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.

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?

3. Does the sapid substance really enter the taste bud at all, or only affect the ends of the cells which form the so-called entrance to the bud?

4. Regardless of what portion of the taste bud is affected by the stimulus, what is the character of the effect produced? Is it mere contact or mechanical stimulation, or is it a chemical process which is set up?

5. Must different types of receiving structures, of whatever form they may be, be assumed for each type of elementary taste sensation?

The structural relations among the parts of the taste bud were discussed in the chapter on Sensory Elements. There it was concluded that the analogy between the sense of taste and that of certain of the other senses, especially sight and smell, is not so close as it has seemed to be. It will be recalled that in these sensory mechanisms there are modified nerve structures, rods and cones in the eye, and the olfactory cells in the nose, which are affected directly by the stimuli, and in which a transformation of the stimulus takes place, with the resultant nerve impulse. This transformation accompanies a chemical change within these structures, hence vision and smell are called chemical senses. In the taste mechanism, also considered a chemical sense, it was natural to see in the gustatory cells of the taste bud structures with functions similar to that of the rods, cones, and olfactory cells. But the analogy between these types of structures breaks down because the gustatory cells do not have the characteristics of nerve tissue, as revealed especially by the use of differential stains. In fact, as has been said above, there seems to be no fundamental difference between supporting and taste cells. Two further facts seem to indicate that the supporting and gustatory cells take no primary part in the taste function. First, there is no more intimate connection between these cells and the nerve fibrils than that of contact, and the contact seems only incidental to the supporting function. The endings of the nerve fibrils are free from the cells. And, second, those free nerve endings in the anterior portion of the tongue seem to arouse taste sensations without the intervention of any structures resembling the taste buds or their cells. Thus, the evidence to date leads to the conclusion that the nerve fibrils alone are the parts affected by the taste stimulus.

If this hypothesis be correct, is it necessary that the sapid substances should actually enter the taste bud, or only affect its peripheral end? Many of the nerve fibrils entering the taste bud pass through its whole length and end quite near the mouth of the gustatory pore. These might be stimulated without the entrance of the stimulus within the taste bud. But there are many of these fibrils which do not reach to the peripheral end of the bud, but stop far short of this point, and then there are others that reach the entrance of the taste pore but turn back and end in the characteristic knob-like formation within the taste bud. In order that these fibrils may be stimulated upon their ends the stimuli would have to enter the taste bud.

The answer to the fourth question is indeed the most difficult of all. What is the nature of this stimulation by which a fluid substance shall start an impulse along the nerve paths to the brain, which shall there produce sensation? About this last stage of the process nothing is known either about taste or any of the other senses. But very well-developed theories exist to account for the transformation of the physical stimulus into physiological nerve impulse. For instance, in the case of vision, the stimulus for which consists of ether vibrations, these ether waves cause chemical changes in certain hypothetical substances within the rods and cones of the retina. It is this chemical change which creates the nerve impulse. In the case of hearing, for which the stimulus consists of air vibrations, these waves, being slightly modified by the more superficial portions of the auditory mechanism, finally cause vibrations of the basilar membrane, which, in turn, produces the impulse in the auditory nerve.

One of the earliest and simplest conceptions of the nature of the process in the taste organ was a mechanical theory proposed by Boyle, about 1675. He thought that the particles of various sapid substances differed in size and shape and that on account of these differences they produced different effects in their simple contact with the sensory ends of taste.

According to Graham, who announced his theory in 1889, sapidity of substances depends on their chemical constitution, colloids being generally insipid and crystalloids being sapid, hence this has been known as a chemical theory. This difference of chemical structure, discussed on page 94, was made to account for the contact, or the lack of it, between the substances and the sensory ends, but does not account for the effect produced upon the sensory ends by the substances reaching them.

Sir William Ramsay prepared an explanation quite analogous to the theories of color vision and called it a dynamic theory. According to him, the real stimulus to the taste organs is molecular vibration, the different taste sensations being due to stimulation by different rates of molecular movement. Just as in the case of the luminiferous ether or of the air there is quite a range of molecular vibration rates, from exceedingly slow to exceedingly rapid. And, just as in the case of vision and hearing, so is the taste mechanism tuned to respond to only the middle range of these molecular vibration rates. Substances may then be insipid, either because their molecular movements are too slow or too fast to affect the receiving mechanism. In vision we have the analogous case of the infra-red and the ultra-violet rays not producing visual sensations, because they are beyond the range of sensitivity of the eye. Yet the effect of these rays can be recorded by other means. The rate of molecular movement depends on the weight of the molecule, so that very heavy or very light molecules would not produce taste sensations.

