The tongue is the muscular organ found in the vertebrate mouth. It is attached via muscles to the hyoid bone, mandible, styloid process, palate, and pharynx and divided into two parts by the V-shaped sulcus terminalis. These two parts, an anterior two thirds, and a posterior one third are structurally and developmentally distinct. The foramen cecum at the apex of the sulcus terminal indicates the site of embryonic origin of the thyroglossal duct.[rx][rx][rx]
The following papillae cover the tongue and are used for taste perception:
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Vallarta papillae – are arranged in a V-shape anterior to the sulcus terminalis and studded with numerous taste buds. Innervation is by the glossopharyngeal nerve (CN IX).
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Fungiform papillae – are mushroom-shaped papillae with erythematous domes, located on the lateral aspects and at the apex of the tongue.
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Filiform papillae – are slim, cone-shaped projections organized in rows parallel to the sulcus terminalis.
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Foliate papillae – are rarely found in humans (vestigial).
Another important part of the tongue is the lingual tonsil, a collection of nodular lymphatic tissue towards the posterior one-third of the dorsum of the tongue.
Structure of The Tongue
The tongue is a muscular hydrostat that forms part of the floor of the oral cavity. The left and right sides of the tongue are separated by a vertical section of fibrous tissue known as the lingual septum. This division is along the length of the tongue save for the very back of the pharyngeal part and is visible as a groove called the median sulcus. The human tongue is divided into anterior and posterior parts by the terminal sulcus which is a V-shaped groove. The apex of the terminal sulcus is marked by a blind foramen, the foramen cecum, which is a remnant of the median thyroid diverticulum in early embryonic development. The anterior oral part is the visible part situated at the front and makes up roughly two-thirds the length of the tongue. The posterior pharyngeal part is the part closest to the throat, roughly one-third of its length. These parts differ in terms of their embryological development and nerve supply.
The anterior tongue is, at its apex, thin and narrow. It is directed forward against the lingual surfaces of the lower incisor teeth. The posterior part is, at its root, directed backward, and connected with the hyoid bone by the hyoglossi and genioglossi muscles and the hyoglossal membrane, with the epiglottis by three glossoepiglottic folds of mucous membrane, with the soft palate by the glossopalatine arches, and with the pharynx by the superior pharyngeal constrictor muscle and the mucous membrane. It also forms the anterior wall of the oropharynx.
The average length of the human tongue from the oropharynx to the tip is 10 cm.[4] The average weight of the human tongue from adult males is 70g and for adult females 60g.
In phonetics and phonology, a distinction is made between the tip of the tongue and the blade (the portion just behind the tip). Sounds made with the tongue tip are said to be apical, while those made with the tongue blade are said to be laminal.
Upper surface of the tongue
The upper surface of the tongue is called the dorsum, and is divided by a groove into symmetrical halves by the median sulcus. The foramen cecum marks the end of this division (at about 2.5 cm from the root of the tongue) and the beginning of the terminal sulcus. The foramen cecum is also the point of attachment of the thyroglossal duct and is formed during the descent of the thyroid diverticulum in embryonic development.
The terminal sulcus is a shallow groove that runs forward as a shallow groove in a V shape from the foramen cecum, forwards and outwards to the margins (borders) of the tongue. The terminal sulcus divides the tongue into a posterior pharyngeal part and an anterior oral part. The pharyngeal part is supplied by the glossopharyngeal nerve and the oral part is supplied by the lingual nerve (a branch of the mandibular branch (V3) of the trigeminal nerve) for somatosensory perception and by the chorda tympani (a branch of the facial nerve) for taste perception.
Both parts of the tongue develop from different pharyngeal arches.
The undersurface of the tongue
On the undersurface of the tongue is a fold of mucous membrane called the frenulum that tethers the tongue at the midline to the floor of the mouth. On either side of the frenulum are small prominences called sublingual caruncles that the major salivary submandibular glands drain into.
| Surfaces | Dorsal (superior) and ventral (inferior) |
| Relations | Anterior and lateral – teeth Superior – hard and soft palates Inferior – mucosa of the floor of the oral cavity, sublingual salivary glands, posterior wall of oropharynx Posterior – epiglottis, pharyngeal inlet Lateral – palatoglossal and palatopharyngeal arches |
| Muscles | Intrinsic – superior longitudinal, vertical, transverse, inferior longitudinal muscles Extrinsic – genioglossus, hypoglossus, styloglossus, palatoglossus muscles |
| Blood Supply | Lingual artery (dorsal lingual, sublingual, deep lingual arteries), ascending palatine, tonsillar, ascending pharyngeal arteries |
| Lymphatics | Marginal, central, dorsal, submandibular, jugulo-omohyoid, deep cervical lymph nodes |
| Innervation | Hypoglossal nerve, pharyngeal plexus, lingual nerve, glossopharyngeal nerve, facial nerve, vagus nerve, chorda tympani |
| Mucosa | Stratified squamous keratinized (dorsal surface) and non-keratinized (ventral surface) epithelia |
| Lingual Papillae | Filiform, fungiform, foliate, circumvallate |
| Taste Buds | Stratified squamous epithelium Structure – gustatory cells, supportive cells, basal stem cells, taste pore |
Blood Supply of The Tongue
Blood supply to the tongue is predominantly from the lingual artery, a branch of the external carotid artery between the superior thyroid artery and the facial artery, which departs at the level of the greater horn of the hyoid bone within the carotid triangle. After branching from the external carotid artery, the lingual artery passes deep to the hyoglossus muscle and superficial to the middle pharyngeal constrictor muscle. It then gives rise to the following four arteries:
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The suprahyoid artery – supplies the omohyoid, sternothyroid, and thyrohyoid muscles. They anastomose with the corresponding vessels from the opposite side.
