§ 1. Taste and Smell.*—The greater number of the sensations which are usually ascribed to taste are in realityodours. "If the nose be held and the eyes shut, it is very difficult to distinguish, in eating, between an apple, an onion, and a potato; the three may be recognised by their texture, but not by their taste." Cinnamon applied to the tongue under the same conditions appears like flour; the taste may appreciate a slight sweetness, but that is all. There are four undoubted taste-sensations—sweet, salt, acid, and bitter. There are two others—the alkaline and the metallic— which are disputed. The alkaline is possibly a mixture of salt and sweet, together with peculiar touchsensations.

*In regard to taste, I have mainly followed Kiesow, "Beitrage zur Physiologischen Psychologie des Geschmacksinnes," Philosophische Studien, X. (1894), pp. 329368, 532561.

All taste-sensations appear to be intermingled with and qualified by tactile sensations. An acid, too slight to be distinguished as such, produces a peculiar touchsensation by its astringent character; and as the acidity is increased the touchsensation becomes stinging, and finally passes into a pain-sensation which completely dominates the special experience of acidity. Salt is also accompanied by a stinging sensation: but this does not reach the same pitch of intensity as in the case of acids. The sensation of softness and smoothness is associated with sweetness; this is appreciable when the sweet substance is present in quantities so small that it cannot be discerned as such. As the sensation of sweetness becomes intensified, the touchsensation is dominated and obscured by it. But it emerges again as the sweetness is further increased. Very intense sensations of sweetness are sometimes accompanied by a biting sensation.

The tip of the tongue is especially sensitive to sweetness, the edges to acidity, and the base to bitterness. The tip and edges are equally sensitive to salts, the base less so. When the mouth has been washed out, and some neutral substance, such as distilled water, is applied to the tongue, the result differs according to the point of application, and varies in different persons. The base of the tongue appears in all cases to respond by a sensation of bitter. In some persons the same sensation is aroused to whatever part of the tongue the distilled water is applied. Others feel no sensation except at the base. Others feel a sensation of sweetness at the tips and of acidity at the edges. There appear to exist among taste-sensations relations somewhat analogous to the contrast of colours. Salt, by a sort of contrast, makes distilled water taste sweet. It has the same effect on solutions of sweet substances which in themselves would be too weak to be appreciable. It also has an intensifying effect on solutions which are strong enough to be appreciable. It operates in this way both when the same part of the tongue is successively stimulated, first by a salt, then by a neutral or sweet fluid, and also when the salt and the sweet are simultaneously applied to homologous parts of the tongue, e.g., to corresponding points on the right and left edges of the tongue. Sweet has a much weaker contrast effect on salt, than salt on sweet. In both forms of the experiment, sweet instead of making distilled water taste salt by contrast, makes it taste sweet. On the other hand, contrast with sweet makes distinctly appreciable a salt solution in itself too weak to be perceived. Similar relations have been observed between salt and acid, and between sweet and acid; but in the case of sweet and acid they are manifested only when the two stimuli arc applied successively to the same part of the tongue, not when they are applied simultaneously to homologous parts. Bitter appears neither to produce contrast effects nor to be affected by them.

The sense of taste can be stimulated only by fluids. Solid substances must be dissolved in the mouth before they can affect it.

The appropriate stimulus for the sense of smell, on the other hand, consists of odoriferous particles conveyed to the membrane in a gaseous medium. The sensations of smell have not been adequately classified or analysed into their primary constituents: there appears to be a very great variety of them. They are often modified by mixture with touch and taste-sensations. The pungency of an odour is not strictly a sensation of smell at all, but a peculiar kind of tactual experience. Odours proper do not appear to produce sneezing: this is due to irritation affecting the sense of touch. Odorous sensations take "some time to develop after the contact of the stimulus with the olfactory membrane, and may last very long. When the stimulus is repeated the sensation very soon dies out: the sensory terminal organs speedily become exhausted. The larger, apparently, the surface of olfactory membrane employed, the more intense the sensation; animals with acute scent have a proportionately large area of olfactory membrane. The greater the quantity of odoriferous material brought to the membrane, the more intense the sensation up to a certain limit; and an olfactometer for measuring olfactory sensations has been constructed, the measurements being given by the size of the superficial area, impregnated with an odoriferous substance, over which the air must pass in order to give rise to a distinct sensation. The limit of increase of sensation, however, is soon reached, a minute quantity producing the maximum of sensation, and further increase giving rise to exhaustion. The minimum quantity of material required to produce an olfactory sensation may be in some cases, as in that of musk, almost immeasurably small."*

* Foster, op. cit., pp. 13891390.

The sense of smell plays an immensely important part in the life of animals. It is to them what sight and hearing are to us. The animal detects its prey and follows it by means of scent. On the other hand the scent of the pursuer warns the prey and guides its efforts to escape. Probably every individual and every species has its own characteristic and distinctive odour. There are some men who can distinguish human beings by smell; dogs and other animals possess this power in a very high degree. The ants of one nest attack those of another nest or of another species who may intrude among them; whereas they never under normal conditions attack ants belonging to their own nests. It has been clearly shown by experiment that this is due to the peculiar and distinctive odours belonging to different nests and their inhabitants. The unfamiliar odour of an ant coming from a strange nest has an exasperating effect. The intruder is attacked and usually killed. If before being introduced into a nest it is first bathed in juice produced by crushing the tenants of the nest, no notice is taken of it however widely it may differ in appearance from these. It is incorrect to say that ants recognise other ants as belonging or not belonging to their own family: all depends on the irritating effect of the unfamiliar odour of strangers.* The comparatively small part played by smell in the mental life of human beings may be accounted for by the fact that trains of ideas constitute so large a part of human experience. Smells are not adapted to ideal revival in serial succession as sounds and sights are.