Content Reviewers:Viviana Popa, MD, Scott Caterine, BSc (Hons.), MSc, MB, BCh, BAO (Hons.)
The cervical viscera are composed of three layers based on their main functions.
The most superficial layer is the endocrine layer which contains the thyroid and parathyroid glands, then there’s the respiratory layer which contains the larynx and the trachea, and finally, the deepest layer is the alimentary layer which contains the pharynx and the esophagus.
So, let’s start with the pharynx which also has a respiratory function because it conducts air to the larynx, from where it passes through the trachea and into the lungs.
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.
It’s located posterior to the nasal and oral cavities and it extends inferiorly past the larynx, where it continues with the esophagus.
Now, the pharyngeal wall consists of three layers: the buccopharyngeal fascia is the most external layer, covering the pharynx and, inferiorly, it is continuous with the pretracheal fascia.
Then there is a muscular layer which has an outer circular part and an inner longitudinal part, and the innermost mucous membrane which has a thick submucosa that contributes to the pharyngobasilar fascia.
Moreover, the posterior wall of the pharynx lies against the prevertebral layer of the deep cervical fascia.
The interior of the pharynx is divided into three parts: the nasopharynx which is located posterior to the nose and superior to the soft palate, the oropharynx which is located posterior to the mouth, and the laryngopharynx which is located posterior to the larynx.
The nasopharynx is the posterior extension of the nasal cavities and communicates with the nose through two paired openings called choanae, which are separated by the posterior end of the nasal septum.
The roof and the posterior wall of the pharynx form a continuous surface that lies inferior to the body of the sphenoid bone and the occipital bone.
On the lateral wall of the nasopharynx, there’s the opening of the pharyngotympanic tube, also called the auditory tube or the eustachian tube which connects the nasopharynx to the tympanic cavity.
Superior to this opening, there is the cartilage of the pharyngotympanic tube, called torus tubarius, and inferior to this opening, there’s an elevation of mucosa as it overlies the levator veli palatini muscle.
From the medial end of the pharyngotympanic tube, a vertical fold of mucous membrane extends inferiorly, called the salpingopharyngeal fold which covers the salpingopharyngeus muscle.
This muscle raises the pharynx during swallowing, while at the same time opening the pharyngeal orifice of the pharyngotympanic tube. This last action helps equalize the pressure in the pharynx to that of the auditory canal.
Finally, posterior to the torus tubarius and the salpingopharyngeal fold there is the pharyngeal recess, which is a slit-like projection of the pharynx that extends laterally and posteriorly.
Now, the pharynx contains abundant lymphoid tissue. In some regions, the lymphoid tissue is aggregated and forms masses called tonsils.
Pharyngeal tonsils, called adenoids when enlarged, can be found in the mucous membrane of the roof and posterior wall of the nasopharynx just above the pharyngeal recess.
Additionally, there are tonsils located in the submucosa near the orifice of the pharyngotympanic tube called tubal tonsils. Let’s take a quick break and try to identify the nasopharynx as well as the structures contained by it.
The second part, or the oropharynx lies posterior to the oral cavity which is bounded superiorly by the soft palate, inferiorly by the base of the tongue and laterally by the palatoglossal and palatopharyngeal arches. It extends from its superior border of the soft palate to the superior border of the epiglottis.
Between the base of the tongue and the epiglottis there is a midline fold of mucosa called the median glossoepiglottic fold, and between the base of the tongue and the lateral border of the epiglottis there is another fold of mucosa called the lateral glossoepiglottic fold.
Between these two folds there is a depression called the epiglottic vallecula - which serves a role in temporarily storing the saliva so as to not trigger the swallowing reflex.
Now, the oral cavity and the oropharynx are separated by a line formed by the palatoglossal arch. Posterior to the palatoglossal arch, there’s the palatopharyngeal arch which descends along the lateral wall of the oropharynx.
These are the two folds you can see in the back of the mouth when someone is opening up to say ‘ahhhhhhh’. Between these two arches there is a space called the tonsillar sinus where the palatine tonsils can be found.
The palatine tonsil is supplied by a branch of the facial artery called the tonsillar artery, which passes through the superior pharyngeal constrictor muscle and enters the inferior pole of the tonsil.
The tonsil is drained by the external palatine vein, which descends from the soft palate and passes close to the lateral surface of the tonsil before it enters the pharyngeal venous plexus.
Now, the palatine tonsils along with the pharyngeal and tubal tonsils located in the nasopharynx, in addition to the lingual tonsil located in the posterior part of the tongue, form an incomplete circular band of lymphoid tissue around the superior part of the pharynx called the pharyngeal lymphatic ring or the Waldeyer ring.
The anteroinferior part of the ring is formed by the lingual tonsil, the lateral parts of the ring are formed by the palatine and tubal tonsils, and the posterior and superior parts are formed by the pharyngeal tonsil. Let’s take a quick break and try to identify the oropharynx as well as the tonsils that form the pharyngeal lymphatic ring.
Finally, the laryngopharynx is posterior to the larynx and extends from the superior border of the epiglottis and the pharyngoepiglottic folds to the inferior border of the cricoid cartilage, where it narrows and eventually becomes the esophagus.
Now, the posterior and lateral walls of the laryngopharynx are formed by the middle and inferior pharyngeal constrictors muscles, and internally the wall is formed by the palatopharyngeus and stylopharyngeus muscles. The laryngopharynx communicates with the larynx via the laryngeal inlet on its anterior wall.
On each side of the laryngeal inlet there is a small depression called the piriform fossa or recess which is covered by mucosa and is separated from the laryngeal inlet by the aryepiglottic fold.
The piriform fossa is bounded laterally by the medial surfaces of the thyroid cartilage and the thyrohyoid membrane, and medially by the lamina of the cricoid cartilage, both covered in mucous membrane.
Deep to the mucous membrane of the piriform fossa, there are branches of the internal laryngeal and recurrent laryngeal nerves which are exposed to injury if a foreign body lodges in the fossa. Let’s take another break and see if you can identify the laryngopharynx, as well as the structures contained by it.
Now, let’s move on and take a look at the pharyngeal muscles. The pharynx has two layers of muscles: an external circular layer composed of three muscles - superior, middle and inferior pharyngeal constrictors, and an internal longitudinal layer composed also of three muscles - palatopharyngeus, stylopharyngeus and salpingopharyngeus.
Now let's take a big gulp, and try to digest these muscles that help us speak and swallow! So first, let’s start with the external layer which has three muscles: the superior pharyngeal constrictor, the middle pharyngeal constrictor and the inferior pharyngeal constrictor, which all come in mirrored pairs.
Although these are skeletal muscles, they contract involuntarily, in sequence from the superior to the inferior constrictor during swallowing to help, well, constrict the wall of the pharynx.
So far so good! The pharyngeal constrictors have a strong internal fascial lining called the pharyngobasilar fascia, and a thin external fascial lining called the buccopharyngeal fascia which, as we mentioned before, blends inferiorly with the pretracheal layer of deep cervical fascia.
The superior pharyngeal constrictor originates on the pterygomandibular raphe, and the adjacent parts of the medial pterygoid plate and mandible.