“How is it that this heat is always the same, and whence can it come unless from some sort of combustion? There is, in fact, going on within us a perpetual combustion, respiration furnishing the necessary air, and food supplying the fuel. To live is to consume oneself, in the strictest sense of the word; to breathe is to burn. In a figurative sense man has long used the expression ‘the torch of life’; but this figurative term proves to be the exact utterance of the truth. Air consumes the torch; it consumes the animal no less; it makes the torch give out heat and light, and it produces in the animal heat and motion. Without air the torch goes out; without air the animal dies. From this point of view the animal may be compared to a highly perfected machine put in motion by heat. It feeds and breathes to produce heat and motion; it receives its fuel in the form of food and burns it in the inmost recesses of its body with the help of the air introduced by breathing. That explains why the need of food is greater in winter than in summer, the body cooling off more rapidly by contact with the outside cold air, and consequently an increased consumption of fuel being required to maintain the normal temperature. A low temperature creates a desire for food; a high temperature lessens the demand. To the hungry Siberians hearty food is necessary; they ask for bacon and other fats, with brandy to drink. But for the people of Sahara a few dates suffice, with a pinch of flour kneaded in the palm of the hand with a little water. Everything that lessens the loss of heat lessens also the need of food. Sleep, rest, warm clothing, all serve to some extent as substitutes for food. And so there is much truth in the saying that he who sleeps dines.” “That may be,” Jules assented, “but I don’t see how hedgehogs and bats can do without food for four or five months at a time. No matter how soundly I might sleep, I couldn’t go without eating so long as that.” “Wait until I finish, and for the present remember this: in every animal life depends on an actual and never-ceasing combustion. Air, as necessary to this combustion as to the burning of wood or coal in our stoves and fireplaces, is taken into the body by breathing. That is what makes breathing so urgent, so incessant. As to the fuel burned, that is furnished by the substance of the animal itself, by the blood made from digested food. Not a finger is lifted by us, not a muscle moved, that does not use up just so much of the fuel furnished by the blood, which itself is made by the food we eat. Walking, running, working, excitement, all forms of exercise or emotion—these literally burn up our blood just as a locomotive burns its coal in dragging behind it the immense weight of a train. That is why activity, hard work, increases our need for food, while rest, idleness, lessens it.