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.
Development of Taste in Infancy and Childhood
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.