learns to distinguish two approximate points on the skin, since it has become conscious of the fact that, between these two excited points, a number of points are situated which are not excited. The greater this number, the more distinctly can we recognise the existence of the points; but there is a number which is not sufficiently great to enable us to recognise the points. By practice, however, this number can be diminished; that is to say, fewer non-excited points between those which are excited will then be necessary to create the impression of a double sensation. Since, during this process in the skin the arrangement of the nervous system has remained the same, practice being nothing more than the act of learning, it must therefore clearly follow, that we have here to

do with a process of the brain, and that the sensory circle of the skin is not really situated in the skin but in the brain.

A very interesting illusion of the sense of touch, which is probably already known to the reader as a trick, is no less a proof of the views just given. It is an experiment which even Aristotle regarded psychologically. If we cross the first and middle fingers and then pick up a pea from the table, we have a distinct impression that we are holding two peas, and even when we look closely and convince ourselves that there is only one pea, we can hardly get rid of the impression. The illusion is, moreover, particularly strong if we roll the pea backwards and forwards between the fingers.

Fig. 6.

The reason of this curious illusion is evidently that the tactile surfaces of the skin are placed in an unaccustomed position. If we were to hold the pea with the first and second fingers in their usual position we should know by experience that there was only one pea. If, however, we cross the fingers our experience not only leaves us in the lurch, but deceives us in the impression produced by the sensation.

The cause of the illusion proceeds in reality from the experience of the brain which has been inculcated by practice, and in such a case has led us into error. Thus when we cross the fingers we bring the outer side of each finger into contact with the pea simultaneously. But these two sides of the fingers are, in the ordinary position of the hand, invariably turned away from each other, and, if when in this position, the two sides are touched simultaneously, the brain knows by experience that it can only be caused by two different objects.

Now this experience of the brain is not affected by a change of circumstances, but remains unaltered in whatever position we place the fingers. If we cross them, and hold between them a small ball, their position will still be the natural one, as far as concerns the sense of touch which the brain has gained by experience, and therefore, our imagination changes the crossed position into the ordinary one. When this takes place our imagination must also convert one ball into two.

This shows us how firmly the representation of the surface of our body is imprinted upon our brain. The brain knows perfectly well the natural position of every part of the body, and the situation of every point of skin, a knowledge which it has gained by many years of prac

tice and experience. It is also able to recognise a great number of the movements of different parts of our body. When the hand is moved over any object, it is placed in many different positions with regard to the body, and by the touch we obtain a true representation of the object, whilst, by experience, we can tell the position of the hand at the same time. We can however only recognise all these positions and movements of our tactual limbs correctly after sufficient practice and experience. But as soon as we bring our limbs into an unaccustomed position, our power of recognising correctly tactual impression ceases, and we are no longer able to estimate rightly the position of the objects touched.

39

CHAPTER III.

The Skin's Sense of Pressure-The Sense of Temperature.

THE skin not only possesses the faculty of perceiving contact with a body, but it is also able to calculate the pressure with which the contact takes place.

If we lay an object upon the outstretched hand, we are able to determine the weight of it by raising and lowering the hand, till we are sufficiently satisfied as to the amount of the weight.

Our decision is founded principally upon the amount of exertion, which the muscles of the arm are obliged to use in lifting the weight; and as we have a distinct sensation of the activity of these muscles, and as this sensation increases with an increase of action in the muscles, we are thus able to determine the difference between

different weights. But in such an experiment the weight laid upon the hand acts not only by the tension of the muscles, which are trying to support it, but it also makes an impression upon the skin, which we feel as well as the tension of the muscles. We have to do, therefore, with two influences which almost invariably work together when we lift an object, but which in a scientific examination of the question must be considered

independently. Let us lift, for example, a weight with a ring attached to it, from the ground with the arms straightened at the side, then the weight acts by pressure upon that part of the skin of the hand which grasps the ring, and, by tension, upon the muscles used in lifting. The tension upon the muscles remains the same with the same weight, but the influence of the pressure upon the hand may differ with the form of the handle by which the weight is held. If the handle consists of a broad ring, then the pressure of the weight will be spread over a large portion of the surface of the hand, and each point of the skin will only experience a very moderate amount of pressure; if, however, we take a very small ring, then the whole pressure is concentrated upon a small surface of skin, and we then experience much more pressure, which may even become painful.

Our decision upon the weight of a body is then guided by these two sensations which work simultaneously, and, therefore, in science we distinguish two separate properties which correspond to these sensations. We designate the property of realizing the amount of muscular action the muscular sense, and the property which estimates the amount of pressure upon the skin, the skin's sense of pressure.

In ordinary life the muscular sense and the sense of pressure almost always act simultaneously whenever we lift an object, and, therefore, through the influence of the sense of pressure we are frequently subject to illusions when estimating the weight of objects. For if we lift a heavy body when a very small portion of it presses upon our skin, it will generally appear heavier than if it pressed upon a larger surface, because the pressure on