106. Burning inside the body.—In every part of a man's body a very slow fire is always burning. The blood brings to the cells food from the intestine, and air from the lungs. The food and air join in a burning. The smoke goes back to the blood and is carried to the lungs, and breathed out with the breath. The ashes, also, go back to the blood, and are carried away by the skin and kidneys. The burning makes no flame or light for it goes on very slowly. You cannot see the smoke, but you can feel the warmth of the burning. Some of the heat is turned to power, and gives the body strength to do work. The body is like a steam engine. It burns up all its food.
107. How the body is warmed.—The body is warmed by the slow burning in the cells. This burning keeps the body always at the same warmth. On a hot summer's day you feel warmer than on a cold snowy morning. But your body is no warmer. Only your skin is warmer.
If the skin is warm, the whole body feels warm, but if the skin is cold, the whole body feels cold. On a hot summer's day the heat is kept in the skin, and we feel warm. On a cold winter's day a great deal of heat passes off from the skin, and we feel cold. Yet our bodies have the same warmth in winter as in summer.
108. How the sweat keeps us cool.—When your hands or feet are wet, they are cold. On a hot summer's day, your body becomes wet with sweat. This cools the body as if water were poured over it. So sweating keeps you from getting too warm, and from being sunstruck.
We are sweating all the time, but the sweat usually dries as fast as it forms. When we are too warm it comes out faster than it dries. On a winter's day we sweat only a little, and so we save the heat. But more heat passes off from the skin into the cold air, and we do not grow warmer.
109. Clothes.—We wear clothes to keep the heat in the body. They do not make heat, but they keep it from going off. Wool and flannel clothes keep the heat in better than cotton. We wear woolen in the winter, and cotton in the summer.
Fur keeps in heat the best of all. In very cold lands only fur is worn.
Linen lets heat out easily. It makes good summer clothes.
110. Where to wear the most clothes.—The face and hands are kept warm by the blood and we do not cover them except in the coldest weather. Our feet are more tender and need to be covered enough to keep them warm. We ought to wear thick-soled shoes or rubbers in damp weather so as to keep the feet dry and warm. We ought to dry the stockings every night, for they will get wet with sweat.
The trunk of the body needs the most clothes. The legs ought to be kept warm, too. If the dress reaches only to the knee, thick underclothing is needed for the lower part of the leg.