The evidence on this point is not absolutely consistent. Some observers, for example, feel impelled to add metallic and alkaline to the group, making six elementary qualities instead of four. Other observers,—most, in fact,—are persuaded that the metallic and alkaline qualities represent mixtures of the salt, sour, sweet, and bitter, along with unavoidable sensations of touch and smell. Thus, by a suitable mixture of strong solutions of salt and sweet substances, the alkaline taste may be very well produced. “It has been suggested that the metallic taste is due to the simultaneous development of salt and sour tastes. The failure to produce exact alkaline and metallic tastes synthetically is in part due to the difficulty of imitating the tactual and other sensations with which they are bound up.” Still other observers are convinced by careful elimination of smell sensation that the unique character of the alkaline and metallic qualities is really a question of odor.
By the application of specific drugs to the organ of taste further indications are secured that these four qualities, unanalyzable to introspection, also function in relative independence. Thus, the juice of gymnema leaves temporarily destroys the qualities of sweet and bitter, while sensitiveness to sour and salt remains unimpaired. “The true acid or sour taste may be separated from the astringent effect which accompanies it by painting the tongue several times with a five to ten per cent solution of cocaine. Cocaine first abolishes the sour taste, and after several minutes begins to abolish the astringent action of the acid solution. Later, the sour sensation begins to return, while the astringent effect is still in abeyance, so that the application of an acid solution at a certain stage during recovery enables the true taste character of sour to be differentiated.” It is also reported that when gymnemic acid and cocaine are applied to the tongue, the one abolishes the sweet and the other the bitter, thus leaving the two other tastes relatively unimpaired. Certain mixtures seem to paralyze both sweet and bitter, but the former sooner than the latter.
Distribution of the Taste Qualities
To these four elementary tastes we are not equally sensitive on all parts of the sense organ. Roughly speaking, sweet is best tasted at the tip of the tongue, and many forms of candy are prepared so as to allow as much as possible the employment of this part of the taste organ. Bitter, on the other hand, is best tasted at the back or root of the tongue, which explains why many substances do not taste bitter until we have swallowed them. The edges of the tongue are most sensitive to sour, while in adults the central area does not commonly yield taste qualities at all. In children, however, the taste buds extend not only over the whole surface of the tongue, but are also found in the walls of the cheek, the palate, and even on the larynx and epiglottis. Titchener has suggested that these facts may explain “the childish tendency to take big mouthfuls.”
Attempts have been made to determine whether these elementary taste qualities depend on separate taste buds or papillæ. Experiments show it to be true that some points respond only to sweet, sour, etc. But there are others which yield two or three or even all four qualities, while some yield no taste at all. In one such experiment thirty-nine papillæ, in a certain region, were separately stimulated by acid, sugar, salt, and quinine. Of these thirty-nine, thirty-one responded to salt, and the same number to sweet, twenty-nine to sour, twenty-one to bitter. Four yielded no taste at all, one responded only to bitter, and one to sweet. In another case of one hundred and twenty-five papillæ examined by solutions of sugar, quinine, and tartaric acid, sixty gave sensations of all three qualities (sweet, sour, bitter), twelve gave both sweet and sour, twelve sour only, seven bitter and sour, four bitter and sweet, and three sweet only. None of them seemed to give a bitter quality alone and this seems to be the general rule.
It is of course difficult in these experiments to restrict the application of the stimulus to single taste buds or even to single papillæ. But these experiments, along with the effects of drugs which we have already described, suggest that the taste buds are not all alike in function, even though they seem quite similar so far as appearance and structure are concerned.
The Vocabulary of Taste
Several investigators have been interested in the study of the taste names found in different languages and communities. It has been suggested that such a study might throw light on the order of development of the various taste qualities.
Kiesow found that both children and adults quite commonly confused bitter with salt and sour. Myers found, in studying the taste names of primitive people in the region of the Torres Straits, that they had no word for bitter. In some primitive languages the same word is used for sweet and salt. When there is a word for salt it is usually some derivative of the word for sea water. Salt and sour are also often confused. In Polynesia, indeed, a single word is used in describing salt, sour, and bitter tastes. This is analogous to the fact that in primitive languages it is often found that the same word is used to indicate blue and black.
Attempts to argue from these facts of vocabulary to facts of sensitiveness and order of development are, of course, open to many sources of error. As Myers points out, “The differences between sour and bitter are considered less striking than their common unpalatability.” It has often been pointed out that in our own language sweet is probably the only taste word that had from its very origin a gustatory meaning. In some languages even the word for sweet means literally “tasting good.”