Any condition affecting the mucous lining of the mouth cavity may be expected to modify taste sensitivity. For instance, burning the tongue with a hot drink will destroy taste for a time. Tumors and abnormal thickening of the epithelium of the tongue will likewise disturb the taste function. These effects are clearly due to a direct action upon the taste buds or the gustatory pores, and may be local or general in character. Under this head might be mentioned the effects of drugs applied to the tongue surface, but they have been discussed elsewhere. Hallucinations of taste likewise, of which there are a great variety, have been described in another connection.
Racial Differences in the Structure and Function of the Taste Organs
A number of races, e.g., Negroes, Japanese, Europeans, etc., have been studied to determine differences in the taste mechanism, but little of significance has been found. The number of papillæ upon the tongue is just about the same in every case. The slight differences of size and arrangement of the papillæ, especially the circumvallate type, are not such as to be of much importance from the functional point of view. The variation in these respects is so large within any one racial group that there is little likelihood of finding significant racial differences.
CHAPTER X
Evolution of Taste
Sensitivity of the Unicellular Organisms
The study of the sense organ of taste in adult human beings consists of an examination of the taste sensations resulting from controlled stimulation of limited parts, supported by the microscopical examination of the structures found in the regions in which these taste sensations can be aroused. The relation of cause and effect is then assumed. Neither method taken alone will suffice, as there always remains the possibility of function in the absence of definitely recognized taste structures and also the possibility of the presence of functionless structures. The difficulties and uncertainties arising in this combined study of structure and function have been discussed in earlier chapters.
But if the determination of the taste organs and their localization offers difficulties in the adult human being, these are multiplied many times when the study is carried to the lower animals. The method of stimulation is greatly limited, to what extent depending upon the kind of animal studied, because the results of the application of stimuli must be interpreted from the forms of behavior exceedingly crude, compared with the language behavior of man. This handicap in the study of taste is great, as compared with sight and hearing, at least, on account of the extremely close relation between the taste and smell organs in position and in the nature of their appropriate stimuli. In case of certain of the lower animals the assumption of distinct states of consciousness corresponding to our experiences of taste and smell is unwarranted, as is even the assumption of any consciousness at all.
The study of taste structures reduces itself largely to the search for sense organs resembling those of man, and in the same neighborhood as they are found in man. Here again the difficulty is especially great in taste, because the taste organs even in man are not very highly differentiated from other structures, and the really essential part of the organ is not definitely known. (See [Chapter VI.]) In the search for taste in lower animals one must rely much upon the expectation of finding the taste mechanism in the mouth or its immediate neighborhood. Structures found here and not known to function otherwise are likely to be looked upon as taste organs. These assumptions from location are then tested by stimulation of the parts with sapid substances and looking for characteristic responses, and further by extirpating these organs and noting the effect upon behavior. Other criteria of sensitivity, which can be used especially in the study of sight and hearing, such as rate of fatigue, reaction time to stimulation, and the like, are of little use on account of the above-mentioned close relation between taste and smell.
When looked at from the evolutionary point of view, all of the senses are seen to have developed through modification of the sensitivity of a single structure, the cell, with its additional properties of conductivity and motility. In the simplest living organisms, for instance, one finds sensitivity to consist in the irritability common to all living cells, and the sense organ to be represented by the whole cell. Still, the amœba, one of these unicellular organisms, reacts differently to the contact of food substances and to purely mechanical stimulation. And the white corpuscles of the blood in the human body are said to adjust their behavior according to the chemical composition of their surroundings. So, even in this earliest stage of evolution, before any differentiation of structure appears, one sees a reaction analogous to the taste reactions of the higher animals.