TASTE Four Chief Tastes.—In man taste may be excited from most surfaces of the mouth and the pharynx, the chief exceptions being the floor of the mouth, the gums, the hard palate, and the inner surfaces of the cheeks. The principal region for taste is the upper surface of the tongue, and the four principal tastes are differently distributed on this surface. Sour is best developed on the sides of the tongue, saline on the sides and tip, bitter at the base, and sweet at the tip.
All surfaces of the mouth from which taste may be elicited are provided with taste-buds and such buds are limited to these parts. Each taste-bud is a microscopic group of cells bud-shaped in outline whose free, bristle-like ends project into a pore that opens on the surface of the tongue ; the deep ends of these cells are associated with nerve fibres that lead to the brain. There is no special nerve of taste but the fibres concerned with this sense enter the brain through at least two nerves, the seventh or facial nerve for those from the tip and sides of the tongue and the ninth or glosso-pharyngeal nerve for those from the base of that organ.
Sour tastes are excited by acids in watery solution. When an acid is dissolved in water its molecules break up into part-mol ecules or ions and in aqueous solutions of acids one of these is always a hydrogen ion. Hence hydrogen ions are believed to be the exciting agents for the sour taste. In a similar way the saline tastes are believed to be called forth by chlorine, bromine, and iodine ions which are produced when the appropriate salts are dissolved in water. The two remaining tastes are as a rule not excited by ions but by whole molecules, the bitter tastes chiefly by the alkaloids such as quinine, strychnine, and the like, and the sweet tastes by the alcohols, and particularly by the sugars. Sweet tastes are also excited by saccharine which, though an organic compound, is chemically quite unrelated to the alcohols or to the sugars, and by the inorganic substance lead acetate which in consequence of this peculiarity is often called sugar of lead.
Taste-buds.—When taste-buds are tested locally some are found to be excited only by acid solutions and to give rise only to the sensation sour ; others are exclusively saline and still others sweet. The same rule probably holds for the bitter taste but thus far the exclusive relation of a special set of taste-buds with this sensation has not been fully demonstrated.
It seems probable, however, that there is a particular type of taste-bud for each of the four or more classes of taste sensations. These types of taste-buds must differ chemically one from an other. Thus the free tips of sour buds must be so constituted chemically as to react with acid solutions but not with solutions of salts, of sweet substances, or of bitter substances, and a cor responding relation must obtain in the other types of taste-buds. Ordinarily one class of chemical substances may be expected to be the means of excitation for one class of taste-buds and such seems to be generally true, but it is conceivable that on the basis of the analysis just given a single substance may be so constituted as to excite two or more classes of buds and thus possess an equal number of tastes. This appears to be true of parabrom benzoic sulphinide which is sweet when applied to the tip of the tongue and bitter at its base.
The Functions of Taste.—Taste is thus essentially a chemical sense in which the materials tasted are in aqueous solution and as such react with the taste-buds, the classes of which are as numerous as the classes of taste sensations. Taste, therefore, is in reality a complex sense in which there are at least four sub senses each with independent terminals and separate sensations. It would be entirely appropriate to speak of a sense of sour, of sweet, of saline and of bitter.
Taste is naturally a most important sense for the choice of food; by means of it animals are enabled to select from the vari ous materials available to them what is appropriate for their nourishment. Organs of taste are therefore commonly found in or about the mouths of most creatures especially in close relation with those parts by which the food is crushed and its juices thereby freed. In some fishes, as for instance the catfishes, taste buds occur not only in the mouth but on the long oral tentacles and even over much of the body.
The organs of taste are also of great importance in initiating digestive operations. Through them may be excited the flow of salivary juice and of other digestive secretions; the act of swal lowing and other movements of the digestive organs, all of which constitute very important steps in the appropriation and assimi lation of food, may also be started directly or indirectly through taste.