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The Sense of Touch

hand, cold, contact, heat and inch

THE SENSE OF TOUCH.

The sense of touch is aroused by stimulation of the nerve terminations, already described, either by mechanical means or by heat or cold.

When we lay our hand on anything, the mere mechanical contact with the body produces a sensation of touch, and if the body be warmer or colder thau the hand a sense of heat or cold is aroused. Touch includes three things: (1) the sense of contact, (2) the sense of pressure, (3) the sense of heat and cold.

(1) The sense of contact is the most impor tant element in touch. By it we gain informa tion as to the form, size, and other characters, smoothness, hardness, &c., of external bodies. The sensitiveness of touch varies in different parts of the skin. Where the scarf-skin (epider mis) is thinnest it is most acute; where it is thickest it is more dull. The absence of epider mis altogether does not render the part more sensitive to sensations of contact. The direct contact with the unprotected true skin occasions pain, which effectually masks the feeling of con tact. The tips of the fingers, the red border of the lips, and the tip of the tongue are the most sensitive parts. Experiments have been made on the degree of sensibility of various parts to touch, by using a pair of compasses, with points blunted by pointed pieces of cork, and deter mining how much the compasses required to be open for the impression of each point to be felt. If the two points were very near, the sensation was of one point only, and in order to produce the sensation of two points the ends of the compasses had to be separated by varying dis tances according to the part of the skin experi mented on. The result showed that two points could be distinguished by the tip of the tongue though they were only nth of an inch apart, by the tip of the forefinger if Ath of an inch apart, by the red surface of the under-lip if *th of an inch apart, by the tip of the nose when fth inch apart, by the palm of the band if moths inch apart; and that the points of the compasses required to be separated lk inch to be perceived as two when placed on the back of the hand, while in the middle of the thigh they required to be separated inches.

(2) The sense of pressure is different from the sense of contact, for sometimes those parts which are less acute for mere sensations of touch are more correct in gauging pressures. It is by the sense of pressure that we estimate differences of weights. Another element is introduced, however, in judging of weight, when the weight is taken in the hand, and the Band moved up and down. The weight offers resistance which the muscles require to over come, and this calls forth what has been called a muscle sense, a sensation produced by the muscles, caused by the resistance offered to their movement.

(3) The sense of skin also judges of heat and cold, but its judgments are in this case liable to serious error. If one hand be very cold and the other very warm, and both be placed in the same basin of tepid water, the warmth of the water will be very different to each hand. To the warm hand it will appear cold, and to the cold hand warm. We cannot, therefore, judge absolutely of tem perature. Then, again, the thickness of the scarf-skin seems to affect the sensitiveness to heat, for parts with thin epidermis can bear less heat than parts with thick epidermis.