number of taste buds, the pharynx also participates in creating taste sensation.
• The pharynx takes part in air conditioning.
• The pharyngeal mucosa with lymphoid tissue aggregates helps to provide local immunity and defense against various harmful antigenes.
• The pharynx provides drainage for secretions of nasal and oral cavities, and the middle ear.
• It also aids in pressure equalization on both sides of the tympanic membrane, as it is connected to the middle ear via the auditory tube.

References:

1. Drake, R., Vogl, W., & Mitchell, A. (2019). Gray’s Anatomy for Students: With Student Consult Online Access (4th ed.). Elsevier.
2. Gray, H., & Carter, H. (2021). Gray’s Anatomy (Leatherbound Classics) (Leatherbound Classic Collection) by F.R.S. Henry Gray (2011) Leather Bound (2010th Edition). Barnes & Noble.
3. Moore, K.L., Dalley, A.F., Agur, A.M. (2018). Clinically Oriented Anatomy, 8th Edition, Lippincott Williams & Wilkins.