By means of, or through, the medium of its vast number of sweat glands, the skin regulates the amount of heat in our bodies. This is a function which is much more important than most people might at first imagine. The temperature of the body in health is about 99 degrees Fahrenheit, if it rises much above this--even a few degrees, indeed--or if it falls much below it, severe illness is indicated, danger is apparent, danger even to life itself. An equable temperature of the body it is therefore evident is alone compatible with perfect health, but if it were not for the perspiratory system, when any extra strain is put upon the body, as by hard work, or hard exercise, heat would accumulate in the system, and the temperature of the body would be raised, to our discomfort, detriment, and danger. But the pores of the skin are our safety valves; from exertion the blood is determined to the surface, the sweat glands are thus excited to increased action, and perspiration is thrown off in abundance, which, passing off in steam, carries with it--in obedience to a law too well known to need explanation--all the extra caloric. In hot weather, a great deal of heat is thus expended through the skin; in cold weather the kidneys are more active, and they excrete the water which otherwise would have passed through the pores, and by storing it for a time in a reservoir designed for the purpose, conserve the heat of the system, and prevent lowering of the animal temperature. By means of these same sweat glands with their ducts or pores, an immense amount of effete matter is carried off from the body in the course of twenty-four hours, which, as already stated, if retained in the system, would tend to lower vitality by poisoning the blood. If the reader recollects that the lungs also perform a renovating function on the blood, and thus on the body, that oxygen is inhaled, and that air loaded with carbonic acid, water, etc, exhaled, he will readily understand how much assistance the respiratory organs receive from a healthy acting skin.