The tongue serves as an organ of taste, with taste buds scattered over its surface and concentrated toward the back of the tongue. In chewing, the tongue holds the food against the teeth; in swallowing, it moves the food back into the pharynx, and then into the esophagus when the pressure of the tongue closes the opening of the trachea, or windpipe. It also acts, together with the lips, teeth, and hard palate, to form word sounds.
Taste is one of the five special senses, in humans and other animals, by which four gustatory qualities (sweetness, sourness, saltiness, and bitterness) of a substance are distinguished. Taste is determined by receptors, called taste buds, the number and shape of which may vary greatly between one person and another. In general, women have more taste buds than men. A greater number of taste buds appear to endow a greater sensitivity to sweetness, sourness, saltiness, and bitterness. In humans, the taste buds are located on the surface and sides of the tongue, the roof of the mouth, and the entrance to the pharynx. The mucous membrane lining these areas is invested with tiny projections of papillae, each of which in turn is invested with 200 to 300 taste buds. The papillae located at the back of the tongue, and called circumvallated, are arranged to form a V with the angle pointing backward; they transmit the sensation of bitterness. Those at the tip of the tongue transmit sweetness, whereas saltiness and sourness are transmitted from the papillae on the sides of the tongue. Each flask-shaped taste bud contains an opening at its base through which nerve fibers enter. These fibers transmit impulses directly to the brain. In order for a substance to stimulate these impulses, however, it must be in solution, moistened by the salivary glands. Sensations of taste have been determined to be strongly interrelated with sensations of smell.