forms, each of which has its particular name. The mode of distribution of those throughout the skin does not enable us to say as yet which of them are specially concerned in the different kinds of tactual sensation (pressure and temperature).1

Tactual sensations are to be carefully distinguished from common sensations which are apt to combine with them. Sensations of tickling illustrate the tendency of the two to coalesce. In the experience of being tickled there is a certain element of true tactual sensation, that of gentle contact, which is rapidly intermittent and which commonly shifts from one point of the skin to adjacent points. But the whole effect with its large element of feeling involves the action of the nerves of common sensation as well.2

§ 16. Degrees of Pressure. The fineness of the tactual sensibility proper is seen in the estimation of degrees of pressure. Here a whole mass of experiment comes to our aid.

It is found, first of all, that different parts of the skin are very unequal in respect of absolute sensibility, or capability of reaction on very weak stimuli. This variation of absolute sensibility, moreover, does not appear to correspond exactly with what we know otherwise of the local variations of tactual sensibility.

Thus, according to Aubert and Kammler, the lightest weight which produces a tactual sensation is '002 grammes for the forehead and temples, and '005 - '015 for the volar side of the fingers. The measurement of absolute sensibility has been carried further by Goldscheider's experiments. By using a very fine point, and applying this gently, he ascertained that a true sensation of pressure is only obtainable at certain minute spots within a given area of the skin, which spots he calls pressure spots. These seem to be most numerous in what we otherwise

know as the most sensitive parts of the organ.

The second and more important mode of tactual sensibility is the discriminative sensibility to different degrees of pressure. Here definite results are difficult to obtain, owing to the fact

1 For a full account of these terminal organs, and of what is known as to their functions, see Ladd, op. cit., pt. i. chap. v. § 8 and following, and pt. ii. chap. iv. § 19 and following.

2 On the distinction between elements of common sensation and tactual sensation proper in our experiences of touching or being touched, see Wundt, op. cit., cap. ix. § 1.

3 A gramme is about 15'4 grains. For a fuller account of the facts here touched on, see Wundt, op. cit., i. p. 367 ff.

For a full account of these experiments, see Ladd, op. cit., p. 346 and following.

that in ordinary cases where we estimate higher degrees of pressure, as in lifting a weight, the tactual sensibility is greatly assisted by the muscular sensations, to be spoken of presently. By supporting the arm or other part experimented on, and then successively applying different degrees of pressure, it has been found possible to some extent to measure the discriminative tactual sensibility of different regions of the skin. Among the results obtained is that the discrimination of pressure pure and simple is much less acute than when the muscular sensations co-operate. The inequalities at different dermal regions, as measured by the smallest difference discernible, correspond to some extent at least to known variations of tactual sensibility.

The experiments have been carried out by Weber and Fechner. A certain weight is laid on the hand or other part, and the experimenter then tries how much must be taken away or added in order that the subject may note a difference in the sensations. As might be expected, the discrimination of pressure is much finer when the same part is used successively than when two parts are used. Among the differences in different regions of the skin, one may mention that the anterior surface of the finger detects one-half of the smallest difference appreciable by the posterior surface (one-sixth as compared with one-third). These differences of discriminative sensibility do not, as already pointed out, correspond to differences of absolute sensibility in the same regions.1

§ 17. Qualitative Differences of Tactual Sensation. The differences of quality among sensations of touch are less numerous than those among sensations of smell. The most important differences, next to that of sensations of pressure and of heat and cold, are those of soft and hard, and rough and smooth.

Here, as already hinted, we have to do with differences of intensity. The contrast between hard and soft, as known purely by touch, is simply that between great and little pressure. It is obvious, moreover, that the terms are relative; the same object being called hard or soft in relation to different objects. The difference between smooth and rough, so far as dependent on pure touch, apart from movement, is connected with continuity and uniformity of pressure in the one case, and discontinuity and inequality in the other. In laying the hand upon, and letting it glide over a polished table, the pressure is uniform at all points of the skin involved and at successive stages of the movement. In laying it on a rough newly-sawn piece of wood there is no such uniformity, but rather a congeries

1 See Ladd, op. cit., p. 367; Wundt, op. cit., i. cap. viii. § 2.

of discrete prickly sensations. Thus the impressions of rough and smooth are closely connected with the two modes of local discrimination already touched on, viz., separation of points and continuous mass or extensity. Lastly, the sensation of moist or wet, regarded by Aristotle as a primary sensation of touch, is probably compounded of a sensation of smoothness and one of cold.1

§ 18. Extensity: Local Distinctness. Touch, as already pointed out, is characterised by a fine appreciation of extensive magnitude, and of local distinctness of sensation. The discriminative sensibility to separateness of point or locality, which is measured by the smallness of the distance between two points, say those of a pair of compasses, just distinguishable as two, is found by the classical experiments of Weber, aided by those of more recent investigators, to vary considerably at different parts. In general, it is finest in those regions, as the fingers and lips, which are known by every-day observation to have high tactual sensibility. It is much finer in the mobile parts, hands, feet and lips, than in the comparatively fixed parts (the trunk). It is about twice as fine on the anterior as on the posterior surface of the fingers. In the former the minimum distance between the points sinks as low as 2 millimetres. It falls off as we go from extremities (fingers or toes) towards the trunk.

