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THREE-FOLD SENSIBILITY OF THE TONGUE.

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4. The precise mode of action whereby the nerves of the tongue are stimulated has not as yet been explained. Taste may be produced by mechanical irritation of the surface, as by a smart tap with the fingers on the tip of the tongue, and by galvanism. A stream of cold air directed upon the tongue gives a cool saline taste, like saltpetre. If we look at the substances that cause taste proper, it appears probable, that their chemical constitution is the determining circumstance, whence it would seem that the action is a chemical one. A certain secretion from the blood vessels that line the papillæ of the tongue combines with the dissolved food, and the act of combination constitutes the stimulus of the nerve fibres. We know that a chemical action on any surface or tissue will suffice to stimulate a nerve and produce sensation; and it is difficult to assign any other mode of stimulus either in taste or in smell.

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5. Having thus considered the external objects of the sense, and the structure of the organ, it remains for us to describe the mental phenomena, that is, the Sensations themselves. From what has been already said, the reader will gather, if he has not otherwise remarked it, that the tongue is the seat of a twofold sensibility, taste and touch. go still further, and ascribe to it a threefold sensibility, viz-touch, taste properly and strictly so called, and relish, or a participation in the alimentary sensations; the reasons are the following. First, there is an obvious continuity of structure in the tongue and alimentary canal, a common character of surface as regards mucous membrane, glands,

trate one another, except with slowness and difficulty; whereas a crystalloid body like sugar or salt penetrates a colloid very readily. Animal membranes belong to the colloid class, and accordingly while they are freely permeated by crystalloid substances, they resist the passage of starch, gum, albumen, g-latine, &c. This would be a sufficient reason for the absence of taste in these bodies. Graham remarks:- While soluble crystalloids are always highly sapid, soluble colloids are singularly insipid. It may be questioned whether a colloid, when tasted, ever reaches the sentient extremities of the nerves of the palate, as the latter are probably protected by a colloidal membrane impermeable to soluble substances of the same physical constitution.'

and papillæ, which would imply some community of action and feeling, in the midst of diversity. We may here allude to a certain gradation that is apparent from the papillæ of touch, through those of taste, to the absorbing villi of the small intestines. Touch shades into taste, and at a lower point sensibility is lost.'-(Todd and Bowman, I., 441.) Secondly, the tongue, besides its power of discriminating niceties of taste that have very little reference to digestibility, can inform us at once whether a substance will agree or disagree with the stomach, and this it can do only by being, as it were, a part of the stomach, affected like it by wholesome or unwholesome contacts. Thirdly, the peculiarity we call relish, is not the same as a mere taste. For the type of taste, I may take such substances as common salt, quinine, soot, Epsom salts; for relishes, I would select butter and animal flesh; the savoury in cookery being made up much more of relishes than of tastes. The condition of the stomach governs the one, but not the other. After an attack of sea-sickness, a person is still in a condition to discriminate sour, bitter, alkaline, or acrid, when the choicest food excites no relish in the mouth. Fresh, disgusting, nauseous, are terms applying to the stomachic sensibility and to that portion of the tongue in sympathy with the stomach, and not to tastes as I understand them. With this explanation, I shall now proceed to examine in detail the sensations of the tongue.

6. Deferring for the present the consideration of the purely tactile sensibility, shared by the tongue in common with the skin and the inner surface of the mouth, we shall have to classify and describe the several kinds of sensations coming under both Taste and Relish. On the general plan of taking the least intellectual sensations first, we should commence with the relishes and disgusts of taste, which constitute its relation with the alimentary sensations already treated of. But these feelings need not be again gone into in the detail; all that appears necessary is to quote a few instances, with the view of illustrating still farther the distinctions we have drawn, between the alimentary sensations of the stomach and those

RELISHES.-DISGUSTS.

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taste.

7. The classification will therefore commence (I.) with Relishes. These are the agreeable feelings arising from the stimulus of food on the organs of mastication and deglutition; they are intense in degree. The substances that produce them. in greatest amount are reckoned savoury by pre-eminence. Animal food has the highest power of exciting a vigorous relish, or that keen sensation so powerful as a stimulus to mastication and the taking of food, rendering the individual voracious for the time being. A healthy digestion and the state of hunger are the necessary conditions of a strong relish, whether in the stomach or in the mouth; from which fact, as already said, we can discern the difference there is between a mere taste and a relish. Butter and oils and fatty substances are relishes, used for that purpose along with the more insipid kinds of food, such as bread. Sugar is both a taste and a relish. Being one of the necessaries of animal life, as is proved by the function of the saliva in producing it from starchy substances, there is a direct craving for it throughout the system; and everything craved for in this way is likely to produce a far deeper impression than a mere sensation of taste.

