Function of Tongue and Salivary Glands
When sapid substances are taken into the mouth as solids, liquids, or gases they either become dissolved in the saliva or mixed with it. The glandular activity, with the resulting secretion of the saliva, as described in chapter V, may begin at the sight of the objects or may not begin until the substances have come into contact with the linings of the mouth cavity or tongue. The breaking down of the solid substances and their mixture with saliva is facilitated by chewing movements and by the movements of the tongue. When the substances have been transformed into the liquid state they move toward the back part of the mouth, from which the swallowing reflex movements will carry them into the gullet and stomach. In the course of this movement the fluids will come into contact with the tip, the superior surface, and sides of the tongue, and with portions of the mucous linings of the mouth. And it is just in these regions that we find that the taste organs are located.
The uneven surface of the tongue, due to the presence of the papillæ, tends to retard the movement of the fluid substances and to give them time to affect the taste organs. It will be recalled that on the tip and the superior surface of the front part of the tongue there are few taste buds found, even where the papillæ of the filiform and fungiform type are numerous, but a tremendous number of free nerve endings are found close to the surface of the epithelial covering of the tongue. They can be affected by the fluids without passing through a gustatory pore into the taste bud. Now, it happens that the latent time of the sweet sense is very short compared with that for bitter. And since it is known that the bitter sensations are aroused especially by stimulating the circumvallate papillæ, which contain real taste buds, it seems quite probable that the free nerve endings in the forward part of the tongue are real sensory ends of taste and are directly affected by the fluid stimuli. It was at one time supposed that sweet tastes could not be aroused on this part of the tongue without the aid of tongue movements. Although this is no longer believed, it is, nevertheless, likely that tongue movements which would press its surfaces against neighboring parts of the mouth cavity would bring the sapid substances into contact with the free nerve endings, and that more quickly than in the absence of any movement.
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:
1. Does the sapid substance affect the taste-bud cells, or only the nerve fibrils that twine around these cells?
2. If it affects these cells, does it affect the gustatory cells only, or both these and the supporting cells?