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-46 ture, do not feel cold. It is, then, the rapid loss of heat that gives rise to the sensation of cold.

The power of the skin in recognizing changes of temperature is very accurate, although the power of judging of the absolute degree of temperature is very slight.

By dipping the finger rapidly into water of varying temperature, it has been found that the skin can distinguish between temperatures which differ by only 1/4 Cent, or l/2° Fahr. The time required for the arrival of temperature impressions at the brain is remarkably long when compared with the rate at which ordinary tactile impulses travel. To judge satisfactorily of the temperature of an object, we must feel it for some time.

There must be special nerve endings which are capable of receiving heat impressions, because warmth applied to the nerve fibres themselves is not capable of giving rise to the sensation of heat. Thermic stimuli, no doubt, do affect nerve fibres, but only cause the sensation of pain when applied to them.

These nerve endings are not the same as those that receive touch and pressure impressions, because the appreciation of differences of temperature is not very delicately developed in the parts where the tactile sensations are most acute. Thus, the cheeks and the eyelids are especially sensitive to changes of temperature, a fact known by people who want a ready gauge of the heat of a body, e. g., a barber approaches the curling tongs to his cheek to measure its temperature before applying it to the hair of his client. The middle of the chest is very sensitive to heat, while it is dull in feeling tactile impressions. The hand is far from being the best gauge of temperature, for heat appreciation is not developed in a due proportion to the keenness of its tactile sensibility. The larger the surface exposed to changes of temperature the more accurate the judgment at which we can arrive - the slightest changes being at once recognized when the entire surface of the body is exposed to them. The foregoing facts are well known to persons in the habit of testing the temperature of a warm bath without the aid of a thermometer; they do not use the limited surface of a sensitive tactile finger tip, but plunge the entire arm into the water. The elbow, indeed, is the common test used by nurses in ascertaining that the water in which they are about to wash an infant is not too warm for that purpose.

Great extremes of heat or cold, such in fact as would act as stimuli to a nerve fibre, do not give rise to sensations of different temperatures, but simply excite feelings of pain. Thus, if we plunge our hand into a freezing mixture or into extremely hot water, it is difficult to say at once whether they are hot or cold - in both cases pain being the only sensation produced.