However, most of the nerves we call sensory nerves convey tactile impressions, and, speaking generally, those parts of the outer skin which have the keenest tactile sense are also those most ready to excite feelings of pain.

The intensity of the stimulation for the sense of touch must be kept within certain limits in order to be adequate, i. e., capable of exciting the specific mental perceptions. If the stimulus exceed these limits, only a general impression, approaching that of pain, is produced.

The power of forming judgments by touch differs very much in different parts of the body, being generally most keen where the surface is richest in touch corpuscles, namely, the palmar aspect of the hands and feet, and especially the finger tips, tongue, lips, and face.

When we feel a thing in order to learn its properties, we make use of all the qualities of which our sense of touch is made up. We estimate the number of points at which it impinges on our finger tips, rub it to judge of smoothness, press it to find out its hardness, and at the same time gain some knowledge of its temperature and power of absorbing heat.

To get a clear idea of our complex sense of touch, we must consider each kind of impression separately.

SENSE OF LOCALITY.

By this is meant our power of judging the exact position of any point or points of contact which may be applied to the skin. Thus, if the point of a pin be gently laid on a sensitive part of the skin we know at once when we are touched, and if a second pin be applied in the same neighborhood, we feel the two points of contact and can judge of their relative position. When we feel anything, we receive impulses from many points of contact bearing varied relationships to each other, and thus become conscious of a rough or smooth surface.

The delicacy of the sense of locality differs very much in dif ferent parts of the skin. It is most accurate in those parts which have been used as touch organs during the slow evolution of the animal kingdom.

The method of testing the delicacy of the sense of locality is that of applying the two points of a compass to different parts of the skin, and by varying their position, experimentally determine the nearest distance at which the two points give rise to distinct sensations. The following precautions must be attended to in carrying out this experiment: 1. The points must be simultaneously applied, or two distinct sensations will be produced at abnormally small distances. 2. The force with which the points are applied must be equal and minimal, because excessive pressure causes a diffusion of the stimulus and a blurring of the tactile sense. 3. Commencing with greater and gradually reducing the distance of the points enables a person to appreciate a less separation than if the smaller distances were used at first. 4. The duration of the stimulus; two points of contact being distinguished at a much nearer distance if the points be allowed to rest on the part, than when they are only applied for a moment. 5. The temperature and material of the points should be the same. 6. Moisture of the surface makes it more sensitive. 7. Previous or neighboring stimulation takes from the accuracy of the sensations produced. 8. The temperature of the different parts of the skin should be equal, as cold impairs its sensibility. The following table gives approximately the nearest distances at which some parts, which may be taken as examples of the more or less sensitive regions of the skin, can recognize the points of contact by their giving rise to two distinct sensations:

Tip of the tongue...

1 mm.

Palmar aspect of the middle finger tip...... 2

Tip of the nose...

Back of the hand..

d

8

If one point of the compass be applied to the same spot, and the other moved round so as to mark out in different directions b the limits at which the points can be distinguished as separate, we get an area of a somewhat circular form, for which the name sensory circle has been proposed. It would be convenient to d

0

i

explain this on the simple anatomical basis that the impressions of this area were carried by one nerve fibre to the brain, and thus but one sensation could be produced in the sensorium. We know this cannot be the true explanation, from the following facts: 1. No such anatomical relationship is known to exist. 2. By practice we can reduce the area of our sensory circles in a manner that could not be explained by the development of new nerve fibres. 3. If the two points of the compass be placed near the edges of two well-determined neighboring sensory circles, and so in relation with the terminals of two nerve fibres, they will not give distinct impressions; they require to be separated as much as if they were applied within the boundary of one of the circles where they also give rise to the double perception.

To explain better the sense of locality, it has been supposed that sensory circles are made up of numerous small areas, forming a fine mosaic of touch fields, each of which is supplied by one nerve fibre, and that a certain number of these little fields must intervene between the stimulating points of the compass in order that the sensorium be able to recognize the two impulses as distinct. For, although every touch field is supplied by a separate nerve fibril which carries its impulses to the brain, and is therefore quite sensitive, the arrangement of the cells in the sensorium is such that the stimuli carried from two adjoining touch fields are confused into one sensation. Thus, when an edge is placed on our skin, we do not feel a series of points corresponding to the individual fields with which it comes in contact, but the confusion of the stimuli gives rise to an uninterrupted sensation, and we have a right perception of the object touched.

