and closer together, near and upon the apex.' 'The smallest papillæ are the most numerous of all. They are minute, conical, tapering, or cylindrical processes, which are densely packed over the greater part of the dorsum of the tongue, towards the base of which they gradually disappear. They are arranged in lines, which correspond at first with the oblique direction of the two ridges of the large papillæ, but gradually become transverse towards the tip of the tongue.'

'These different kinds of papillæ are highly vascular and sensitive prolongations of the mucous membrane of the tongue. When injected, they seem to consist almost entirely of capillary vessels; the large papillæ, containing many vascular loops, whilst the smallest papillæ are penetrated by only a single loop. Nerves proceed in abundance to those parts of the tongue which are covered with papillæ, into which the nerve-tubes penetrate.' The papillæ are undoubtedly the parts chiefly concerned in the special sense of taste; but they also possess, in a very acute degree, common tactile sensibility.'-QUAIN.

The nerves supplied to the tongue are the glosso-pharyngeal on the back part, and twigs of the fifth pair on the fore part. The former must be considered as in all probability the nerve of taste proper. The fifth pair, being a nerve of touch, can confer that high tactile sensibility distinguishing the tip of the tongue; but there are no facts decisively showing any portion of this nerve to be the medium of pure taste. It is true that some so-called tastes, as the sour or acid, can be discerned by the tip, but these are properly of the nature of pungent or fiery stimulation, capable of acting on nerves of touch. A bitter taste, which appeals to the strict gustatory sensibility, is felt principally in the back part of the tongue. Fiery, cooling, and astringent tastes may arise through the lips and the gums, showing that they are merely effects on our common or tactile sensibility. Mustard acts on any tactile surface with variations of degree merely. It has not been possible to excite a pure gustatory sensation by irritating the fifth pair of nerves.

3. With regard to the precise localities of the tongue where the sensibility resides, there has been some difference of opinion. We conclude generally,' say Messrs. Todd and Bowman, 'with regard to the tongue, that the whole dorsal, or upper, surface possesses taste, but especially the circumferential parts-viz., the base, sides, and apex. These latter regions are most favourably situated for testing the sapid qualities of the food; while they are much less exposed than the central part to the pressure and friction occasioned by the muscles of the tongue during mastication. The central region, as a whole, is more strongly protected by its dense epithelium, and is rougher, to aid in the comminution and dispersion of the food.' But in addition to the tongue, 'the soft palate and its arches, with the surface of the tonsils, appear to be endowed with taste in various degrees in different individuals.'-I., 443.

The increasing sensibility of the tongue, from tip to back, serves as an inducement to move the food gradually onward in the direction of the pharynx, in order to be finally swallowed. The same sensibility, acting according to the general law of feeling-guided action, or volition, keeps up the mastication, whereby the sapid action of the food is increased by solution and comminution of parts. Thus it is that mastication is purely a voluntary act, while deglutition or swallowing is purely reflex and involuntary.

Among the conditions of taste, in addition to solubility, it is noticed that taste, like touch, is much influenced by the extent of surface acted on; and is also heightened by the motion and moderate pressure of the substance on the gustatory membrane.' In order to taste, also, the tongue must not be in a dry or a parched condition. The impression of cold

air deadens the sense of taste.'*

* Another condition of taste, brought to light by the researches of Graham on 'Dialysis,' is that the substance should belong to the 'crystalloid' class of bodies, and not to the 'colloid' class. The colloids are represented by starch, the gums, caramel, tannin, albumen, gelatine, vegetable and animal extractive matters. Now, it is a law that these colloids do not pene

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, gelatine, &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.

143 of the mouth, and between both and the proper sensations of 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.

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The relish in the mouth is much more intense or acute than the feeling in the stomach, although this last may 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