3 January, 2001
The larynx and pharynx both have their own functions but do share common functions. This leads to some structural similarities and differences.
The pharynx is a 12 cm fibro-muscular tube that extends from the base of the skull to the oesophagus. Its functions are conduction of food to the oesophagus and conduction of air to the larynx and lungs. It has no anterior wall so that it can communicate with the nose, mouth and larynx. The three connections give rise to three areas: the nasopharynx, oropharynx and laryngopharynx. The posterior part lies against the prevertebral fascia.
The larynx lies from the cervical vertebral levels of C3 to C6 and connects the pharynx to the trachea. It functions as a respiratory organ, a protective sphincter for the air passages and also in phonation.
The laryngeal epithelium develops from endoderm of the 4th and 6th pharyngeal arches, the cartilages develop from mesenchyme derived from the neural crest. The pharyngeal muscles are derived from the endoderm of the 4th and 6th pharyngeal arches. These developmental similarities may give rise to anatomical similarities.
The pharynx has no individual skeleton except for a small cartilaginous area for the auditory tube to open into. The larynx has a skeleton of cartilages. Three single ones; the epiglottis, which is elastic cartilage and used to keep food out, the cricoid and the thyroid. The paired ones are: the aretynoids, corniculates and cuneiforms. The laryngeal skeleton has a few functions; firstly it maintains the integrity of the pharynx, the epiglottis keeps food material out of the larynx and lastly, the cartilages can be moved by muscles to alter the vocal ligaments’ length and tension. The aretynoids are a good example of this and a change in length and tension of the vocal ligaments can alter the sound produced, an useful property for phonation. The pharynx has no individual skeleton so that it can remain collapsible for when food is being propelled to the oesophagus. A fractured laryngeal skeleton can therefore produce problems in the larynx that will not be seen in the pharynx; for example, respiratory obstruction and an inability to speak if the laryngeal nerves are damaged.
The pharyngeal muscles function to move food into the oesophagus and to raise the larynx and pharynx during phonation. There are two muscle layers; an external circular layer of constrictors and an internal longitudinal layer. The superior, middle and inferior constrictor muscles of the pharynx contract involuntarily so that contraction is sequential propelling food into the oesophagus. The pharyngeus muscles (Palato-, stylo- and salpingopharyngeus) elevate the pharynx and larynx in phonation and swallowing.
With the laryngeal muscles, the infrahyoids depress the hyoid bone and the suprahyoids and stylopharyngeus (which is a pharyngeal muscle) raise the larynx and hyoid bone. With the intrinsic laryngeal muscles; the cricothyroid stretches and tenses the vocal fold, the posterior cricoarytenoid abducts the vocal fold, the lateral cricoarytenoid adducts the vocal fold, the thryoaretynoid relaxes the vocal fold, the transverse and oblique aretynoids close the inlet of the larynx by moving the aretynoid cartilages closer and the vocalis alters the vocal fold during phonation. The laryngeal muscles are not arranged in layers of circular and longitudinal muscle. They are divided into extrinsic and intrinsic groups and each muscle has a different function. The laryngeal muscles have to meet more complex needs functionally so the arrangement is more complex whereas the pharyngeal muscles have a relatively easier task.
In the larynx there are elastic ligaments and membranes to hold the cartilages together such as the thyrohyoid membrane, cricothyroid ligament, cricotracheal ligament, quadrangular membrane and cricoarytenoid ligament. The larynx also has a series of folds which function as sphincters. The epiglottis separates the pharynx from the larynx, the vestibular folds (false vocal cords) act as an outlet valve to raise intra-thoracic pressure and the vocal folds act in phonation.
The pharynx does not seem to have foldings like these or any type of holding elastic ligaments. It does have a layer of pharyngobasilar fascia that blends with the buccopharyngeal fascia to from the mucous membrane. The pharynx would not have folds within to prevent any unnecessary resistance to food when swallowing.
The pharynx has arterial branches derived from many arteries; ascending pharyngeal, ascending palatine, lingual and the superior and inferior laryngeal arteries. Venous blood is collected into the pharyngeal venous plexus, which drains into the internal jugular vein.
In contrast, the larynx is supplied solely by the laryngeal arteries, branches of the superior and inferior thyroid arteries. Venous drainage is through veins that accompany the arteries, the superior laryngeal vein drains into the internal jugular vein and the inferior laryngeal vein joins the inferior thyroid vein or enters into the venous plexus.
The laryngeal lymphatic vessels superior and inferior to the vocal folds go with the superior and inferior laryngeal arteries respectively and drain into the cervical lymph nodes. Lymph from the pharynx passes to the retropharyngeal lymph nodes and drains to the deep cervical nodes just like the larynx.
Nervous Supply
Sensory Innervation: The internal laryngeal nerve supplies the mucous
membrane of the larynx above the vocal folds and the recurrent laryngeal nerve
supplies the membrane below the folds. These nerves are both branches of the
vagus (X) nerve. The pharynx innervation is mostly from the pharyngeal plexus
where the sensory fibres are derived from the glossopharyngeal (IX) nerve,
which supplies the mucosa of most of the three parts of the pharynx. The mucous
membrane supply in the nasopharynx is from the maxillary nerve (V2).
All the laryngeal nerves are derived from the vagus (X) nerve via branches of the superior and recurrent laryngeal nerve. The pharyngeal nerves in contrast are derived from a variety of cranial nerves. The pharyngeal plexus of nerves is formed from the pharyngeal branches of the vagus (X) and glossopharyngeal (IX) nerves and it also has sympathetic input from the cervical ganglion. Pharyngeal plexus motor fibres are carried by the vagus nerve to the muscles of the pharynx and are derived from the accessory nerve. The stylopharyngeus is the exception as the glossopharyngeal (IX) nerve supplies it and the tensor veli palatini is supplied by the facial nerve (VII).
The parts of the pharynx subject to abrasions is lined with nonkeratinised stratified squamous epithelium and the rest is lined with pseudostratified colmunar epithelium with goblet cells. The larynx is mostly comprised of pseudostratified columnar ciliated epithelium but the vocal folds have stratified squamous epithelium which is less resistive to air flow. There are also ciliated cells in the larynx which serve a defensive function.
In conclusion, I can say that the larynx and pharynx show many anatomical analogies, they also show differences in structure, which facilitate their main functions in life. The arrangement is ideal because having separate tubes to the mouth and nose would be space inefficient. I believe in my essay, I have shown at leaast some similarities and differences between the larynx and pharynx. Without the arrangement that they adopt, man would not have been as well adapted to his environment and Darwinian evolutionary pressures would push man out.