taken place, and since we have convinced ourselves a thousand times, that this sensation is only caused by contact with the skin, since, further, we have no token. that the excitement passes through a long nerve to the brain, therefore our imagination, in consequence of the experience gained, refers the entire action of the sensation to the spot where the recognizable cause of the irritation is present.

With the aid of these observations we can explain the sense of locality possessed by the skin, ie. the power of determining correctly the locality of the irritation. From all parts of the skin run nerve-fibres, isolated like telegraph wires, to the nervous centre of the brain in which the consciousness of sensation takes place. The brain is, therefore, the terminus of these nerve-branches, and, as it were, receives and explains the messages. transmitted to it. The brain, however, distinguishes very clearly by what branch such a message has come, and just as a clerk in a telegraph office, where a great number of wires meet from all sides, knows by experience from what direction each wire brings its message, so the brain also knows, by the experience it has gained, where an irritation has occurred, when it reaches it by a certain nerve-fibre, and therefore refers the whole sensation to that part of the skin where the irritation took place. It is probable that the brain, by its imaginative faculty, has formed a complete picture of the surface of the body, which has been obtained gradually and by experience, and is always being more highly perfected. The nerve-fibres are in a definite relative position to this picture, or rather with the part of the brain where this picture is formed, and we can imagine that every spot

upon the surface of our body is connected with its corresponding spot in the picture of the brain. When, therefore, an irritation is conveyed by a certain nervefibre, there arises in the brain a picture of the point of the skin where the irritation has taken place.

It is evident that if a true localisation of the sensation is to take place, all the connections of the skin with the brain must be in good order. If the irritation were to pass from one nerve-fibre to another the brain would no longer be able to distinguish the place from which it came, and could not localise the irritation.

A very interesting observation is often made in men, in whom, for the purpose of an operation, a piece of skin is removed from its original position and made to grow in another place. In order, for instance, to replace a nose, which has been destroyed by disease, or has been wanting from birth, surgeons detach a triangular piece of skin from the forehead, so that it is only attached by its apex to the root of the nose between the eyebrows; they then fold the piece over downwards, and sew it on the skin of the face in the position of the nose. The new nose after a short time grows on completely; if, however, it is pricked with a needle, the person does not feel the prick on the nose, but on the forehead--the original position of the skin. This sensation does not continue long after the operation, and the person gradually becomes conscious that the locality of the sensation is now a different one, and thus, by experience, learns to determine the locality correctly. A similar and no less interesting observation is made in the case of persons who have undergone amputation. They very frequently make the

remark that they have sensations in the amputated legs or arms, just as if the limbs were still existing. They feel in them the effects of frost and warmth; they say they feel pain in this or that toe of the lost leg; that they feel tickling, itching, etc., and distinguish the exact point where these feelings take place. The explanation of these facts is quite simple. In the stump of the amputated leg lie the divided nerve-trunks, which have provided the entire limb with sensory nerves. In the healed scar the nerve-stems are often irritated; and since the irritation of the nerves is conveyed to the brain, it causes sensation, and simultaneously produces-we might almost say from habit-the picture of the same part of the body in which they naturally end. The brain, therefore, refers all these sensations, from the experience it has gained, to the same limbs in which the irritated nerves originate, even when the limb itself is wanting.

This very remarkable phenomenon is a manifest proof that the action of sensation takes place only in the nervous centre of the brain. Since even when a part of the body is wanting, the sensation of its existence and irritation does not disappear.

CHAPTER II.

Sensibility of the Sense of Touch-The Sensory Circles of the Skin-Relation of the Sense of Touch to the Activity of the Brain-- Delusions of the Sense of Touch.

THE power of touch is not equally developed in all parts of the body. We know from experience that it is fully developed in the hands, since we use these organs for the purpose of touching objects. The hand, by means of its mobility and its articulated structure, is more especially adapted for this action than all the other parts of the body; and it is a general fact in the animal kingdom, that all organs adapted for touching are endowed with the greatest mobility, such as the feelers of an insect, the trunk of an elephant, and the tongue of all animals. Mobility alone, however, is not sufficient to give such organs the function of a tactile organ, but its surface must be especially provided with a fine sense of touch, in order to render it capable of perceiving fine distinctions of space.

Other parts of the body than the hand, especially the feet, are very little adapted for touching, not only on account of their inconvenient position and defective form, but because their skin possesses a far less cultivated sense of locality. It would, for instance, be very difficult to recognise with closed eyes the form of a

body, however simple, by the aid of the arm alone; with. the surface of the feet it could be done more easily, while with the hand we should very quickly make our decision. It seems extremely natural that the members most capable of motion should possess the finest sense of touch; and it may with justice be said, that it is most probably developed by use, and, further, is transferred by inheritance. Apes, who use the feet for grasping just as much as the hands, have the sense of touch equally cultivated in both members, since both are used to the same extent for touching. In mankind, whose foot is transformed into an instrument of progression, the power of touch in this member returns to the hand; but it is interesting to observe that occasionally in men, who are without arms, the sense of touch can be highly developed in the feet by practice, and as the mobility increases in an equal degree, they are able to write and execute many kinds of work.

The delicacy of the sense of touch, or, more exactly, the sense of locality in different parts of the skin, has been measured by Ernst Heinrich Weber, by a very ingenious method. It consists of the following highly interesting experiment, which everyone can perform with the greatest ease. Two persons are required for this experiment, one of whom tests the sense of touch of the other. For this purpose a pair of compasses are taken, whose points, somewhat blunted, are placed at a certain distance from one another on a part of the skin of the other person. The latter must then say, with closed eyes, whether he feels the contact of two separate points, or whether both points seem to be merged into one.