§ 2. Cutaneous Sensations.—These are principally of three kinds—pressure, temperature, and certain others allied in their nature to organic sensations, among which the most prominent are those which from their peculiarly disagreeable character are called pains. This last class will be best considered in connexion with organic sensations in general.

* See Albrecht Bethe's Durfen wir den Ameisen und Bienen psychische Qualitaten zuschreiben (Archiv fur die gesammte Physiologic Bd. 70). Bethe also shows that ants find their way to and from their nests by means of smell. In moving they leave an odorous track behind them.

"The sensation caused by pressure is at its maximum soon after its beginning, and thenceforward diminishes. The more suddenly the pressure is increased, the greater the sensation; and if the increase be sufficiently gradual, even very great pressure may be applied without giving rise to any sensation. A sensation in any spot is increased when the surrounding areas of skin are not subject to pressure at the same time. Thus if the finger be dipped into mercury the pressure of the mercury will be felt more at the surface of the fluid adjoining the skin which is not in contact with the mercury, than in the parts of the skin wholly covered with the mercury; and if the finger be drawn up and down, the sensation caused will be that of a ring moving along the finger."* It should be noted that this applies only to sensations of pressure sufficiently marked to attract attention. As a matter of fact, pressure-sensations are present over the general surface of the body in every moment of our lives, and their presence can be detected as soon as we turn attention to them. Such conditions as the circulation of the blood, etc., furnish a constantly present stimulus within the body itself, and such uniform contact as that with the clothes we wear produces sensations which ordinarily escape notice, but are quite discernible when we attend to them.

The different areas of the skin are sensitive to pressure in varying degrees. The tips of the fingers, the lips, and the surface of the forehead, discriminate the smallest differences. The sole of the foot, the arm, and the back, have comparatively little power of discrimination.

* Foster, op. cit., p. 1413.

Bodies of the same shape, weight, size, and temperature, produce different pressure-sensations according to their various textures. Thus contact with a smooth surface and contact with a rough surface yield specifically different experiences. Similarly, we distinguish sharpness and bluntness, hardness and softness, wetness and dryness. All these peculiar qualities of sensation are due to varying combinations of pressure, to variations in the relative intensity of the constituent pressures, in the mode of their spatial distribution, and in their successive changes. A smooth surface produces a uniform pressure at every point; a rough surface produces a pressure which is discontinuous and irregular. The difference between hard and soft is connected with successive changes in the intensity of the pressure sensations. Sharpness and bluntness are differences in the extent of surface touched. These various qualities are presented to consciousness, not merely as varying combinations of pressure, but as having a specific character of their own, which does not appear to be capable of further analysis. We are here confronted with the same fact which has met us in other departments of sensation. Just as the partial tones combined in a musical note produce by their union a specific experience distinct from the quality of any of them taken separately, or of all of them taken together, so the combination of pressures which we experience when velvet comes in contact with the skin, produces those peculiar modifications of consciousness which we call softness and smoothness.

Temperature-Sensations are of two classes—the cold and the hot. The sensation of cold is as specifically distinct from that of warmth, as the sensation of black from that of white. A sensation of heat sufficiently marked to attract attention seems only to take place when the temperature of a region of the skin, which has previously been fairly constant, is raised; and it is also a necessary condition that the rise in temperature should not be too gradual. "Our skin has a certain temperature which varies from time to time, according to circumstances, and is not the same in all regions of the skin at the same time. A given spot of skin at a given time will have a certain temperature; that temperature does not give rise to a distinct sensation though its effects may enter into what we may call general sensibility; we may not be directly conscious, for instance, that the forehead has one temperature and the hand another, though the two temperatures may differ widely."* As the stimulus for sensations of heat is a more or less sudden rise in the temperature of the skin; so the stimulus for sensations of cold is a more or loss sudden fall in the temperature of the skin. This applies only to sensations conspicuous enough to attract attention. A general experience of heat or cold is always present, and discernible whenever we are interested in taking note of it.

* Op. cit., p. 1416.

The sensations of heat and of cold and of pressure respectively are produced at different points of the skin. "If a blunt pointed but otherwise fine needle be used to exert pressure, a little exploration will ascertain that at some points the amount of pressure can readily be recognised—the sense of touch is acute—while at other points, and these may be quite near the others, the amount of pressure cannot be recognised, and indeed no sensation is experienced until the pressure is excessive and then the sensation felt is not one of touch proper but of pain. Similarly, if heat or cold be applied by means of a metal tube or rod narrowed to a point, it will be found that some points of the skin are very sensitive to changes of temperature, while other points are insensitive to temperature, the application of heat or cold giving rise to pain only and not to specific sensations of heat or cold. Further, the points of the skin which are sensitive to pressure are those which are not sensitive to heat or cold, and vice versa."* It appears also that the points sensitive to heat are not identical with those sensitive to cold. The separation in this case does not seem to be so complete as that between temperaturespots and pressurespots. Some points peculiarly sensitive to cold seem also in a less degree to be sensitive to heat, though this result of experiment may be illusory. It is possibly due to spreading of the stimulation over neighbouring parts of the skin. Further investigation is needed, but the general result, so far, is: the skin is the seat of three distinct senses,—the sense of heat, the sense of cold, and the sense of pressure.

*Op. cit., p. 1428, 1429.