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.
About the fifth month there begins a multiplication of the cells in the germinative layer of the epithelium covering the tongue, which marks the beginning of the taste buds. At the eighth month the development has advanced so far that the gustatory cells can be distinguished from the supporting cells, and the whole taste bud has separated itself somewhat from the surrounding tissue. The nerve fibrils found in the fully developed taste bud come into connection with the gustatory cells very early in their history—in fact, the presence of the fibrils is considered by some authorities as the stimulus to modification of the epithelial cells into those special forms and the consequent formation of the taste buds.
At birth the most essential parts of the taste mechanism seem to be fully developed and able to function. The taste buds and the taste pores by which they communicate with the tongue surface have the same structure as in the adult. The papillæ, however, according to Stahr, have not attained the adult form. From an examination of the tongues of a large number of newborn babies he reports that the foliate and the circumvallate papillæ are not complete, the depression surrounding the latter being broken in places and one papilla not clearly distinguished from others. The fungiform type, too, does not at this stage resemble a toadstool, as it does in the adult, but they are more like very large filiform papillæ. Microscopical examination shows both of these types of papillæ to carry their taste buds on the top, rather than far down upon their sides. In no case did Stahr find a fungiform papilla without one or more taste buds.
The distribution of the taste buds over the surface of the mouth cavity of the fetus and the newborn, as determined by histological examination and experiment, is very extensive, as compared with the adult. The presence of taste buds and of taste sensitivity has been reported upon the arches of the palate, over the whole dorsal surface of the tongue, and the inside of the cheeks. Buds are especially numerous upon the tip of the tongue.
After birth certain changes occur in the taste organs. During the whole period of lactation the tip of the tongue remains especially sensitive to sweet stimuli, a valuable condition for the promotion of the food-taking reactions in the infant. During this period also the whole superior surface of the tongue remains sensitive to taste stimuli, but with increasing age there appears a region of insensitivity just back of the sensitive tip. At the same time there is said to be an increasing sensitivity upon the edges of the tongue.
The insensitive zone differs in extent for the different elementary tastes, but is fairly large for all. These progressive changes in sensitivity have been explained as the result of the appearance of the teeth and their importance in the bodily economy. While the individual lacks teeth and subsists on a milk diet the process of nourishment is best accomplished by having the food pass directly back over the superior surface of the tongue until it reaches a point where the swallowing reflexes are set up. However, after the teeth appear and mastication of food is necessary, then it is better that the food should pass from the tip of the tongue to either side, where it will come between the teeth! Thus, seeking the maximum taste sensation from the food is equivalent to placing it in a position to be thoroughly masticated. Further, the presence of food, and especially sour food, upon the sides of the tongue forms the stimulus to the flow of saliva, a necessary factor in preparing the food to be swallowed and digested. Now, if the superior surface of the tongue were highly sensitive to taste stimuli there would be a tendency to hold the substances upon that part in order to prolong the pleasure, thus retarding the proper mastication of the food.
A simpler explanation of the decreasing sensitivity of the upper surface of the tongue and the increasing sensitivity of the sides is the fact that, after the teeth appear, chewing would tend to collect the dissolved food substances at the sides of the tongue, rather than upon its dorsal surface, hence the greater importance of the taste buds upon the sides of the tongue.
It is difficult to see why the inside of the cheeks should be sensitive to taste stimuli in young children and why this sensitivity should be lost in the adult. Titchener has suggested that its presence in children may account for their desire to take large mouthfuls of food, thereby getting maximum taste sensation.
Whatever the biological interpretation of these changes may be, certain structural changes have been noted which seem to account in part, at least, for the changed sensitivity. The circumvallate and foliate papillæ become complete during the first few months and the taste buds, instead of being located upon the top of the papillæ, are now found rather low down upon their sides. The change in the fungiform type is more pronounced. These, more or less conical in shape at first, begin to acquire an enlarged head (by the growth of secondary papillæ, according to Stahr). Not only this, but the taste buds in all of them are now found to have moved from the top to the sides of the papillæ, and many of the buds have disappeared altogether, some of the papillæ apparently losing all taste function. There are various opinions as to how the buds change their position or are lost, but none of these are well enough established to need description.
Thus, by the completion of growth in the circumvallate and the foliate papillæ and by the degeneration of the fungiform (loss of taste buds) the region of greatest sensitivity is transferred from the tip to the sides and back part of the tongue, while the zone just back of the tip becomes entirely taste blind.