THE SENSE OF PRESSURE.

There seems to be a reason for separating the perception of differences in the degree of pressure exercised by a body from the simple tactile or local impression. If we support a part of the body so that no muscular effort be called into play in the support of an increasing series of weights placed upon the same area of skin, we can distinguish tolerably accurately between the different weights. It has been found that if a weight of about

30 grammes be placed on the skin a difference of about 1 gramme can be recognized—that is, we can distinguish between 29 and 30 grammes, if they are applied soon after one another. If the weights employed are smaller, a less difference can be detected; if larger weights are used, the difference must be greater, and it appears that the weight difference always bears the same proportion to the absolute weight used. We can perceive a difference between 74 and 71, 14 and 15, 29 and 30, 58 and 60, etc., the discriminating power decreasing in proportion as the absolute degree of stimulation increases.

One of the reasons why the sense of pressure is regarded as distinct from that of locality is that the former is found not to be most keenly developed in the parts where the impressions of locality are most acute. Thus, judgment of pressure can be more accurately made with the skin of the fore arm than the finger tip, which is nine times more sensitive than the former to ordinary tactile impressions, while the skin of the abdomen has. an accurate sense of pressure, though dull to ordinary tactile sensation.

It has been said above that the weights by which pressure sense is to be tested should be applied rapidly one after the other. This fact depends upon the share taken in the mental judgment by the function we call memory. In a short time the recollection of the impression passes away, and there no longer exists any sensation with which the new stimulation can be compared.

At best we can form but imperfect judgments of pressure by the skin impressions alone. When we want to judge the weight of a body we poise it in the free hand, which is moved up and down so as to bring the muscles which elevate it into repeated action. Hereby we call into action a totally different evidence, namely, the amount of muscle power required to raise the weight in question, and we find we can arrive at much more accurate conclusions by this means. The peculiar recognition of how much muscular effort is expended is commonly spoken of as muscle sense, which may arise from a knowledge of how much voluntary impulse is expended in exciting the muscles to action, but more probably it depends upon afferent impulses arriving at the

sensorium from the muscles. By its means we aid the pressure sense in arriving at accurate conclusions of the weight of bodies, so that in the free hand we can distinguish between 39 grm. and 40 grm.

TEMPERATURE SENSE.

We are able to judge of the difference in temperature of bodies which come in contact with our skin. Since our sensations have no accurate standard for comparison, we are unable to form any exact conception of the absolute temperature of the substances we feel. The sensation of heat or cold, derived from the skin itself, without its coming into contact with anything but air of moderate temperature, varies with many circumstances, and because of these variations the powers of judgment of high or low temperature must be imperfect. The skin feels hot when its blood vessels are full; it feels cold when they are comparatively empty. An object of constant temperature can thus give the impression of being hot or cold according as the skin itself is full or empty of warm blood. But, independent of any very material change in the blood supply of the cutaneous surface of a part, any change in the temperature of its surroundings causes a sensation of change of temperature, which is, however, a purely relative judgment. Thus, if the hand be placed in cold water, we have at first the sensation of cold; to which, however, the skin of the hand soon becomes accustomed and no longer feels cold; if, now, the hand be placed in water somewhat warmer -but not higher in temperature than the atmosphere we have a feeling of warmth. If the hand be placed in as hot water as the skin can bear, it feels at first unpleasantly hot, but this feeling soon passes away and the sensation is comfortable. If from this hot water it be placed again in the water of the air temperature, this-which before felt warm-feels very cold.

An important item in the estimation of the temperature of an object by the sensations derived from the skin depends upon whether it be a good or a bad conductor of heat. Those substances which are good conductors, and therefore, when colder than the body, quickly rob the skin of its heat, are said to feel cold, while badly-conducting bodies, of exactly the same tempera