As the fluids pass over the sides and superior surface of the tongue still farther back they meet the foliate and the circumvallate papillæ. The character of these papillæ is well adapted to retard the fluids in their passage and give ample time for stimulating the taste nerves. The former does this by holding the fluid in its long folds, or ditches, and the latter by collecting it in the circular ditches surrounding the papillæ proper. In these two types of papillæ real taste buds are found, with their taste pores leading from the surface into the interior of the taste bud. It is necessary, then, that the fluid be retained long enough to reach these hidden parts. As might be expected, there is a rather long latent time for the sensations aroused in these parts, namely, sour and bitter. Tongue movements would be of service here, perhaps even more than in the forward portion of the tongue, in forcing the fluids more rapidly through the taste pore. But the tongue movements are said to be of use in still another way. The bases of the papillæ beneath the epithelial layer are supplied with a rich network of small veins. Now, tongue movements increase the flow of blood to the tongue and these veins become congested with blood. Thus, the veins form a kind of erectile mechanism through which the papillæ become swollen, and at the same time the crevices in the epithelial tissue are opened wider, and easier access to the taste buds results. This hypothesis of the erectility of the papillæ is not generally accepted. The devices in connection with the circumvallate and foliate papillæ, the circular and linear ditches, for retarding the fluid, may account for certain other characteristics of taste sensations, namely, the difference in duration of the taste sensations. Since the depressions and the taste pores become filled with the sapid substances, the taste sensations ought to last as long as the fluid remains, or until the taste organs become adapted to them, and thus interfere with the production of new and different sensations. But a corrective device for this defect has been assumed by certain investigators in the form of the numerous secretory glands found in the mucous membrane of this part of the tongue. These glands are said to pour their secretions through ducts into these depressions and flush them out, thus removing stimulating fluids and making way for new ones. This mechanism would prevent the confusion which would necessarily occur from the mixture of old and new sapid solutions in the depressions of these papillæ. _The Function of the Taste Buds_ Real difficulties and differences of interpretation come when explanation of what takes place in the taste bud is attempted. Such questions as the following arise, none of which has received a perfectly definite answer. Theories of various sorts are all that can be offered in this connection: