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Core
The primary function of the mouth is to act as a receptacle for food and drink, and to initiate digestion.
The opening to the mouth is known as the oral fissure. Immediately posterior to this is the vestibule, which is a slit-like space between the lips and the gums and teeth. Beyond the teeth is the oral cavity proper.
The boundaries of the oral cavity proper are:
- Anterior boundary - Teeth
- Lateral boundaries of the oral cavity proper - the dental arches. The cheeks, containing buccinator, form the outer lateral walls of the oral vestibule.
- Roof - Hard and soft palate
- Floor - Tongue and mylohyoid muscles
- Posterior boundary - Palatoglossal arches
The palate forms the roof of the oral cavity. The anterior 2/3 of the palate is made of bone and is called the hard palate. The soft palate makes up the posterior 1/3 of the palate; it is moveable and made up of soft tissue. Most muscles of the soft palate are innervated by the vagus nerve (CN X) through the pharyngeal plexus. The exception is tensor veli palatini, supplied by the nerve to medial pterygoid from the mandibular division of the trigeminal nerve (CN V3). Suspended from the soft palate is the uvula, which has roles in speech, lubrication of the mouth and stimulation of the gag reflex.
The soft palate is continuous with the wall of the pharynx and extends laterally to form two arches: the palatoglossal arch (anteriorly) and palatopharyngeal arch (posteriorly). The space between these arches is called the palatine fossa, which accommodates the palatine tonsils.
Diagram - Basic anatomy of the mouth
Public Domain Source [Public domain]
The tonsils are aggregations of lymphoid tissue found within the oropharynx and nasopharynx. They are therefore part of the immune system and specifically protect against ingested and inhaled pathogens.
There are 6 tonsils, which form an arrangement called Waldeyer’s ring:
- Lingual Tonsil - located within the submucosa of the posterior 1/3 of the tongue.
- Palatine Tonsils (x2) - located within the palatine fossae.
- Tubal Tonsils (x2) - located in the lateral walls of the nasopharynx, near the opening of the Eustachian tube.
- Pharyngeal Tonsil (adenoids) - located in the roof of the nasopharynx.
The pharynx and tonsils can become inflamed due to infection, resulting in pharyngitis and/or tonsillitis respectively. Most cases of tonsillitis are viral, however, it can also be caused by the bacterium Streptococcus pyogenes, which produces a more severe infection. The likelihood of a tonsillitis being bacterial in nature can be estimated using the Centor criteria or FeverPAIN score. Viral cases are managed conservatively whereas bacterial tonsillitis is treated with an antibiotic such as penicillin V. For more information, check out our article on Streptococci.
Image - Inflamed palatine tonsils with a purulent exudate
Public Domain Source by Michaelbladon [Public domain]
The primary function of the teeth is mastication (chewing), which increases the surface area of food for digestive enzymes to act on and enable bolus formation. The teeth also play a role in articulation of speech.
Humans have 2 sets of teeth during their lifetime: deciduous (primary) and permanent (secondary). Children have 20 deciduous teeth, which shed between the ages of 6-12 years as the permanent teeth erupt and take their place. Adults normally have 32 permanent teeth, most of which erupt by age 12 years. The 3rd molars (wisdom teeth) can take up to the age of 25 to erupt and may not even be present in some individuals.
Teeth are classified by the characteristics and function:
- Incisors (8) - located centrally, these thin knife-like teeth act to cut food and break off food in the initial bite.
- Canines (4) - these cone-shaped teeth are designed for shearing food.
- Premolars (8) - these have 2 cusps and a flatter surface for mashing and grinding.
- Molars (12) - these back teeth have 3 or more cusps and a flat surface for mashing and grinding.
Each tooth has a crown, a neck and a root. The crown is the externally visible part of the tooth which projects from the gingiva. The neck is the region between the crown and root. The root is fixed in the tooth socket by some connective tissue called the periodontium. The majority of the tooth is composed of dentine. The crown is lined with enamel (the hardest substance in the body!) and the root is lined with cementum. The root canal transmits nerves and blood vessels.
Diagram - Anatomy of the tooth
Creative commons source by KDS4444 [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Bacteria in the oral cavity produce acid and enzymes which may break down the enamel of the tooth, resulting in dental caries. Left untreated, this decay can extend into the pulp cavity resulting in infection and inflammation of the pulp, which presents with considerable toothache. This could eventually lead to abscess formation and loss of the tooth. Treatment involves removal of the decayed tissue and replacement with a prosthetic filling.
Bacterial deposits in the mouth can also irritate the gums, leading to gingivitis. This presents with swollen, red gums which are tender and bleed easily. Gingivitis can potentially progress to periodontitis and abscess formation. Management involves maintaining good oral hygiene (including flossing) and chlorhexidine mouthwashes can also be helpful when used short term. The dentist or hygienist can also offer more specialist cleaning treatments.
The tongue is a very mobile, muscular organ which occupies most of the oral cavity at rest. It has important roles in taste, speech and swallowing.
The tongue is lined with mucosa, which contains projections called papillae. The papillae increase the surface area of the tongue and are covered with taste buds.
The tongue is anchored to the floor of the mouth via the lingual frenulum.
Muscles of the tongue are grouped as either intrinsic or extrinsic:
- Intrinsic - there are 4 pairs of intrinsic muscles which act to alter the shape of the tongue.
- Superior longitudinal
- Inferior longitudinal
- Transverse
- Vertical
- Extrinsic - these muscles connect the tongue to surrounding structures and act to alter the position of the tongue.
- Genioglossus - important in protruding the tongue.
- Hyoglossus
- Styloglossus
- Palatoglossus
Most of the muscles of the tongue are innervated by the hypoglossal nerve (CN XII). The exception is palatoglossus, which is innervated by the vagus nerve (CN X).
Diagram - Muscles of the tongue
Creative commons source by OpenStax [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The special sensory innervation of the tongue (i.e. taste) is the following:
- Anterior 2/3 - chorda tympani, a branch of the facial nerve (CN VII)
- Posterior 1/3 - glossopharyngeal nerve (CN IX)
The general sensory innervation of the tongue is:
- Anterior 2/3 - lingual nerve, a branch of the mandibular division of the trigeminal nerve (CN V3)
- Posterior 1/3 - glossopharyngeal nerve (CN IX)
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