Taste, the sense by which we distinguish the sapid properties of bodies, through the sensory apparatus in the mouth. Though the tongue takes the principal cognizance of gustatory sensations, the soft palate and its arches and the fauces share in this office. The nerves of taste are the lingual branch of the trifacial or fifth pair of cerebral nerves, distributed to the mucous membrane of the anterior two thirds of the tongue, and the glosso-pharyngeal nerve, which supplies the base of the tongue, the soft palate, pillars of the fauces, and upper part of the pharynx. The glossopharyngeal nerve is also regarded as the channel by which disagreeable impressions producing nausea and vomiting are propagated to the medulla oblongata. The exact seat of the sense of taste has been determined by placing in contact with various parts of the mucous membrane small sponges moistened with some sweet or bitter fluid, like a solution of sugar or quinine. It is thus found that the power of perceiving savors resides in the whole upper surface (dorsum) of the tongue, its point and edges, the soft palate, the fauces, and part of the pharynx.
The most acute sensibility to taste is found in the base, tip, and edges of the tongue, while it is less marked in the middle of its upper surface, and almost or entirely wanting in its inferior surface. These parts are also supplied with general sensibility by the same nerves which communicate to them the sense of taste; and in the tip and edges of the tongue the general sensibility is even unusually acute, as compared with the external integument or other mucous membranes. (See Tongue.) - Owing to the existence of these two kinds of sensibility in the organs of taste, we must distinguish between the different impressions produced upon them by foreign substances. The sapid qualities, properly speaking, which we distinguish by the sense of taste alone, are such as we designate by the terms sweet, sour, alkaline, salt, bitter, etc, besides various compound savors, like those of cooked meats, vegetables, and fruit. But other physical qualities are often mingled with these, which are of a different character, and are perceived by the general sensibility of the mucous membrane, here developed to an unusual degree. Thus, what is called a viscid, watery, or oleaginous taste is simply a certain modification in consistency of the substance under examination.
An oil may have a well marked taste; but this is in consequence of its partial rancidity, or of its containing other impurities or sapid ingredients. An oil which is perfectly pure and fresh is almost or entirely destitute of taste, and conveys to the mucous membrane of the mouth only the sense of its oleaginous consistency. Other substances have an irritating or pungent quality, like alcohol, red pepper, and mustard; and this pungency is also perceived by the general sensibility of the mucous membrane. Most of the condiments in ordinary use produce their effect principally by means of their pungency, mingled with a small proportion of true sapid qualities. Many articles of food also have their taste modilied or heightened by the presence of volatile ingredients perceived by the sense of smell; and this mixture of sapid and odoriferous qualities gives to the substances in question the properties which we know as their flavors. In this way are produced the flavors of wines, of tea and coffee, of cooked meats, etc.
How much of the effect produced by these substances upon the senses is due to their odoriferous qualities, may be ascertained by holding the nose while swallowing them, so as to prevent the passage of air through the nasal passages. - An essential condition of the sense of taste is, that the sapid substance should be in a state of solution. In the solid form a substance even of well marked sapid quality, like crystallized sugar, produces no effect upon the taste, and is perceived when applied to the tongue only as the physical contact of a foreign body. It is only when it is presented in the liquid form, or is gradually dissolved in the fluids of the mouth, that it impresses the nerves of taste, and its sapid qualities are accordingly perceived. This is probably because sapid substances excite the sense of taste only by being actually absorbed by the mucous membrane, and thus coming in contact with the extremities of the gustatory nerves. This absorption requires time for its accomplishment, and especially requires that the substance, to be taken up by the mucous membrane, should be in a proper condition of fluidity. It is also on this account that a free secretion of saliva is so essential an aid to the sense of taste.
When the internal surface of the mouth is in a dry condition, the savor of the food is imperfectly perceived. The salivary fluids, being themselves partly composed of organic materials, are especially adapted for rapid absorption, and, as they penetrate the mass of the food undergoing mastication, they become impregnated with its sapid ingredients, and cause them to penetrate readily the substance of the mucous membrane. The sense of taste is also materially aided by the movements in mastication, and particularly by those of the tongue; since a combination of movement and pressure is always favorable to the absorption of fluids by the animal membranes. The full effect of sapid substances is not obtained until the moment of actual deglutition. It is only after mastication is complete, and the food is actually in the involuntary grasp of the fauces and pharynx, to bo swallowed into the stomach, that all parts of the gustatory mucous membrane are brought in contact with it at once, and their sensibility heightened by the simultaneous contraction of the muscles of deglutition.