It consists of a free part known as the tip, a body, and a hinder fixed part or root. The under-surface lies upon the floor of the mouth, whilst the upper surface is curved from side to side, and still more from before backwards so as to adapt it to the roof of the mouth. At its root, the tongue is in contact with, and firmly united to, the upper edge of the LARYNX; so that in some persons who can depress the tongue readily the tip of the EPIGLOTTIS may be seen projecting upwards at its hinder part.
The substance of the tongue consists almost entirely of muscles running in various directions. A muscle on each side unites it to the lower jaw-bone just behind the chin, enabling the tongue to protrude from the mouth; other muscles, which retract the tongue, attach it to the hyoid bone, the larynx, the PALATE, and the styloid process on the base of the SKULL.
The mucous membrane on the under-surface of the tongue is very thin. In the mid-line, a fold of mucous membrane, the frenum, passes from the under-surface to the floor of the mouth. On the upper surface or dorsum of the tongue, the mucous membrane is thicker, and in its front two-thirds is studded with projections or papillae, most of which are conical. Some of them end in long filaments, and are then known as filiform papillae. On the tip, and towards the edges of the tongue, small, red, rounded fungiform papillae are seen, which act as end-organs for the sense of taste – as do circumvallate papillae, each of which is surrounded by a trench along which open numerous taste-buds. These taste-buds are also found in the fungiform papillae, scattered over the throat, FAUCES, and palate. Five nerves, originating from the fifth, seventh, ninth, tenth and 12th cranial nerves supply the tongue.
The chief uses of the tongue are:
to push the food between the teeth for mastication, and then mould it into a bolus preparatory to swallowing;
as the organ of the sense of taste, and as an organ provided with a delicate sense of touch; and
to play a part in the production of speech. (See VOICE AND SPEECH.)
It is usual to classify any taste as: sweet, bitter, salt and acid, since finer distinctions are largely dependent upon the sense of smell. The loss of keenness in taste brought about by a cold in the head, or even by holding the nose while swallowing, is well known. Sweet tastes seem to be best appreciated by the tip of the tongue, acids on its edges, and bitters at the back. There are probably different nerve-fibres and end-organs for the different varieties of taste. Many tastes depend upon the ordinary sensations of the tongue.
Like other sensations, taste can be very highly educated for a time, as in tea-tasters and wine-tasters, but this special adaptation is lost after some years.