The most striking characteristics of taste in the adult as compared with that of the child, then, is what appears to be a gradual reduction in the extent of distribution of taste sensitivity and a shifting of the region of the greatest sensitivity. No doubt there are other changes, e.g., a general reduction in delicacy of taste analogous to the sort of change which is noted in the other senses as age advances. In extreme old age such a condition is quite pronounced. In hearing, for example, there is not only a dulling sensitivity but a shortening of the range of audible pitches, especially in the region of the higher pitches. So, on the tip of the tongue, sensitivity may be very much reduced or disappear entirely. These changes in taste are not commonly brought to one’s attention as are those of sight and hearing, because they affect our life less vitally. But there is little doubt that careful tests would reveal them.
It has been suggested that individuals differ considerably in the distribution and function of their taste mechanism. In the search for general laws these variations within the limit of normality have been looked upon as troublesome exceptions and not of much interest. And these differences are no doubt responsible for the lack of agreement among investigators on many points. For instance, there are persons whose sensitivity on the tip of the tongue is so poor that sweet tastes can be aroused only with the aid of tongue movements. This and other similar cases have given cause for the belief in the necessity of tongue movements for arousing all taste sensations. Some investigators have found the tonsils and the uvula sensitive, while others have found them insensitive. When one finds such conflicting statements from men like Nagel and Kiesow, individual differences seem a plausible explanation. No special studies have been made of these individual differences. Consequently, about all that one can say is that the distribution of the taste buds in different people is subject to considerable variation and that on this account one cannot definitely mark out the limits of their distribution which shall hold for all persons. The same is true of the distribution of sensitivity to specific taste stimuli, sweet, sour, bitter, and salt substances. Such vague statements as are usually made, namely, that sweet is best tasted upon the tip of the tongue, sour upon the sides, bitter upon the back, and salt over nearly the whole tongue, are true, but when one attempts to assign definite limits to these regions then great individual differences appear.
More extreme variations in taste are frequent as an accompaniment of disease or congenital malformation of the cerebrum. These variations may be in the nature of absence of sensitiveness, dull (hypo) sensitivity, or very high (hyper) sensitivity. Only one case, so far as we have found, has been reported of congenital taste blindness, either for all taste sensations or for one or two of them, such as would correspond to hereditary color blindness of the various sorts. The apparent absence of taste in certain idiots is not a form of taste blindness to be compared with color blindness. Such persons will eat sulphate of quinine with as much enjoyment as sweet food, but so will they try to eat wood or stones or paper. Nor are the cases, which are more or less common, of dulled taste sensitiveness as a result of cerebral malformation to be compared with real color blindness, which, as far as is known, at least, depends upon no such cerebral abnormality.
There are cases of acquired taste blindness, either general or for special qualities, which resemble in general character acquired color blindness. In both senses the blindness results from pathological changes in the sensory mechanism, either in the sense organ, its centers within the brain, or its connecting nerve trunks. The nature of the sensory defect depends upon the extent of the pathological change and its location. Thus, there may be general taste blindness if both of the cerebral centers are involved, a defect on only one side of the tongue if one cerebral hemisphere is involved. And if the disturbance is in the nerve trunks, only the anterior two-thirds of the tongue may be affected, or only its posterior third. Again, if the lesion is in the sense organ itself, one or more specific taste qualities only may be lost, or all qualities for a very small portion of the tongue. These latter cases are of much importance in developing an adequate theory of taste function. Nagel describes a case in which all sorts of taste stimuli produced only a sensation of salt on one side of the tongue, while on the other side taste was normal. This peculiar condition was followed by a total loss of sensitivity on the affected side.
Epileptics are said to show taste abnormalities of varying character after an attack, and lasting for hours in some cases. There is usually a loss or dulling of the sense, most prominent for salt and least noticeable for bitter. Such conditions are not peculiar to the taste sense, for there is usually a disturbance of the other special senses also after an epileptic attack.
All sorts of taste abnormalities are found in hysteria, but little is known of their underlying causes in the taste mechanism. There may be either hyper- or hypo-sensitivity, although the latter is more common; and both sides of the tongue or only one side may be affected, or even only a small portion of the tongue may be involved. Here, too, the taste abnormalities are accompanied by disturbances of the other senses.
Criminals, especially those recognized as degenerate, show taste abnormalities, usually extreme dullness of taste, along with the same sort of defect in the other senses. Attempts have been made to find a positive correlation between keenness of taste and number of stigmata of degeneration, but with no success.
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.
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.