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Oral cavity

The oral cavity (Latin: cavitas oris) is the initial part of the gastrointestinal tract. It is a space located in the lower part of the face and head right below the nasal cavity. In the oral cavity starts the digestion process, and it is the entrance to the digestive system. It also has an essential role in speech formation and respiration processes. The oral cavity consists of multiple anatomical parts, and it houses such anatomical structures as the teeth and tongue that efficiently provide functions and normal physiological processes related to chewing, digestion and speech.

Oral cavity anatomy

Although the oral cavity is a small anatomical structure, it is a significant part of the digestive system. It has two parts, four walls and two openings. The structures that form or are found within the oral cavity include the following:

Oral cavity parts

The oral cavity anatomically consists of two main parts. The anterior region is called the oral vestibule, while the posterior portion is the oral cavity proper. Both are separated by the teeth, gums and alveolar arches of the mandible and maxilla. If the mouth is open, the oral cavity proper and oral vestibule connects through the gap between the maxillary and mandibular teeth. But if the maxillary and mandibular teeth are in contact, the connection happens via a small aperture behind the last molar teeth.

Oral vestibule

The oral vestibule is a horseshoe-shaped anterior region of the oral cavity. Externally, it is bounded by the lips and cheeks, but internally by the teeth, gums and the inner aspects of the cheeks. The oral vestibule allows the external environment to reach the oral cavity proper.

Oral cavity proper

The oral cavity proper is the posterior part of the oral cavity located behind the teeth. The oral cavity proper extends from the alveolar arches of the maxilla and mandible anteriorly to the entry into the oral part of the pharynx (oropharynx) posteriorly. The oral cavity proper houses the tongue, soft and hard palate, minor salivary glands and three pairs of major salivary glands such as parotid, sublingual and submandibular glands. The tongue occupies most of the oral cavity proper.

Oral cavity walls

The oral cavity is composed of four walls - a roof, two lateral walls and a floor, and it presents with two openings instead of walls anteriorly and posteriorly. Overall, the anatomical bone frame of the oral cavity is formed by the maxilla, mandible, palatine bone, hyoid bone and parts of the sphenoid and temporal bones.

The roof or superior wall is formed by the hard palate anteriorly and the soft palate posteriorly. It separates the nasal cavity from the oral cavity. The mucous membrane and soft tissue of the cheeks, mastication and facial muscles (mostly buccinator) make the inside of the lateral walls. The floor or inferior wall of the oral cavity is mainly formed by the mylohyoid and geniohyoid muscles. Also, within the floor lie the sublingual and submandibular salivary glands.

Oral cavity openings

The oral cavity has two openings - one anterior and one posterior. Anteriorly, the oral cavity begins at the border between the facial skin and lips, and it opens through the oral fissure that connects the external environment with the oral vestibule. The food is taken into the oral cavity through the oral fissure, and several muscles control its movements. These muscles include the orbicularis oris, depressors and elevators of the lips and zygomaticus major and minor muscles. Also, the risorius and buccinator muscles provide motions of the anterior opening.

The posterior opening connects the oral cavity proper with the middle part of the pharynx, known as the oropharynx. The junction between the oral cavity and pharynx is called the oropharyngeal isthmus or the oropharyngeal opening, also the isthmus of the fauces. It is a narrow opening leading to the throat. Overall, the oropharyngeal isthmus is formed by the uvula, palatine and lingual tonsils, root of the tongue and palatopharyngeal arch.

Mucous membrane of oral cavity

The surfaces of the oral cavity are lined by a thick mucous membrane called the oral mucosa. It consists of two layers - external and internal. The external layer is the epithelium, but below it lies the internal layer known as the lamina propria - a thin layer made of loose connective tissue.

Opposite to the skin, the surfaces of the oral cavity are lined mainly by the stratified squamous non-keratinized epithelium. The mucosal part of the lips and cheeks, part of the soft palate that faces the oral cavity, the floor of the oral cavity and the ventral surface of the tongue are covered by a non-keratinized epithelium.

