Tongue

The tongue (lingua; Greek, glotta or glossa) is an oval muscular organ covered with mucosa that is mainly made up of highly developed striated musculature with extremely variable mobility. The tongue is highly perfused with blood and carries nerves for the senses of taste and touch. It helps in eating and during chewing, sucking, and swallowing, and it is used for the purposes of speech because it is highly mobile.The tongue almost completely fills the oral cavity and extends at the rear as far as the epiglottis. The body of the tongue can be divided into the root, dorsum, and apex (or tip), while the surfaces and edges are described as the superior surface (facies superior), the lateral margins (margines laterales), and the inferior surface (facies inferior). Lifting up the tongue reveals the anterior apex and lateral margins on the underside, while the central part at the root is seen to be fused to the floor of the mouth over a wide area.

The frenulum of the tongue (frenulum linguae) is a thick fold under the tongue that extends forward to the mandible and is moved powerfully during tongue movements. Because the frenulum of the tongue is moved a great deal during speaking, chewing, and swallowing, a denture border must not restrict the frenulum.

The root of the tongue (radix linguae) is the posterior part of the body of the tongue, which is divided from the dorsum by a V-shaped groove, the terminal sulcus (sulcus terminalis). At the posteriorly directed apex of the sulcus, there is a depression (foramen caecum linguae), a rudimentary remnant of the thyroglossal duct.

The dorsum of the tongue carries not only the free nerve endings for the sense of touch but also numerous differently shaped papillae mainly for the sense of taste (Fig 6-49). The mucosa on the dorsum is firmly (hence immovably) joined to the inner musculature of the tongue by bundles of fibers. Based on form and function, there are four different types of papilla on the mucosa of the body and root of the tongue (Fig 6-50):

1. Filiform papillae (papillae filiformes) are distributed over the whole of the dorsum and give it a velvety, rough, or coarse surface. The posteriorly directed papillae are horny at their tip and contain tactile nerves for mechanical functions, temperature sensation, and pain sensitivity.
2. Reddish, fungiform papillae (papillae fungi-formes) are mainly located at the margins and apex of the tongue and contain taste buds, while special mucous glands known as gustatory glands are absent. Instead, these papillae contain a lot of lymph cells.
3. Circumvallate papillae (papillae vallatae) are wart-shaped taste papillae that lie mainly in a V-shaped formation in front of the terminal sulcus. Each papilla is surrounded by a circular groove in whose epithelium the taste buds lie and where ducts from mucous glands end. There are branches of the taste nerve (nervus glossopharyngeus) located in the taste buds, and a few fibers of the nerve also run between the taste buds.
4. Foliate papillae (papillae foliatae) are mucosal folds running roughly crosswise along the posterolateral border of the tongue. Taste buds are densely distributed in these foliate papillae, with openings from serous gustatory glands.

The four taste qualities are sweet, sour, salty, and bitter, and each is assigned to a specific region of the tongue surface. As well as enabling the sensation of foodstuffs, the sense of taste also helps in the detection of harmful and inedible substances. Sweet is perceived as a pleasant stimulus from edible substances and functionally is checked preferably with the tip of the tongue, while salty is checked at the anterior margin, both by fungiform papillae; sour is checked at the middle margin by foliate papillae, while bitter is checked by circumvallate papillae at the base of the tongue. Poisons (eg, alkaloids such as strychnine) taste bitter, and the stimulus threshold for this taste nuance is therefore low, triggering protective reflex reactions. Because most of the taste buds sensitive to bitter tastes are located at the base of the tongue, the protective reflex is often retching.

The sense of taste is a chemically active sense, which means that the taste receptors react to substances dissolved in saliva. The finely graduated taste sensations are based on the interaction of the sense of smell and taste, which is why a head cold will dull the sense of taste. The tactile and temperature sensations as well as the sense of pain in the lips, tongue, and palate can also influence the sensation of taste, for instance with crunchy foods, alcohol, or excessively strong spices.