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The dorsal lingual arteries – arise beneath the hyoglossus muscle and pass to the posterior part of the dorsum of the tongue. They supply the mucous membrane of this region as well as the glossopalatine arch, lingual tonsils, soft palate, and epiglottis. They anastomose with their corresponding vessels on the opposite side.
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The sublingual artery – branches at the anterior border of the hyoglossus muscle before passing between the genioglossus muscle and mylohyoid muscle to the sublingual gland. It supplies the sublingual gland before giving branches to the mylohyoid muscle. One branch from the sublingual artery passes posterior to the alveolar process of the mandible and anastomoses with the corresponding artery from the other side. A second branch of the sublingual artery pierces the mylohyoid muscle and anastomoses with the submental branch of the facial artery.
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The deep lingual artery – which is the termination of the lingual artery, passes between the genioglossus muscle and inferior longitudinal muscle.
Nerves
The hypoglossal nerve (CN XII) provides motor innervation to all of the intrinsic and extrinsic muscles of the tongue except for the palatoglossus muscle, which is innervated by the vagus nerve (CN X). It runs superficial to the hyoglossus muscle. Lesions of the hypoglossal nerve cause deviation of the tongue to the ipsilateral (i.e., damaged) side.
Innervation of the tongue consists of motor fibers, special sensory fibers for taste, and general sensory fibers for sensation.
- Motor supply for all intrinsic and extrinsic muscles of the tongue is supplied by efferent motor nerve fibers from the hypoglossal nerve (CN XII), with the exception of the palatoglossus, which is innervated by the vagus nerve (CN X).
Innervation of taste and sensation is different for the anterior and posterior part of the tongue because they are derived from different embryological structures (pharyngeal arch 1 and pharyngeal arches 3 and 4, respectively).[rx]
- Anterior two-thirds of tongue (anterior to the vallate papillae) – Taste to the anterior two-thirds of the tongue is achieved through innervation from the chorda tympani nerve, a branch of the facial nerve (CN VII). General sensation to the anterior two-thirds of the tongue is by innervation from the lingual nerve, a branch of the mandibular branch of the trigeminal nerve (CN V3). The lingual nerve is located deep and medial to the hyoglossus muscle and is associated with the submandibular ganglion.
- Taste: chorda tympani branch of the facial nerve (CN VII) via special visceral afferent fibers
- Sensation: lingual branch of the mandibular (V3) division of the trigeminal nerve (CN V) via general visceral afferent fibers
- Posterior one third of tongue: On the other hand, taste to the posterior one-third of the tongue is accomplished through innervation from the glossopharyngeal nerve (CN IX), which also provides general sensation to the posterior one-third of the tongue.
- Taste and sensation: glossopharyngeal nerve (CN IX) via a mixture of special and general visceral afferent fibers
- Base of tongue
- Taste and sensation: internal branch of the superior laryngeal nerve (itself a branch of the vagus nerve, CN X)
Taste perception also is performed by both the epiglottis and the epiglottic region of the tongue, which receives taste and general sensation from innervation by the internal laryngeal branch of the vagus nerve (CN X). Damage to the vagus nerve (CN X) causes contralateral deviation (i.e., away from the injured side) of the uvula.
Muscles
The tongue’s intrinsic muscles include the following:
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The superior longitudinal lingual muscle, which shortens the tongue and curls it upward.
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The inferior longitudinal lingual muscle, which shortens the tongue and curls it downward.
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The transverse lingual muscle, which elongates and narrows the tongue.
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The vertical lingual muscle, which flattens the tongue.
The tongue’s extrinsic muscles include the following:
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The genioglossus muscle – which protrudes the tongue, and is innervated by the hypoglossal nerve (CN XII).
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The styloglossus muscle – which draws up the sides of the tongue to create a trough for swallowing following adequate mastication. The pair of styloglossus muscles works together on each side to retract the tongue. The styloglossus muscle is innervated by the hypoglossal nerve (CN XII).
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The hyoglossus muscle – which depresses and retracts the tongue and is innervated by the hypoglossal nerve (CN XII).
