CHAPTER VII
Taste-Producing Substances
Adequate and Inadequate Stimuli
Ever since the doctrine of the specific energy of sensory nerves was presented by Müller, and since modified into the specific energy of sense organs or of cerebral centers, two sorts of stimuli for a sense organ have been spoken of. There are those called adequate,—for which the sense seems to be especially adapted,—and those called inadequate, to which the sensory mechanism is sensitive by virtue of its possession of a general irritability or sensitivity. It is a matter of common knowledge that one sees because his visual mechanism is stimulated by light, or rather by the vibrations of the luminiferous ether, and that this is the appropriate stimulus for visual sensations. But it is just as well known that if one receives a blow upon the head he will see “stars,” or if he presses his finger upon his eyeball he will see patches of light. The sensations produced in this latter fashion are due to the mechanical stimulation of the sensitive visual mechanism, which responds with its specific kind of sensation. The questions to be answered in this chapter are,—What is the kind of stimulus to which the taste mechanism is especially adapted? and, Are there other or inadequate stimuli which can produce taste sensations? In answering these questions it is well to keep in mind the biological function of the sense organ of taste. Situated as it is at the entrance of the alimentary canal it has been called the “eye of the stomach,” whose duty it is to prevent the entrance into the body, by way of the mouth, of harmful substances. If this is so, the adequate stimulus for taste would be any kind of substance which might be taken for food.
Adequate Taste Stimuli
All substances may be classed either as sapid, tastable, or insipid, tasteless. And one of the main conditions for sapidity is solubility. A substance to be tasted must enter the mouth cavity as a fluid or else after being taken into it must be dissolved in the saliva. Thus, the adequate stimulus for the taste organ may be said to be a fluid. Recalling the structure and location of the sensory ends of the taste mechanism, it is at once inferred that only fluids can enter the taste pore and stimulate there the nerve endings of taste. It might then be assumed that all fluids should produce taste sensations. But all soluble substances are not sapid or tastable. Consequently, other conditions of sapidity have been sought, among them being chemical constitution.
One of the most interesting attempts to solve the question of the conditions of sapidity is that which makes the only condition necessary, the contact of the substance with the nerve endings within the taste buds. Now, Graham pointed out that all tastable substances belong to the class of crystalloids, while tasteless substances belong to the colloids. It is known, too, that living membranes are permeable by certain solutions and not by others. Colloid membranes, of which all of the mucous membranes of the body are examples, are impervious to colloids in solution, while the passage of crystalloids in solution is rapid. Hence, no colloids, even those in solution, could actually reach the free nerve endings of the taste buds. As Nagel says, however, the truth of this assumption is not easy to settle, since it is very difficult to get pure colloids and to make sure that their chemical constitution is not modified by the saliva before contact with the taste nerves. Other possibilities will be discussed in the chapter dealing with the theories of the taste function. It will be sufficient here to say that the adequate stimulus to taste is a liquid, or a solid, or gas, which may be dissolved in the saliva. Gases such as chloroform vapor, carbonic acid gas, and the like, were at one time thought to act directly upon the taste buds, but a safer view is that the gases are first dissolved in the saliva before acting on the taste mechanism. A few experiments have been reported in which the gases were said to have produced taste sensations when the mucous membrane was dry, but it is practically impossible to produce this condition, since the small glands of the tongue open directly into the fissures containing the taste buds and tend to keep the neighborhood of the taste pores in a moist state.
To say that the adequate stimulus for taste is a fluid is to give only an incomplete description of adequate stimuli. In the case of vision the adequate stimuli are ether vibrations; and the different visual sensations, reds, yellows, blues, etc., are attributed to ether vibrations of different rate.
And in the sense of hearing, the adequate stimulus of which consists of air vibrations, it is the different vibration rates that account for the qualitative or pitch differences in sound sensations. What characteristics of the stimulating fluids are responsible for the specific sensations sweet, sour, bitter, and salt? The answer to this question has been sought in the chemical constitution of the sapid substances. There is found to be a certain relation between chemical groups and the taste sensations they produce. Kiesow and others have pointed out that acids are sour; that many chemical salts have a salt taste; that many carbohydrates taste sweet, and that most of the alkaloids are bitter. There are too many exceptions to these simple relations between chemical structure and sensation quality to have them serve as an answer to our question. There are chemical salts which taste sweet, there are acids which do not taste sour, and there are chemical substances whose tastes differ according to their concentration and even according to the part of the tongue which they affect.
Sternberg, who has made a very extensive study of the relation between chemical structure and sensation quality, has recognized the impossibility of finding a simple relation between chemical groups and sensation quality. His method of study consisted in cataloguing separately all those substances which taste sweet, sour, bitter, and salt, and then looking for similarities of structure within the same sensation group and differences among different groups. This is just the opposite of the customary procedure which was to take the chemical groups as a starting point and examine into the tastes aroused by them. He finds no difference in the molecule of a substance producing a sweet taste and a molecule of another substance producing a bitter taste, and finds similarities among the molecules of substances producing different tastes. He is forced to the conclusion that the tastes are due to the character of the intramolecular vibrations; that the taste mechanism is capable of responding to the relations among the atoms which have escaped the other senses, even when their keenness is increased by all sorts of artificial devices. Nagel, in reviewing this work of Sternberg, says that, in spite of the interesting facts which have been accumulated by him and others, very little has been contributed to the solution of the question of the stimulus for the different taste qualities.