About the same time Richet and Gley performed a series of experiments which seemed to show that the molecular weight of the substance was an important factor in producing taste sensations. They found that the intensity of the effect produced by different salts was in proportion to their molecular weight; that if account was taken of the different molecular weights of the salts used as stimuli the threshold stimulus would be the same for all of the salts. But if solutions were prepared according to the absolute weight of the salts these threshold stimuli appeared to be quite different for the different salts. Later experiments have shown that the same relation does not hold for sour-producing substances, certain sours of very small molecular weight having the sourest taste.

Sternberg, whose work was mentioned in chapter VII, recognized only two elementary taste sensations, sweet and bitter, and he found no differences in the molecule of substances producing these two sensations. He concludes that it is the intramolecular vibrations that form the real taste stimuli. By transferring the seat of the activity from molecular vibration to intramolecular vibration the whole matter becomes more difficult of solution and verification. Still, the modification of the theory of Ramsay, made necessary by the work of Sternberg, would leave its essentials, namely, that vibrations of some sort form the stimulus and that the sense organ of taste is tuned to receive only a limited range of vibration rates.

Granting that a dynamic theory as outlined above, with possible modifications to meet new discoveries, is correct, still another question requires an answer. In the visual organ affected by ether vibrations of different rates every theory assumes certain parts of the mechanism sensitized especially to certain vibration rates. To take the theory of Hering, for example, there are three photochemical substances, one decomposed by a relatively slow vibration rate, another by a more rapid vibration rate, and so on. And in the auditory mechanism there is the basilar membrane, capable of vibration in parts in sympathy with different rates of vibration of the air. The answer to this question concerning the taste sense is peculiarly difficult, in that it is impossible to stimulate individual taste buds, and even difficult to stimulate a single papilla, which may contain many buds. And then it must be further borne in mind that each bud contains many nerve endings, so that what is stimulated in experimental work is really a very complex portion of the sense organ.

As mentioned in another connection, certain papillÆ when stimulated with various substances respond to but one, e.g., sweet, sour, bitter, or salt, while others respond to two or three stimuli. Further, it is well known that certain portions of the tongue when stimulated produce a predominance of certain kinds of taste sensations. On the other hand, an examination of the taste buds or of the nerve endings within the taste buds shows no differences among them. Still, in this respect taste does not differ from sight. The cones of the retina, which are sensory ends of vision, show no differences in structure, and it is generally believed that all of the elementary sensations can be produced by the stimulation of one single cone. If it be true that the nerve fibrils in the taste buds themselves receive the stimulus, then any specific characteristics of the receiving mechanism would have to be looked for in these nerve endings.

Oehrwall has accounted for the facts of the specific sensitivity of different parts of the tongue and of different individual papillÆ mentioned above by the assumption that each taste bud has a specific function and that certain papillÆ of the tongue have buds all of one sort, other of two sorts, and so on. Nagel prefers to modify this view to the extent that each taste bud is capable of arousing every elementary sensation, but is adapted to respond best and easiest to a certain specific stimulus with a certain elementary sensation. This view is analogous to that of the visual photochemical substances which, according to the Helmholtz theory, were each sensitive to all wave lengths of light, but not equally.

The previous review of theories does not consider the possibility of the specific energy of brain centers, rather than of the peripheral sense mechanism—that is, the possibility that the sensation qualities, sweet, sour, etc., may be due to structural characteristics of brain centers, rather than of sense organs. This is a question which is unsolved for other senses and consequently is not peculiar to taste. The present tendency seems to be toward attributing the different sensation qualities to the sensitivity of the sense organ, or else to divide it between brain center and sense organ. The following facts are considered as indirect evidence of the specific energy of the sensory ends of taste: (1) The distribution of taste sensitivity over the tongue. (2) The effect of certain drugs, e.g., cocaine, which destroys the taste sensations one at a time. This suggests that the sensory ends that have to do with the different sensations are differently affected by the drug. (3) The fact that the same substance as it passes over the surface of the tongue arouses different sensations, e.g., sodium sulphate, which is sweet on the tip and bitter on the back of the tongue.