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.