These experiments were initiated by Weber, who employed a pair of compasses, varying the distance between the points. Weber himself explained the results by help of a system of 'sensation-circles' which cover the skin. These circles are supposed to vary greatly in size, and to some extent also in form, and to overlap one another in a somewhat intricate manner. More recently, Goldscheider, who has tested the dermal discrimination of points by experiments of his own, explains the fact by supposing that we obtain two distinct sensations only when two 'pressure spots' are stimulated. If, on the other hand, one point touches a pressure spot and the other some contiguous area free from such spots, two tactual sensations do not arise.

Reference may also be made here to some experiments made by Prof. G. Stanley Hall on the "motor sensibility" of the skin, that is to say, the capability of judging of the movement of a point over the skin by dermal sensations alone. Here too considerable differences were found to exist at different parts, though the differences did not accord with those of local discrimination as measured by Weber.3

1 On the question whether roughness and smoothness, sharpness and bluntness,

etc., involve qualitative difference, see Wundt, op. cit., cap. ix. § 1.

2 A millimetre is th part of a metre, and is equal to '0393 of an inch.

* For a fuller account of Weber's experiments, see Ladd, op. cit., p. 405 and following; Wundt, op. cit., ii. cap. xi. § 2, p. 6 ff.; Goldscheider's and Stanley Hall's experiments are described by Ladd, op. cit., p. 410, etc.

$ 19. Thermal Sensations. The sensations of hot and cold obtained by contact of different parts of the skin with bodies of various temperatures constitute a second main group of sensations usually included under the sense of touch. Sensations of heat and cold may arise in any part of the organism, and are in this respect closely allied to common sensations. More particularly they are experienced through variations in the temperature of the skin. In certain dermal areas they are finely distinguishable in their degree, and in this respect they constitute, along with the specialised discriminative sense. of pressure, a special mode of sensibility. There is good reason to suppose that, just as they differ qualitatively from sensations of pressure, so they involve distinct nervous fibres.

The mode of stimulation in the case of thermal sensations is not precisely ascertainable. The common supposition is that they depend on an expansion or contraction of certain nervous elements with the rise or fall of the temperature of the part. That thermal sensations are the same in kind as sensations of pressure has been hastily assumed on the ground that in certain cases we are apt to confuse the two, as when we take a cold body resting on the skin to be heavier, and a warm body to be lighter, than they really are. The fact of the differentiation of special nervous structures for the sense of temperature has been established with some certainty by the experiments of Goldscheider and others to be spoken of presently.

The sensations of temperature received by way of contact of bodies with the skin present a clearly-marked contrast of quality, viz., that of hot and cold. As already pointed out, the extremes of heat and cold tend to approach one another: that is to say, in the case of very hot and very cold bodies we are no longer able to recognise the qualitative peculiarity of the sensation. Between these extremes many degrees of hot and cold are distinguishable. In this way we get a scale of thermal sensations analogous to that of rough and smooth, and hard and soft, with a neutral or indifferent point, known as the zeropoint, in the median region of the scale.1

This zero-point appears to be related to the normal temperature of the part of the skin stimulated. Bodies having the same temperature as the skin yield us no distinct sensations either of hot or of cold. It may be added that the discrimination of temperature is only fine in the median region of the scale near the zero-point.

1 See Ladd, op. cit., p. 347.

The discrimination of temperature, like that of pressure, varies considerably at different parts of the skin. These variations do not run parallel with those of sensibility to pressure. Since, moreover, the normal temperature of the skin varies at different parts, e.g., at the finger-tips and the inside of the mouth, the zero-point is not the same for all dermal areas.

The sensations of hot and cold are known to be highly subjective or relative. Thus they vary with the changing temperature of the part affected.

Weber showed that if

the hand be held in water of the temperature 54'5° Fahr. and then plunged in water at 64'4°, it will feel this last to be hot, whereas if the hand had been put into the second at the outset it would have felt it to be cold.1

Weber's explanation of these facts was that when the temperature of the skin is rising we have a sensation of heat; when sinking, a sensation of cold. Hering, on the other hand, connects the sensations of heat and cold, not with change in the temperature, but with differences of temperature above or below the zero-point of the part affected.

Our knowledge of the thermal sensations of the skin has been recently increased by some experiments of Goldscheider and others. These have elicited the fact that there are temperature-spots distributed over the area of the skin analogous to the pressure-spots, but not coinciding with these in their mode of distribution. Hence localities highly sensitive to pressure may be dull in thermal discrimination, and vice verså. Still more curious, it has been ascertained that these temperature-spots are of two kinds, viz., 'heat spots' and 'cold spots,' some being sensitive to heat and not to cold, others exclusively to cold. Thus the forehead, though highly sensitive to cold, is only moderately so to heat. These researches leave the question how the nerves are stimulated in the case of thermal sensations very obscure."

20. Value of Sense of Touch. Our examination into the sensations of touch shows us that this sense is capable of yielding us a variety of finely-graduated differences. In spite of the few qualitative dissimilarities, as compared with those of the higher senses, hearing and sight, it furnishes us with an exact knowledge of some of the more important qualities of bodies. This result depends first of all on its fine discrimination

1 The relativity of these sensations is further seen in the fact that the temperature of a body was found by Weber to vary with the extent of dermal surface acted upon. Water at 29 R. seemed to the whole hand warmer than water at 32° R. to a single finger. This fact is curious as illustrating the effect of extensity in modifying quality.

2 On these researches, see Ladd, op. cit., p. 348, etc.; cf. 370. Also Wundt, op. cit., cap. ix. § I.