Tongue


The tongue is a muscular organ in the mouth of a typical tetrapod. It manipulates food for chewing and swallowing as part of the digestive process, and is the primary organ of taste. The tongue's upper surface is covered by taste buds housed in numerous lingual papillae. It is sensitive and kept moist by saliva and is richly supplied with nerves and blood vessels. The tongue also serves as a natural means of cleaning the teeth. A major function of the tongue is to enable speech in humans and vocalization in other animals.
The human tongue is divided into two parts, an oral part at the front and a pharyngeal part at the back. The left and right sides are also separated along most of its length by a vertical section of fibrous tissue that results in a groove, the median sulcus, on the tongue's surface.
There are two groups of glossal muscles. The four intrinsic muscles alter the shape of the tongue and are not attached to bone. The four paired extrinsic muscles change the position of the tongue and are anchored to bone.

Etymology

The word tongue derives from the Old English tunge, which comes from Proto-Germanic *tungōn. It has cognates in other Germanic languages—for example tonge in West Frisian, tong in Dutch and Afrikaans, Zunge in German, tunge in Danish and Norwegian, and tunga in Icelandic, Faroese and Swedish. The ue ending of the word seems to be a fourteenth-century attempt to show "proper pronunciation", but it is "neither etymological nor phonetic". Some used the spelling tunge and tonge as late as the sixteenth century.

In humans

Structure

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. The average weight of the human tongue from adult males is 99g and for adult females 79g.
In phonetics and phonology, a distinction is made between the tip of the tongue and the blade. Sounds made with the tongue tip are said to be apical, while those made with the tongue blade are said to be laminal.
Tongue posture is the resting position of the tongue in the mouth. Evidence demonstrates that the tongue plays a role in mouth and face development.

Upper surface

The upper surface of the tongue, the dorsal surface, 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 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 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 for somatosensory perception and by the chorda tympani for taste perception.
Both parts of the tongue develop from different pharyngeal arches.

Undersurface

On the undersurface, the ventral surface, 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.

Muscles

The eight muscles of the human tongue are classified as either intrinsic or extrinsic. The four intrinsic muscles act to change the shape of the tongue, and are not attached to any bone. The four extrinsic muscles act to change the position of the tongue, and are anchored to bone.
Extrinsic
The four extrinsic muscles originate from bone and extend to the tongue. They are the genioglossus, the hyoglossus the styloglossus, and the palatoglossus. Their main functions are altering the tongue's position allowing for protrusion, retraction, and side-to-side movement.
The genioglossus arises from the mandible and protrudes the tongue. It is also known as the tongue's "safety muscle" since it is the only muscle that propels the tongue forward.
The hyoglossus, arises from the hyoid bone and retracts and depresses the tongue. The chondroglossus is often included with this muscle.
The styloglossus arises from the styloid process of the temporal bone and draws the sides of the tongue up to create a trough for swallowing.
The palatoglossus arises from the palatine aponeurosis, and depresses the soft palate, moves the palatoglossal fold towards the midline, and elevates the back of the tongue during swallowing.
Intrinsic
Four paired intrinsic muscles of the tongue originate and insert within the tongue, running along its length. They are the superior longitudinal muscle, the inferior longitudinal muscle, the vertical muscle, and the transverse muscle. These muscles alter the shape of the tongue by lengthening and shortening it, curling and uncurling its apex and edges as in tongue rolling, and flattening and rounding its surface. This provides shape and helps facilitate speech, swallowing, and eating.
The superior longitudinal muscle runs along the upper surface of the tongue under the mucous membrane, and functions to shorten and curl the tongue upward. It originates near the epiglottis, at the hyoid bone, from the median fibrous septum.
The inferior longitudinal muscle lines the sides of the tongue, and is joined to the styloglossus muscle. It functions to shorten and curl the tongue downward.
The vertical muscle is located in the middle of the tongue, and joins the superior and inferior longitudinal muscles. It functions to flatten the tongue.
The transverse muscle divides the tongue at the middle, and is attached to the mucous membranes that run along the sides. It functions to lengthen and narrow the tongue.

Blood supply

The tongue receives its blood supply primarily from the lingual artery, a branch of the external carotid artery. The lingual veins drain into the internal jugular vein. The floor of the mouth also receives its blood supply from the lingual artery. There is also a secondary blood supply to the root of tongue from the tonsillar branch of the facial artery and the ascending pharyngeal artery.
An area in the neck sometimes called the Pirogov triangle is formed by the intermediate tendon of the digastric muscle, the posterior border of the mylohyoid muscle, and the hypoglossal nerve. The lingual artery is a good place to stop severe hemorrhage from the tongue.

Nerve supply

Innervation of the tongue consists of motor fibers, special sensory fibers for taste, and general sensory fibers for sensation.
Innervation of taste and sensation is different for the anterior and posterior part of the tongue because they are derived from different embryological structures.
The tip of tongue drains to the submental nodes. The left and right halves of the anterior two-thirds of the tongue drains to submandibular lymph nodes, while the posterior one-third of the tongue drains to the jugulo-omohyoid nodes.

Microanatomy

The upper surface of the tongue is covered in masticatory mucosa, a type of oral mucosa, which is of keratinized stratified squamous epithelium. Embedded in this are numerous papillae, some of which house the taste buds and their taste receptors. The lingual papillae consist of filiform, fungiform, vallate and foliate papillae, and only the filiform papillae are not associated with any taste buds.
The tongue can divide itself in dorsal and ventral surface. The dorsal surface is a stratified squamous keratinized epithelium, which is characterized by numerous mucosal projections called papillae. The lingual papillae covers the dorsal side of the tongue towards the front of the terminal groove. The ventral surface is stratified squamous non-keratinized epithelium which is smooth.