Sternberg, whose work was mentioned in chapter VII, recognized only two elementary taste sensations, sweet and bitter, and he found no differences in the molecule of substances producing these two sensations. He concludes that it is the intramolecular vibrations that form the real taste stimuli. By transferring the seat of the activity from molecular vibration to intramolecular vibration the whole matter becomes more difficult of solution and verification. Still, the modification of the theory of Ramsay, made necessary by the work of Sternberg, would leave its essentials, namely, that vibrations of some sort form the stimulus and that the sense organ of taste is tuned to receive only a limited range of vibration rates.
Granting that a dynamic theory as outlined above, with possible modifications to meet new discoveries, is correct, still another question requires an answer. In the visual organ affected by ether vibrations of different rates every theory assumes certain parts of the mechanism sensitized especially to certain vibration rates. To take the theory of Hering, for example, there are three photochemical substances, one decomposed by a relatively slow vibration rate, another by a more rapid vibration rate, and so on. And in the auditory mechanism there is the basilar membrane, capable of vibration in parts in sympathy with different rates of vibration of the air. The answer to this question concerning the taste sense is peculiarly difficult, in that it is impossible to stimulate individual taste buds, and even difficult to stimulate a single papilla, which may contain many buds. And then it must be further borne in mind that each bud contains many nerve endings, so that what is stimulated in experimental work is really a very complex portion of the sense organ.
As mentioned in another connection, certain papillæ when stimulated with various substances respond to but one, e.g., sweet, sour, bitter, or salt, while others respond to two or three stimuli. Further, it is well known that certain portions of the tongue when stimulated produce a predominance of certain kinds of taste sensations. On the other hand, an examination of the taste buds or of the nerve endings within the taste buds shows no differences among them. Still, in this respect taste does not differ from sight. The cones of the retina, which are sensory ends of vision, show no differences in structure, and it is generally believed that all of the elementary sensations can be produced by the stimulation of one single cone. If it be true that the nerve fibrils in the taste buds themselves receive the stimulus, then any specific characteristics of the receiving mechanism would have to be looked for in these nerve endings.
Oehrwall has accounted for the facts of the specific sensitivity of different parts of the tongue and of different individual papillæ mentioned above by the assumption that each taste bud has a specific function and that certain papillæ of the tongue have buds all of one sort, other of two sorts, and so on. Nagel prefers to modify this view to the extent that each taste bud is capable of arousing every elementary sensation, but is adapted to respond best and easiest to a certain specific stimulus with a certain elementary sensation. This view is analogous to that of the visual photochemical substances which, according to the Helmholtz theory, were each sensitive to all wave lengths of light, but not equally.
The previous review of theories does not consider the possibility of the specific energy of brain centers, rather than of the peripheral sense mechanism—that is, the possibility that the sensation qualities, sweet, sour, etc., may be due to structural characteristics of brain centers, rather than of sense organs. This is a question which is unsolved for other senses and consequently is not peculiar to taste. The present tendency seems to be toward attributing the different sensation qualities to the sensitivity of the sense organ, or else to divide it between brain center and sense organ. The following facts are considered as indirect evidence of the specific energy of the sensory ends of taste: (1) The distribution of taste sensitivity over the tongue. (2) The effect of certain drugs, e.g., cocaine, which destroys the taste sensations one at a time. This suggests that the sensory ends that have to do with the different sensations are differently affected by the drug. (3) The fact that the same substance as it passes over the surface of the tongue arouses different sensations, e.g., sodium sulphate, which is sweet on the tip and bitter on the back of the tongue.
CHAPTER IX
The Development of Taste in the Individual
Development Before Birth
The structures concerned with the taste sensations develop early in the life of the human embryo. As early as the twelfth day the rapid development of the head end of the embryo causes an infolding of the ectoderm (outer layer of the embryo), which later forms the mouth and nasal cavities. And about the twelfth week the mouth structure is fairly complete. It has at this stage become separated from the nasal cavity through the growth of the hard and soft palate.
At the twelfth week the tongue is also fairly well developed as a single organ, although originating from three or four separate parts which grow together. The anterior two-thirds of the tongue—that is, all of that portion back to and including the rows of circumvallate papillæ of the adult tongue,—grows from the floor of the original mouth structure. It is in this portion alone that papillæ are to be found. At the end of the fourth week the circumvallate papillæ begin to appear and at the twelfth week the fungiform and filiform papillæ begin to appear.