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The palatoglossus muscle – which elevates the posterior tongue, closes the oropharyngeal isthmus, aids in the initiation of swallowing, and prevents the spill of saliva from the vestibule into the oropharynx by maintaining the palatoglossal arch. It is the only extrinsic muscle of the tongue that is not innervated by the hypoglossal nerve; instead, it is innervated by the vagus nerve (CN X).
Functions of The Tongue
The functions of the tongue include taste, speech, and food manipulation in the oral cavity.
Taste Functions
Chemicals that interact with the taste buds in the tongue are referred to as “tastants.” Taste buds themselves are found within the various papillae of the tongue. Tastants interact with gustatory cell receptors in the taste buds, resulting in the transduction of a taste sensation. The five broad categories of taste receptors are (1) sweet, (2) salty, (3) sour, (4) bitter, and (5) umami. The lingual papillae are divided into the village (or circumvallate), fungiform, filiform, and foliate papillae. More than half of the taste buds are located on the vallate papillae at the junction of the oral and oropharyngeal tongue or tongue base.
Speech Functions
Speech is produced in part by manipulation of the tongue in the mouth against the teeth and palate within the oral cavity. The intrinsic muscles of the tongue are involved primarily in shaping the tongue for speech.
Humans also use the tongue’s movability for speaking. Only when tongue, lips and teeth work together do sounds from the throat turn into understandable letters and words. The tongue is extremely agile and quick: It can produce more than 90 words per minute, using more than 20 different movements. The tongue is essential for pronouncing the consonants “t,” “d,” “l” or the rolling “r.” When pronouncing the letter “k” the tongue is slightly narrowed at the back. And when we say “s,” the tip of the tongue moves backwards. If the tip of the tongue remains between the teeth, we hear a typical lisp.
The fact that the tongue is essential for speaking can also be seen in the ability of parrots to imitate human language: Although they only have a very simple organ of speech with fewer muscle groups than humans, they have an extraordinarily thick tongue. This helps them to produce the sounds of human language. They can touch the tip of their tongue to certain points of articulation in their mouth to imitate human words in a deceptively similar way.
Food Manipulation Functions
The tongue moves food around the mouth within the oral cavity by pressing it against the hard palate and out to the sides to enable mastication. It enables the formation of the food bolus in the oral preparatory phase of swallowing. It also takes part in the oral phase of swallowing by elevating and sweeping posteriorly to propel the food bolus past the anterior tonsillar pillar, triggering the swallowing reflex.
Mastication
The tongue is an important accessory organ in the digestive system. The tongue is used for crushing food against the hard palate, during mastication and manipulation of food for softening prior to swallowing. The epithelium on the tongue’s upper, or dorsal surface is keratinized. Consequently, the tongue can grind against the hard palate without being itself damaged or irritated.[rx]
Intimacy
The tongue plays a role in physical intimacy and sexuality. The tongue is part of the erogenous zone of the mouth and can be used in intimate contact, as in the French kiss and in oral sex. It is used for pleasuring of the vagina.
Eating and drinking
Being extremely movable, the tongue’s main job is helping us eat: It enables us to suck, turns solid food into a mash that can be swallowed (bolus) and starts the act of swallowing. The tongue can also differentiate many tastes and flavors, which helps us tell whether the food is good for us.
Sucking
The tongue is vitally important, particularly for babies when breastfeeding. It works like a piston, with the cavity of the mouth being the cylinder: When then tongue moves backward in the closed mouth it produces low pressure, which sucks in fluid for drinking.
Chewing, grinding, pressing, salivating
When we chew, the tongue and the cheeks work together to constantly move the food between the teeth so that it can be chewed. The tongue presses the crushed food against the palate and moves this bolus, which is then ready to be swallowed, to the throat. The movements of the tongue also massage small glands directly underneath it and squeeze out saliva. This starts pre-digestion of the food, and the bolus can glide down the esophagus more easily.
Swallowing
The tongue presses the bolus into the throat, which starts the process of swallowing.
Tasting
The tongue’s mucous membrane contains many taste receptors to test the things we eat and drink. They are found in the taste buds where they are arranged like orange sections around a fluid-filled funnel. This is where the chemical substances responsible for taste are washed up and recognized by the sensory cells. The sense of taste used to be vital to our survival because it was the only way to test food and tell the difference between good and poisonous or bad food. Many taste stimuli also trigger increased production of saliva and stomach acid to start digestion.
Touching
The tip of the tongue is the part of the body that is most sensitive to touch. This fine sensitivity to touch has two main tasks: On the one hand, it tests the mechanical characteristics of the food. This high level of sensitivity is the reason why small stones, bone splinters or fish bones feel much larger than they really are. This magnifying effect of the tongue protects us. On the other hand, the tongue searches the entire mouth for remaining rests of food after the first bite.
Defending
All the defense cells of the tongue are collectively called the lingual tonsil (tonsilla lingualis). It is found in the back of the mouth at the base of the tongue and is part of the lymphatic tonsillar ring. Together with the palatine tonsils and the adenoids, the lingual tonsil is responsible for defending the body against germs that can come in through the mouth.
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