The tongue is made up of seventeen muscles, acting in three planes—vertical, longitudinal, and transverse. Separating these muscles one from another are layers of fatty tissue, enabling the muscles to glide easily over each other. Fifteen of these muscles are extrinsic in the sense that one end of each has its point of attachment outside of the tongue. It is these muscles especially that give the tongue its great motility. By the contraction of single muscles or contraction in various combinations the tongue is protruded or drawn back, the tip raised or lowered, the dorsal surface pressed against the roof of the mouth or withdrawn toward the floor of the mouth, the tongue protruded and turned to one side or the other. In addition to these extrinsic muscles there is a pair of intrinsic muscles, each having both points of attachment within the tongue. By their contraction the sides of the tongue are raised and drawn together. These muscles are richly supplied with blood vessels and receive an especially large supply of nerve fibers from the hypoglossal, which is known as the motor nerve of the tongue, and some from the lingual branch of the seventh, or facial, nerve. It is generally believed that the tongue movements serve the sense of taste only as they facilitate contact of the sapid substances with the real taste endings; for instance, by pressure of the tongue against the roof of the mouth, and by its protrusion from the mouth to receive the stimuli upon its surface. However, it has been asserted by some investigators that the movements are of more direct use in the taste process, in that tongue movements in themselves tend to increase the sensitivity of the taste mechanism. But careful experiments in which the tongue was rendered motionless during tasting show that taste sensitivity is just as great as when the tongue is free to move.