The relish in the mouth is much more intense or acute than the feeling in the stomach, although this last may be more influential upon the general tone of the system by its amount. That the two interests are not altogether identical is shown by the circumstance that many tongue-relishes are hard of digestion. But I am not aware of any case where what passes in the mouth is found nauseous to the digestion; so far the two senses would seem to be in accord.

8. Relishes imply their opposite, Disgusts. This sensation is inspired by certain substances as part of their nature; at particular times it may arise from any contact whatever, the alimentary surface being in a state of distemper. Oily substances, when cold and solid, are relishes; but, when hot and liquid, readily disagree with the palate. Repletion renders

any kind of food distasteful, and some kinds absolutely nauseous. In every point of view, this feeling is as much dependent on the condition of the alimentary canal as on the material tasted.

The different degrees of relish and nausea exhaust all that part of taste in sympathy with digestion; what follows, next in order, belongs (II.) to the distinctive sensibility of the tongue.

9. Sweet tastes. At the head of these, we must class the sugary taste, as being the most prevalent of all forms of sweetness. The sweetness of every kind of fruit, of bread, of milk, of alcoholic liquors, and of confectionery in general, is known to arise from sugar. Besides the relish, it acts strongly upon the sense of taste proper; but no pleasure of mere taste can be compared in amount and influence to an agreeable alimentary feeling. We can lay it down as a rule, that the pleasures of taste proper have as a whole a less influential action than the other class, and this must serve as a defining circumstance for every individual of them. The feeling of a sweet taste is acute, but does not inspire the energy of volition that follows up a savoury morsel. When digestion is satisfied, there remains the enjoyment of sweets, and when the taste for these becomes cloyed by repetition, it is by an independent effect on the gustatory nerves.

But the great distinction of this feeling, and of all other feelings of taste proper, relates to the intellect, or to the power of discrimination belonging to this organ, whereby an indefinite number of substances can produce impressions recognized by us as totally different in character, which impressions of difference can remain or be recalled, after the original is gone, to compare with new cases that may arise, and to give that sense of agreement or disagreement whereou all our knowledge of the world is based. In the case of sweetness, for example, not only can we be affected with the pleasurable feeling or emotion belonging to it, but we can be distinctively affected by a great many substances possessing the quality; we can identify some, and feel a want of identity

SWEET AND BITTER TASTES.

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in others; and we can so far retain the impression of a taste of yesterday as to compare it with a taste of to-day. This feature distinguishes the feelings of the mouth from organic feelings; it distinguishes in some degree tastes from relishes, although these last are also discriminated to a considerable extent; and it is the point of superiority which sight, hearing, and touch, have to a still greater degree over organic sensations.

These are exemplified by quinine,

10. Bitter tastes. gentian, or bitter aloes. This, and not sourness, is the proper contrast of sweet. As sweetness is the pleasure proper to taste, so bitterness is the peculiar or distinctive form of pain. inflicted through this sense. Without having the bulk and influence of the massive forms of pain, this sensation is highly intense in its own limited region, expressing itself by wryness. and contortion of the features. The sweet and the bitter represent the two characteristic modes of acting on the pure gustatory nerves. They are distinct from relish on the one hand, which involves sympathies with the stomach, and from the modes of tactile sensibility on the other. The classes that remain involve (III.) in a greater or a less degree the nerves of touch.

11. Saline tastes. Common salt may be taken as an example of this class. Mineral waters, containing salts of soda, magnesia, and lime, have a saline taste. This taste is rarely an agreeable one, in many cases it is very disagreeable, but we should be disposed to describe the feeling, in most instances, as singular and characteristic rather than as either pleasing or the reverse. Of it, as of all that follow, the character is best expressed by saying, that it can be discriminated from every other.

The repulsive taste of Epsom salts would be termed a compound of the saline and the bitter.

12. The alkaline taste is usually more energetic than the saline, as might be expected, seeing that a salt is a neutralized alkali. But if the remark above made be correct, namely, that salts owe their taste principally to their base, the alkali

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