The tongue has several uses in the body. The saliva it produces helps to soften food and make it more swallowable, it helps to maintain negative pressure required to suck, and it enables food to be chewed so that it can be swallowed more easily. Without the tongue, speech is difficult to produce or understand.
The tongue is an organ made up of muscles which are interwoven, striped muscles and glands, interspersed with fat. The tongue is covered by a mucous membrane and attached to a number of bones, including the hyoid bone at the back of the throat, the epiglottis by three folds of mucous membrane and the pharynx by extrinsic muscles and the soft palate. The tongue is attached by the frenulum at the midline to the bottom of the mouth. The sublingual caruncles are small prominences that drain away saliva from the major salivary submandibular glands. The posterior pharyngeal part of the tongue is around one third of the full length of the organ and is closest to the throat. The anterior oral part of the organ is the visible part of the tongue. The terminal sulcus is a v-shaped groove which divides the anterior and posterior parts. The left and right sides of the organ are divided by the lingual septum, vertical fibrous tissue along the length of the tongue apart from the very back of the throat.
Some mammals such as frogs have adapted the tongue to help them catch prey. Some reptiles use their tongues as sensory organs, but cats use their tongue for grooming. In humans, the edges of the tongue touch the teeth, which helps the person to swallow and talk coherently. The human tongue has eight muscles attached: four paired intrinsic muscles, which are not attached to a bone and help to change the shape of the tongue for speech, eating and swallowing. These are the superior longitudinal muscle, the inferior longitudinal muscle, the vertical muscle and the transverse muscle. The four extrinsic muscles which are attached to bone that help the organ to change its position, allowing it to be extended beyond the mouth (genioglossus attached to the mandible), brought back inside and depressed (hyoglossus, attached to the hyoid bone), the sides of the tongue to be drawn up, creating a trough for swallowing (styloglossus from the styloid process of the temporal bone) and elevating the back of the tongue for swallowing, depressing the soft palate and moving the palatoglossal fold towards the midline (palatoglossus attached to the palatine aponeurosis).
The blood supply of the tongue is mainly through the lingual artery via the external carotid artery. Lingual veins lead into the internal jugular vein. Some blood is also supplied through the facial artery. Muscle movement for the tongue is supplied by efferent motor nerve fibres via the hypoglossal nerve. The palatoglossus is supplied by the vagus nerve. Nerve supply for taste and sensation is different depending on whether it is the anterior or posterior part of the organ. This is because different embryological structures form them. The posterior one third has sensation and taste supplied by the glossopharyngeal nerve. The anterior two thirds sensation is supplied by the lingual branch of the mandibular division of the trigeminal nerve, through general visceral afferent fibres. The taste is supplied by the chord tympani branch of the facial nerve.The base of the tongue is brought both taste and sensation through the superior laryngeal nerve from the vagus nerve.
The upper surface or dorsum of the tongue is covered by rough projections of the mucous membrane, known as papillae. These help to increase the surface area, help to grip food and protect the taste buds. The taste buds are found between papillae and are sensory receptors for taste. The taste buds detect chemicals which are from food in the mouth, dissolved by saliva. The number of taste buds varies from person to person. It can be as many as 1,000 per square cm on the tip of the tongue, while other people may only have a few individual taste buds. There can be as many as 10,000 taste buds in the throat, palate and tongue or as few as 2,000. They also are abundant during childhood, but reduce in number as a person ages. Types of papillae include fungiform papillae, which can be found at the dorsal surface and sides of the tongue. They are supplied by the facial nerve. The foliate papillae are the grooves and ridges which are found at the lateral borders of the posterior part of the organ. They are supplied by the facial nerve and glossopharyngeal nerve. The circumvallate papillae are few in number (10-14 as an average) and can be found at the the back of the oral part in a circular row just in front of the terminal sulcus. They are activated by the glossopharyngeal nerve. There is a fourth type of papillae which are not part of the sense of taste. The filiform papillae are present in highest numbers of all the papillae but they do not have taste buds. They help to provide abrasion.
Taste buds can detect five tastes, including salty, which is caused by salts including sodium chloride and sodium bicarbonate, sweet caused by carbohydrates, including fructose, sucrose and artificial sweeteners. Food that causes the bitter taste are usually unpalatable or undesirable, sour flavours are caused by acidic compounds, including vinegar and critic acid and umami which is savoury, found in foods that taste ‘meaty’, including cheese, mushrooms, meat and monosodium glutamate.