It is a curious fact, that the chemical combination M2 03, or two atoms of a metal with three of oxygen (termed sesquioxides) causes sweetness. Alumina is an illustration; for alum is known to be sweet as well as astringent. The oxide of chromium is still sweeter. Glucina is the sweetest of all, and has its name from this quality.

The salt of silver, termed hypo-sulphite, and its combinations with hypo-sulphites of the alkalies, are the sweetest bodies known.

The salts of lime are bitter.

The organic alkalies are all intensely bitter; quinine, morphine, strychnine, are instances. The taste of strychnine is apparent when diluted with water, to the degree of one in a million.

There is a certain class of vegetable compounds, neutral bodies, which are at present characterized as the bitter and extractive principles of plants. I quote a few examples from the list given in GREGORY'S Organic Chemistry, p. 457.

Gentianine, from Gentiana lutea, forms yellow needles, very bitter. Absinthine, from Artemisia absintheum, or wormwood, is a semi-crystalline mass, very bitter, soluble in alcohol Tanacetine, from tanacetum vulgare, is very similar to it. Syringine is the bitter principle of the lilac, syringa vulgaris. Colocynthine, the active principle of colocynth, is amorphous, intensely bitter and purgative.

Quassine is a yellow, crystalline, and very bitter substance, from the wood of quassia amara. Lupuline is the bitter principle of hops. Liminine, or Limine, is a bitter crystalline matter, found in the seeds of oranges, lemons, &c.

With regard to vegetable and animal substances in general, Gmelin remarks:-Some organic compounds, as gum, starch, woody fibre, white of egg, &c., have no taste; others have a sour taste (most acids); or a rough taste (tannin); or sweet (sugar, glycerine, glycocol); or bitter (bitter principles, narcotic substances, and many acrid substances, also many resins); or acrid (acrid oils and camphors,

acrid resins, acrid alkaloids); or fiery (alcoholic liquids, volatile oils, camphors).' (Chemistry, Vol. VII, p. 66.)

Not only are the different classes of vegetable and animal products distinguished by their taste, as apples from apricots, wine from cider, flesh from fat, but in every such class there are many distinguishable varieties. The class of wines, based on the common ingredient, alcohol, spreads out into innumerable kinds from the presence of sapid substances in quantity so small as to elude the search of the chemist. It is shown by this and by many other facts, that an extremely minute portion of a sapid substance may make itself acutely felt to the taste. The bitter element of soot, for example, can be distinguished in cookery to a very high degree of dilution.

Acids and bitters are said to be the most readily detected of all sapid substances; then saline, and lastly, saccharine. It has been found that one part of sulphuric acid in 10,000 of water, and one of sulphate of quinine in 33,000 of water, can be detected, when carefully compared with pure water. Sugar cannot be tasted when there is less than one in 80 or 90 of water; and of common salt, one part is necessary to 200 of water (Marshall's Physiology, I., 481).

2. The organ of Taste is the tongue, and the seat of sensibility is the mucous membrane covering its surface.

The upper surface of the tongue is covered all over with numerous projections, or eminences, named papillæ. They are found also upon the tip and free borders, where however they gradually become smaller, and disappear towards its under surface.' These papillæ are distinguished into three orders, varying both in size and in form.

The large papillæ, eight to fifteen in number, are found on the back part of the tongue, arranged in two rows, which run obliquely backwards and inwards, and meet towards the foramen cæcum, like the arms of the letter V.' 'The middlesized papillæ, more numerous than the last, are little rounded eminences scattered over the middle and fore part of the dorsum of the tongue; but they are found in greater numbers

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 vaserlar 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 mem. brane impermeable to soluble substances of the same physical constitution.'