Now this is how the furnace of the living body is kept smouldering on in its gentle and even way. Little blasts of air are constantly puffed in upon the burning fuel. That out-and-in play of your chest as you breathe,—that is the puffing of air blasts into certain chambers of your living laboratory, to keep up its smouldering fires. The more quickly and deeply you breathe, the warmer your body becomes; and the more slowly and softly you breathe, the colder that body remains. The same action which blows a fresh wind through the living frame to clear away its impure vapors, also serves to fan its hidden flames, and keep its fuel burning. When the breathing is stopped the fires of the body go out, just as those in a common furnace do, when their air-blasts are arrested, and the body becomes dead cold.
You will remember that when the fresh air is drawn into your lungs as you breathe, it enters a large quantity of little cavities or chambers, which have, all of them, a fine net-work of the supply-pipes stretched out upon their walls; and that as the blood rushes on in its course through these supply-pipes, it sucks air into itself from the air-cavities, and carries it, in its own streams, to all parts of the living structure. Air goes with the blood to that strong force-pump, the heart, and is then pumped out with the blood to every crevice and fibre of the body. Every part of the body therefore receives, by means of the supply-pipes and in the blood, heat-fanning air, as well as supporting food.
When air reaches the living flesh and nerves, by thus flowing to them in the blood-streams of the supply pipes, it sets up those changes of substance in their structures which lead to the production of movement, and feeling, and other kinds of living power. When it reaches the dissolved fuel, contained in the blood and in the various little furnace-chambers of the laboratory, it sets up those changes in the fuel which lead to the production of warmth. The fuel is slowly burned in the blood and in the chambers of the frame, and there gives out warmth, as a fire does whilst it is burning in a grate. This warmth consequently heats the blood, and the warm blood carries its heat wherever it goes. The entire body thus becomes as warm as the blood, or nearly so.