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Anatomy of the Tongue
The tongue is laterally symmetrical -- divided in half by a median septum. It is made up of extrinsic muscles and intrinsic muscles. Extrinsic muscles attach the tongue to the body's skeletal system and aid in its movement while keeping it firmly in place, and the intrinsic muscles allow the tongue to expand, contract, and change its shape. The intrinsic muscles are especially important in speech and swallowing food. The tongue is covered with a double-layer mucous membrane that helps block microbes from entering the body. The epithelial layer of the mucous membrane also secretes mucus that moistens both food and tongue.
Digestion
The tongue is primarily an accessory digestive organ, meaning it works in conjunction with the cheek muscles to move food around the mouth, and it keeps the food between the upper and lower teeth until it has been properly masticated. Once the food has been chewed into a consistency that can be safely swallowed, the tongue expands and contracts in such a way to shape the masticated food. It then guides the food toward to the esophagus to begin the process of digestion.
Taste
Most people associate the tongue with the ability to taste food. Although the tongue does detect tastes, it also is sensitive to tactile, thermal, and painful stimuli that can come from eating certain foods. The bumpy structures that cover the tongue are commonly thought to be taste buds, but they are actually goblet-shaped elevations called papillae. These create friction between the tongue and food; the actual taste buds are much smaller and are within the folds of the papillae. Each taste bud contains about 50 gustatory receptor cells that respond to primary tastes such as sweet, salty, bitter, savory, sour and fat.
The Tongue and Speech
The muscular articulation of the tongue also gives humans the ability to speak. The tongue is so vital to human speech that it has become an alternative term for "language." Phoneticians use the position of the tongue to classify universal vowel sounds. For example, they have traditionally used descriptions such as "high vowel" and "low vowel" to describe the shape of the tongue when vowel sounds are made. "High vowel" refers to sounds made when the tongue is arched to the roof of the mouth, and "low vowel" sounds are made when the tongue is low and flat in the mouth. This system has been updated in recent years to describe eight cardinal vowels, each made by one of eight different positions that the tongue takes during speech. These vowel sounds are universal, and no human language is complete without them.