Many attempts have been made to associate the sensation-qualities of the various odours with the chemical or physical properties of their odorants, with but little success as yet. To excite the sense of smell, a gas must be at least a little heavier than air. No volatile body, it is stated, is so heavy as to be odourless; on the contrary, speaking generally, heavy molecules are more stimulating than light. The quality of a smell-sensation would therefore appear to depend upon the period of vibration of the molecules of the substance which evokes it; but, as already stated, a consideration of the apparatus which responds to stimulation by odoriferous particles does not help us to an understanding of the way in which the particles act upon it.
Fig. 26.—Highly Magnified Section through the Wall of a Circumvallate Papilla
of the Tongue, showing Two Taste-Bulbs.
These sense-organs are groups of elongated epithelial cells, set vertically to the surface. Their cells are of two kinds—the one fusiform, slender, bearing each a bristle-like process which projects through a minute pore left between the superficial cells of the general epithelium; the other thicker and wedge-shaped. Nerve-fibres are connected with the fusiform cells.
Taste is far more limited in its range of sensations than smell. The back of the tongue is sensitive to bitters, the tip to sweets and salts, the sides to acids. Mixtures of these qualities are distinctly analysable by the sense of taste. Our sensations of taste do not fuse. Slight differences in the way in which the organs on the different parts of the tongue react to stimulation enable us to recognize that a sapid substance is a mixture. When, with a great flourish of trumpets, saccharin was introduced as a safe sweetener for gouty people, an attempt was made to provide them with saccharin-sweetened jam. The effect of the jam upon the person who consumed it was truly humorous. First a suspicion of tartness, then its adequate suppression, followed by nauseating sweetness. The sense-organs which subserve the sense of taste are clusters of fusiform epithelial cells, collected in “taste-bulbs” ([Fig. 26]). Each gustatory cell bears a minute bristle, which projects through the pore left by the cells of the surrounding epithelium which constitute a globular case for the bulb. As in the nose, eye, and ear, a second thicker variety of epithelial cell is also present. The nerve-fibres of the taste-bulbs are not, as in the olfactory membrane, processes of their cells, but branches of the fifth nerve which ramify amongst them. On the back of the tongue taste-bulbs are much more numerous than elsewhere. They are not as sensitive as the cells of the olfactory membrane; nevertheless, they enable us to detect 1 part of quinine in 2,000,000 parts of water.
Sensations of taste and smell endure for a long time after stimulation, because the odorous or sapid substance remains in contact with the sense-organs. This accounts for the confusion into which a man is thrown if he sip alternately port and sherry. After a short time he cannot tell the one from the other. The organs are quickly fatigued, using the term loosely. How intolerable patchouli would be to the ladies who use it were it otherwise! If for some time one sniffs the odour of mignonette, it ceases to be recognizable; whereas, turning to a rose, the olfactory membrane is found to be as sensitive as usual. When the sense is fatigued for a particular smell, it is dull for others of the same group, thus affording an opportunity of classifying smell-sensations according to their qualities; but the method is difficult to apply. Taste-organs are greatly affected by temperature. Quinine is not tasted just after drinking ice-cold water. Alcohol, ether, or chloroform paralyses the organs much in the same way. Castor-oil slips down the throat unnoticed if the mouth, just before swallowing it, has been rinsed with brandy or with a strong solution of tincture of chloroform.
Englishmen make but little use of their sense of smell. It might teach them much regarding the various emanations from putrid matter which are produced by bacterial action; but, dreading drains, they decline to cultivate proficiency in the exercise of this sense. The nose is valued for the warning it gives of “nasty smells,” but is not allowed to analyse them. Burnt milk, soap-boilings, rancid oils, are taboo, because they are associated with bungling in the kitchen. With moderated ardour, we allow our sense of smell to distinguish foods and beverages, but we are not a race of epicures. The perfumes of flowers are classed as “nice smells.” The idea of greediness is not associated with their enjoyment; besides, they remind us of gardens, sunshine, pretty forms and colours. When bottled, musk, orange-blossom, violets, lavender, are valued not so much for their own sweetness, as for their singular efficiency in obscuring nasty smells. Few persons practise the recognition and distinction of even pleasant odours. Very few, on first coming across a scented herb or shrub, pay sufficient attention to its perfume to impress it on their memories. They note the shape of its leaves and the colour of its flowers, but they are unable to identify it by its odour when they meet with it again. It is not much to be wondered at, therefore, that this slighted sense tends to leave us after middle life. It has been asserted—and probably the statement is justified—that rarely is the olfactory bulb of a man over forty free from signs of atrophy. We have no statistics concerning the brains of Japanese, who regard the sense of smell as one of the chief avenues of pleasure; but it may be that in this respect their brains present a contrast to our own. Yet the deadening of the sense is scarcely noticed, since its results are of little consequence as compared with those which follow loss of sight or loss of hearing. Many a man, as he grows older, declares that the cook of his club has lost his cunning, or frankly asserts that he “no longer cares for kickshaws. Cold beef, beer, and pickles, are good enough for him.” He little suspects that his palate has lost its power of distinguishing the flavours of dainty meats and wines. Others continue to be exacting, because their imaginations still endow food with the qualities which they remember, just as people eat preserved asparagus or tinned peas because they look—however little they taste—like the gifts of Spring.
Taste accompanies the reception of food in the mouth. We have no knowledge of the situation of our own olfactory membranes, and therefore we suppose that a flavour, whether it be due to stimulation of taste-bulbs or olfactory membrane, is in the mouth. The odour of a flower we mentally project to a distance, because we associate the sight of a flower with its perfume. A dog, able to judge the freshness or staleness of a scent, must project its sensations of smell in the same way in which we project our sensations of sight. It forms an estimate, of a sort, of the time that it will take in reaching the source of the scent. Its excitement increases as the trail grows fresher.
Taste and smell are heavily laden with affective tone. When disagreeable, the feeling which they evoke is near akin to pain. It may gather head until, like hunger, it causes the discharge of motor neurones; but under its influence food is ejected, instead of preparation being made for its reception.