55. And now you ought to be able to understand how it is that we live on the food we eat.

Food, inasmuch as it can be burnt, is a source of power. In burning it gives forth heat, and heat is power. If we so pleased, we might burn in a furnace the things which we eat as food, and with them drive a locomotive or work a mill; if we so pleased, we might convert them into gunpowder, and with them fire cannon or blast rocks. Instead of doing so, we burn them in our own bodies, and use their power in ourselves.

Food passing into the alimentary canal is there digested; the nourishing food-stuffs are with very little change dissolved out from the innutritious refuse; they pass into and become part and parcel of the blood.

The blood, driven by the unresting stroke of the heart’s pump, courses throughout the whole body, and in the narrow capillaries bathes every smallest bit of almost every part. Kept continually rich in combustible material by frequent supplies of food, the blood as well at every round sucks up oxygen from the air of the lungs; and thus arterial blood is ever carrying to all parts of the body, to muscle, brain, bone, nerve, skin, and gland, stuff to burn and oxygen to burn it with.

Everywhere oxidation, burning, is going on, in some spots or at some times fiercely, in other spots or at other times faintly, changing the arterial blood rich in oxygen to venous blood poor in oxygen. From most places where oxidation is going on, the venous blood goes away hotter than the arterial which came; and all the hot blood mingling together and rushing over the whole body keeps the whole body warm. Sweeping as it continually does through innumerable little furnaces, the blood must needs be warm. This is why we are warm. But from some places, as from the skin, the venous blood goes away cooler than the arterial which came, because while journeying through the capillaries of the skin it has given up much of its heat to whatever is touching the skin, and has also lost much heat in turning liquid perspiration into vapour. This is why so long as we are in health we never get hotter than a certain degree of temperature, the so-called blood-heat, 98° Fahr., and why we make warm the clothes which we wear and the bed in which we sleep.

Everywhere oxidation is going on, oxidation either of the blood itself or of the structures which it bathes, and whose losses it has to make good. Everywhere change is going on. Little by little, bit by bit, every part of the body, here quickly, there slowly, is continually mouldering away and as continually being made anew by the blood. Made anew according to its own nature. Though it is the same blood which is rushing through all the capillaries, it makes different things in different parts. In the muscle it makes muscle; in the nerve, nerve; in the bone, bone; in the glands, juice. Though it is the same blood, it gives different qualities to different parts: out of it one gland makes saliva, another gastric juice: out of it the bone gets strength, the brain power to feel, the muscle power to contract.

When the biceps muscle contracts and raises the arm, it does work. The power to do that work, the muscle got from the blood, and the blood from the food. All the work of which we are capable comes, then, from our food, from the oxidation of our food, just as the power of the steam-engine comes from the oxidation of its fuel. But you know that in the steam-engine only a very small part of the power, or energy, as it is called, of the fuel goes to move the wheel. By far the greater part is lost in heat. So it is with our bodies: all the force we can exert with our bodies is but a small part of the power of our food; all the rest goes to keep us warm.

Visiting all parts of the body, rebuilding and refreshing every spot it touches, the blood current also carries away from each organ the waste matters of which that organ has no longer any use. Just as each part or organ has different properties and different work, so also is the waste of each not exactly the same, though all are alike inasmuch as they are all the results of oxidation. The waste of the muscle is not exactly the same as the waste of the brain or of the liver. Possibly the waste things which the blood bears from one organ may be useful to another, and so be made to do double work, just as the tar which the gasworks throw away makes the fortune of the colour manufacturer.

Be this as it may, the waste products of all parts, travelling hither and thither in the body, come at last to be brought down to very simple things, with all their virtue gone out of them, with all, or all but all, their power of burning lost, fit for nothing but to be cast away, come at last to be urea or ammonia, carbonic acid, and salts. In this shape, the food, after a longer or shorter sojourn in the body, having done its work, having built up this or that part, having helped the muscle to contract or the liver to secrete, having by its burning given rise to work or to heat, goes back powerless to the earth and air from which it came. And so the tale is told.