In contrast, part of the hard palate that faces the oral cavity, gums, most of the dorsal surface of the tongue and the outer part of the lips are lined by the orthokeratinized (entirely keratinized) or parakeratinized (partially, incomplete keratinized) epithelium. Opposite other anatomical structures covered with the stratified squamous epithelium, the teeth have decalcified tissue.

Oral cavity functions

Overall, the oral cavity provides functions related to three physiological processes. It helps to accomplish digestion, respiration and the formation of speech.

Digestion

As the oral cavity is the initial part of the digestive system, its primary function is to provide digestion. Overall, the oral cavity starts two different types of digestion processes - mechanical and chemical food processions. After the person has seen or smelt the food, salivary glands begin to produce saliva that later helps with the food processing, hydration and lubrication. Once the food is uptaken, it is chewed and crushed into smaller particles and pieces, and this process is called the mechanical food procession. It is mainly provided by the teeth, muscles of mastication and hard palate.

The chemical food procession in the oral cavity is mainly provided by the salivary gland produced saliva as it contains an enzyme called amylase that starts to digest carbohydrates already in the mouth. More precisely, amylase starts to break down and hydrolyze starch into smaller molecules. Starch is found in many products such as potatoes and bread.

The tongue that occupies most of the oral cavity has an essential role in the digestion process as well. It helps to form a bolus (food that has been chewed and mixed with saliva) from chewed food by pressing it against the hard palate and moving it further to the pharynx. Nevertheless, the tongue also helps to taste the food as it contains many lingual papillae on its dorsal surface.

Speech formation and respiration

Besides the primary oral cavity function, it has two more essential roles. The oral cavity modifies the sound produced in the larynx, and it is responsible for speech formation. The lips, soft and hard palates, teeth and tongue are classified as articulators located in the oral cavity. All these structures help to resonate voice and support word formation.

Besides speech formation, the oral cavity is involved in the respiration process. Together with the nasal cavity, the oral cavity provides air passage. The air from the external environment via the oral cavity is transmitted to the lower airways of the respiratory system.

Oral cavity disorders

There is a wide variety of disorders causing abnormal functioning of the oral cavity and its anatomical changes. If left untreated, oral diseases can have a substantial negative and even life-threatening impact on other organ systems. Many disorders associated with the teeth can lead to serious heart issues, as poor dental health and hygiene can increase the risk of infections that can get into the bloodstream and cause bacterial endocarditis.

Finally, the oral cavity is visible to other people, and disorders affecting it can significantly impact mental health of a person. Most conditions of the oral cavity are easily preventable and treatable unless diagnosed early. All disorders can be divided into two groups - lifelong disorders and genetic disorders. The most common risk factors causing various diseases in the oral cavity include the following:

Poor and unhealthy diet that is rich in sugars and contains fast food
Use of tobacco and alcohol
Poor hygiene
Obesity
Low social status
Genetic predisposition
Age
Stress
Medications that reduce the amount of salivary gland produced saliva
Teeth clenching or grinding
Presence of chronic or systemic diseases (such as diabetes)

Lifelong disorders

The most common conditions include dental caries or tooth decay (also dental cavities), periodontal disease (gum disease), infectious diseases such as HIV manifestations in the oral cavity or oral herpes, various traumas and oral cancers. Oral cancers are among the most common cancers, affecting any part of the oral cavity. Mainly they affect older people and men.

Genetic disorders

Although genetic disorders are not as common as lifelong diseases, various oral manifestations of genetic diseases are common. Cleft palate and cleft lip are among the most common oral presentations and congenital disabilities associated with inherited genetic disorders. This condition happens during the development of the anatomical structures of the face in an unborn baby when the structures do not completely fuse and close. Down syndrome can present with cleft palate and cleft lip.