different kinds of meat with perfect ease by their odour, which seems agreeable to us when we are hungry, but disagreeable, and even revolting, when we have appeased our appetite.

Up to the present time science has not been able to offer any explanation of the existence of the different kinds of sensations of smell, or what difference there is between the irritations produced by good and bad smells. From analogy with the kinds of sensation of light, we should be led to suppose, that in the olfactory nerve, there must also be different kinds of terminal apparatus, which transmit the different kinds of smell to those nerve-fibres which are connected with them. we should have to assume the existence of an exceedingly large, or even infinite, number of such nerves, when we cannot even classify distinctly the different sensations of smell. We must leave all questions upon this subject, which, as yet, is but little understood, for future research

to answer.

But

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CHAPTER II.

The Sense of Taste.

ALTHOUGH the sense of taste is most necessary to the enjoyment and welfare of man, science as yet knows but little of its nature with certainty. Even the extent of its diffusion in the cavity of the mouth has not yet been satisfactorily determined. It is, indeed, certain, that the tongue is the principal seat of the organ of taste, and that the sensation of taste is most intense at the back or root of the tongue. The tip of the tongue which every one must

also possesses a sense of taste, know from experience. The opinions of the different experimenters are greatly at variance as to the properties of the remaining portions of the surface of the tongue. According to the greater number, the under surface of the tongue possesses no power of taste, or a very dull one, though in most cases the edges of the tongue possess this power. Observations are, however, very difficult to make, and uncertain in their result, because substances placed upon a certain spot of the tongue, will not readily remain isolated, but spread very rapidly, and since the slightest trace is sufficient to be recognised, we are exceedingly liable to deception.

On this account the result of experiments upon the delicacy of the power of taste possessed by the palate is still more doubtful, although it is commonly considered to have an extremely delicate sensation of taste. Many observers assert that the whole of the soft palate and the pillars of the fauces possess a sensation of taste, and yet it has been observed by other careful experimenters that all sensation is wanting when a substance is applied with a pencil upon an isolated point, and every movement of the root of the tongue is carefully avoided. It is possible that this may be caused by individual peculiarities, and that the sensitiveness is more strongly developed in some persons than in others. It seems, in short, only a confirmation of the old saying, 'There is no accounting for taste.'

The anatomical and microscopical investigations of the organ of taste afford a much safer stand-point. The entire surface of the tongue is covered with little elevations called gustative papilla, which are visible to the naked eye. Some of them terminate in a bundle of fibres, and others are broad and bushy on their surface. At the root of the tongue a semicircle is formed by larger papillæ, each of which is surrounded by a circular mound. Small depressions have been observed surrounding these circumvallate papillæ. The papillæ stand in the depressions formed by the mounds, and are filled internally with oblong cells, which are connected by prolongations with nerve-fibres. Similar organs have been observed upon the other papillæ of the mucous membrane of the tongue, and it is probable that in them we must look for the true instruments of taste.

It is not so easy to decide whether there be a special

nerve of taste as was the case with the other senses. There is certainly a nerve, the glosso-pharyngeal nerve, which must, without doubt, be regarded as the most important nerve of taste, but its gustative fibres are connected with innumerable motor nerves of the lower part of the head, whilst the optic, auditory, and olfactory nerves are entirely free from any foreign admixture. When this nerve has been severed it has been observed, that animals after this operation will devour food, even when mixed with the bitterest substances, which an animal in a normal condition would refuse to touch.

Besides the nerve named above, another sensory nerve is found in the tongue, the lingual nerve, which provides it with the sense of touch and with sensitiveness. It is still uncertain whether it possesses gustative fibres, besides the ordinary sensory fibres. At any rate it can certainly be excited by sapid substances, when they are of a sharp caustic nature, such as strong acids, alkali, strong roots, etc.

One of the qualities of the sensation of taste is, that sweet and bitter substances seem to produce a distinct sensory impression, apart from any other irritation of the sensory nerves. No concentration, however strong, of these sensations will ever amount to pain, whilst a sour taste will produce a contractive and painfully burning feeling. They stand in direct opposition to each other, for the sweet taste appears to us the pleasant, and the bitter taste the unpleasant sensory impression. It is the sweetness of the milk which has such a charm for the infant, and which, when it is hungry, conduces to its reception of nourishment. The bitter and sour taste

which we allow to a certain extent in our food, would

be distinctly refused, by the much more sensitive gustatory organs of the infant, as an unpleasant sensation.

The sweet taste is the opposite to the bitter and sour taste, inasmuch as we are able to mitigate the unpleasantness of the two latter by the former, when, for instance, we mix sugar with food which has a bitter or sour taste. Since we thus, as it were, correct the taste without allowing the sugar to react chemically upon the bitter and sour substances, it seems to us that some kind of interference with the sensations must take place for which we can find no explanation. It is also possible for the sweet taste to be combined with the sour and bitter tastes, and produce a pleasant one. But it is well known, on the other hand, that a saline taste is not mitigated by the addition of sugar, and that they never combine so as to produce a pleasant sensation.

The contrast between these sweet and bitter tastes is shown by the following phenomena. After having tasted any bitter or saline substance, pure water, if taken immediately afterwards, will appear to be sweet; and sometimes a sour taste will remain in the mouth even after we have eaten a large amount of sugar. We cannot help thinking of the great similarity between these phenomena and that of the contrast of colours, but we have as yet no stand-point upon which to follow out such a comparison.

We ought first to enquire what is the cause of the differences in the sensations produced by different gustative irritations, but unfortunately no positive information can be given upon this point. We might, indeed, very well assume the existence of several kinds of nerves with different terminal organs, one producing a