Page:Popular Science Monthly Volume 10.djvu/685

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feels the cold. It is the same with the hair of animals, and the feathers of birds, they are also without nerves.

Id proportion as our heat-losses increase, while the creation of heat in our interior remains about the same, we feel the necessity of diminishing the rate at which the heat leaves our immediate neighborhood. This kind of regulation is somewhat taken care of involuntarily even by the naked body. In consequence of the cold, the nerves which act on the calibre of the blood-vessels of our surface contract them, and lessen the quantity of blood in them, so that less heat comes to the surface, and we need not be afraid of becoming also inwardly colder if we feel cold, even very cold.

The sensation of cold on the skin does not necessarily give the measure of our internal temperature. In the cold stage of ague, for instance, the temperature of the internal organs rises considerably, while by a kind of spasmodic contraction of the superficial blood-vessels the flow of heat toward the skin is less than normal. The above-mentioned regulation of heat-loss by the capillary system of our skin is not all-sufficient either in point of time or degree. The cold may be too strong, and the regulator get overworked and paralyzed, so that additional clothing is required to delay the departure of our heat, and to spare the nerves of the blood-vessels. We help ourselves by additional clothing, and the underlying article of clothing stands in the same relation to the outer one as the skin to its first covering. From this point of view you have to consider the sequence of shirt, under-clothing, coat, overcoat, etc., etc., an arrangement by which we save the vasomotor nerves the greater part of their work.

It is an open question, which the incompleteness of our hygienic knowledge prevents us from answering quite satisfactorily, how far we ought to hand over the regulation of our heat-loss to our dress, or how far we should go in deputing it to our organism, and its capability of transferring more or less heat from the centres to the surface of our bodies. This self-help of the organism and the readiness for it resulting from frequent exercise of this function are generally called hardening one's self; the contrary, making one's self tender. The former we can never quite dispense with, but I believe that too high a value is sometimes put and too large claims made on it. One ought to possess the capability and the readiness, but not to make use of them continually.

All human aim must be to obtain the greatest effect at the smallest expense. We ought to choose those means which attain the end without exhausting our power, which should be preserved for higher purposes. These principles ought to guide us in approaching the question. It is not only superfluous, but positively injurious, to use one's self up.

I believe that it is now evident to you that a part of the heat of our body radiates from the surface of our clothing; but we must now consider whether this radiation does not vary according to the nature, quality, or color, of the material. Experiments which have been