The four systems of the general sense are all homogenous systems (§ 5, 5). This shows that the sense is genetically earlier than the others, whose systems are all complex. The sensations of pressure from the external skin, and those due to the tensions and movements of the muscles, joints, and tendons, are generally grouped together under the name touch-sensations, and distinguished from the common sensations, which include sensations of hot, cold, and pain, and those sensations of pressure that sometimes arise in the other internal organs.
This distinction, however, has its source in the relation of the sensations to ideas and concomitant feelings, and has nothing to do with the qualities of the sensations in themselves.
7. The ability of the different parts of the general sense-organ to receive stimulations and give rise to sensations, can be tested with adequate exactness only on the external skin. The only facts that can be determined in regard to the internal parts, are that the joints are in a high degree sensitive to pressures, while the muscles and tendons are much less so, and that sensations of hot, cold, and pain in the internal organs are exceptional, and noticeable only under abnormal conditions.
On the other hand, there is no point of the external skin and of the immediately adjoining parts of the mucous membrane, which is not sensitive at once to stimulations of pressure, hot, cold, and pain. The degree of sensitivity may, indeed, vary at different points, in such a way that the points most sensitive to pressure, to hot, and to cold, do not, in generally, coincide. Sensitivity to pain is everywhere about the same, varying at most in such a way that in some places the pain-stimulus acts on the surface, and in others not until it has penetrated deeper. On the other hand, certain approximately punctiform cutaneous regions appear to be most favorable for stimulations of pressure, hot, and cold. These points are called respectively, pressure-spots, hot-spots, and cold-spots. They are distributed in different parts of the skin in varying numbers. Spots of different modality never coincide; still, temperature-spots always receive sensations of pressure and pain as well; and a pointed hot stimulus applied to a cold spot, always causes a sensation of hot, while hot-spots do not seem to be stimulated by pointed cold stimuli. Furthermore, hot-spots and cold-spots react with their adequate sensations to properly applied mechanical and electrical stimuli.
8. Of the four qualities mentioned sensations of pressure and pain form closed systems which show no relations either to each other or to the two systems of temperature-sensations. These last two, on the other hand, stand in the relation of opposites; we apprehend hot and cold not merely as different, but as contrasted sensations. It is, however, very probable that this is not due to the original nature of the sensations, but partly to the conditions of their rise, and partly to the accompanying feelings. For, while the other qualities may be united without limitation to form mixed sensations -- as, for example, pressure and hot, pressure and pain, cold and pain - hot and cold exclude each other because, under the conditions of their rise, the only possibilities for a given cutaneous region are a sensation of hot or one of cold, or else an absence of both.
When one of these sensations passes continuously into the other, the change regularly takes place in such a way that either the sensation of hot gradually disappears and a continually increasing sensation of cold arises, or vice versa the sensation of cold disappears and that of hot gradually arises.
Then, too, elementary feelings of opposite character are connected with hot and cold, the point where both sensations are absent corresponding to their indifferent zone.
In still another respect the two systems of temperature-sensations are peculiar. They are to a great extent dependent on the varying conditions under which the stimuli act upon the sense-organ. A considerable increase above the temperature of the skin is perceived as hot, while a considerable decrease below the same is perceived as cold, but the temperature of the skin itself, which is the indifference zone between the two, can adapt itself rapidly to the existing, external temperature within fairly wide limits. The fact that in this respect too, both systems are alike, favors the view that they are interconnected and also antagonistic. B. SENSATIONS OF SOUND.
9. We possess two independent systems of simple auditory sensations, which are generally, however, connected as a result of the mixture of the two kinds of impressions. They are the homogeneous system of simple noise-sensations and the complex system of simple tone-sensations.
Simple noise-sensations can be produced only under conditions that exclude the simultaneous rise of tonal sensations, as when air-vibrations are produced whose rate is either too rapid or too slow for tone-sensations to arise, or when the sound-waves act upon the ear for too short a period.