CHAPTER XII
SMELL AND TASTE
In Man the chief function of these senses is to guard the entrances to the respiratory and digestive tracts. In this they are not conspicuously efficient, since various poisonous gases, salts, and powders, escape their vigilance. Merely a selection of the substances which occur in air and in food are recognized as having odour or flavour. Smell and taste are only partially distinguished in ordinary parlance. No odorous substance is spoken of as tasteless when taken into the mouth. Its volatile constituents, escaping to the chambers of the nose, are said to afford a certain flavour. On the other hand, it is recognized that substances which stimulate the tongue alone—bitters, acids, sweets, and salts, unmixed with volatile bodies—have no odour.
Biassed as we necessarily are by the paltry rôle assigned to smell in our mental life, it seems a little unworthy of the present functions of the great brain that it should have developed in association with the nose. Yet smell and taste are the oldest of the senses. Their origin goes back to the days of chemiotaxis, when the organism, having no specialized sense-organs, was attracted to its mate or to its food, and repelled from conditions unsuitable for its well-being, by particles in solution acting as chemical stimulants. An amœba is chemiotactically drawn towards its food, one spore of an alga is attracted to another, by the particles of matter which drift across the interval between them.
In the life of many animals smell plays as important a part as that of either of the other senses. One has but to watch a dog “looking” for its master, already full in view, with its nose, to realize that smell is the sense on which a dog chiefly relies. We describe it as looking, because in ourselves the eye has so far outdistanced the other senses as a channel of information that we speak of “looking” when we mean seeking, and say that “we see” when we wish to imply that we understand.
The difference between smell and taste is, in fishes, a difference in the quality of the sensation, and not in its “modality” or kind; but in terrestrial animals the olfactory membrane of the nose has become specialized for the recognition of particles suspended in air, the tongue for substances dissolved in water. The olfactory membrane, which lines the upper two of the three chambers of the nose, is covered with elongated cells of two kinds: (a) Columnar cells, fairly thick; and (b) fusiform cells, each carrying at its free extremity a bunch of exceedingly minute hairs. The fusiform cells are neuro-epithelial cells of the most primitive type. Before nerve-cells, properly so called, appeared, certain favourably-placed epithelial cells were connected by protoplasmic bridges with muscle-fibres, to which they delivered the impulses which were generated in them by external forces. Later some of the neuro-epithelial cells sank beneath the surface, where, as ganglion-cells, they served as intermediaries between groups of sensory cells on the surface and the nerve-net which lay more deeply in the tissues. The olfactory membrane perpetuates the earlier stage; in so far as it consists of elements which are combinations of sense-cells and nerve-fibres. Each of its fusiform cells sends inwards a nerve-filament, which, traversing the submucous tissue of the nose and the bone (cribriform plate) on the base of the skull, between the orbits, enters the olfactory bulb. The olfactory bulb is a part of the local nervous mechanism of smell. It is the ganglion of the nerve of smell plus nerve-elements which in all segments behind the eye have been withdrawn from the neighbourhood of the sense-organ into the central nervous system ([cf. p. 333]).
The way in which odorous particles in air stimulate the fusiform cells is unknown. The quantity which suffices as a stimulant is so small as to put chemical stimulation out of the question. A few grains of musk will scent a room for years. 0·00000004 milligramme of mercaptan (sulphur-alcohol) is recognizable in a litre of air. This is a dilution to 1 in 50,000,000,000. Probably even such figures as these would be thrown into the shade if we could estimate the minimum amount of human effluvium which will enable a dog to follow his master’s trail. Explanations have been sought in alterations in the vibrations of molecules of air caused by the presence amongst them of relatively heavy molecules of volatile substances; but the difficulty of accounting for the generation of nerve-impulses in the sensory cells remains as great as ever. The hairs borne by olfactory cells are so short that it is impossible that they should project beyond the film of moisture on the surface of the membrane. This seems to preclude an answering vibration. Yet an increase in the thickness of this layer and in its density, due to the presence in it of mucus secreted during a catarrh, renders the sense-cells incapable of responding to odorous particles.
Smell in an animal is not a test of the quality of the air it is breathing, but a source of information as to the direction in which it may seek its prey; or, although far more rarely, as to the direction from which the advance of a foe is to be feared. Hunting animals depend for the most part on the nose. Hunted animals rely chiefly on the eye.
If we attempt to analyse our smell-sensations, we find that we can pick out a number of varieties which appear so unlike as to have nothing in common: Putrid meat, burning indiarubber, sulphuretted hydrogen, ammonia, roses, onions, lemon verbena, methylated spirit. Everyone can make for himself a list of typical odours which seem to have specific qualities—odours so distinct that he never confuses one with another. He can also class together scents about which he is often uncertain. The type-odours he can distinguish when present in a mixture; whereas odours which are less distinct reinforce or modify one another. It has been found, by careful experiment, that certain type-odours even tend to neutralize each other. Musk and bitter almonds, for example, if present in small quantities and properly proportioned, produce a very dim sensation, whether supplied as a mixture to both nostrils, or the one assertive odour to one nostril and the other to the other. This last observation is of great importance. It proves that their mutual destruction does not occur on the olfactory membrane. It is not due to physical interference. The sensation of musk is delivered to one side of the brain, the sensation of bitter almonds to the other; but when attention is directed to these two sensations there is found a quality in the one which is irreconcilable with the quality of the other.
In certain persons and under certain pathological conditions, sensitiveness to particular odours, or groups of odours, is absent, while for the rest the sense is normal. Methylated spirit, prussic acid and mignonette, constitute a group which not infrequently drops out. Instances have also been reported of persons unable to smell vanilla (to which some are hyper-sensitive), and of others insensitive to violets, although normally sensitive to the scents of other flowers. The notes sounded in consciousness extend over a long gamut; but there are reasons for thinking that the number of keys on the clavier which odoriferous substances strike is limited. Eleven is the number provisionally adopted. The effect in consciousness varies according as one key or another is struck, or several at the same time with varying degrees of force.