But interesting and instructive though the operation of the locomotive may be, it is not for its own sake that I have mentioned it. It is rather in order to point out a remarkable parallel between its operation and that of a human body. A parallel, indeed, between the operation of a complex inanimate engine of iron and steel, and a still more complex living engine of flesh and bone and blood; both obeying the law of the conservation of energy, as well as the other laws of physics and chemistry. Consider now a human body as a living engine. That man is more than matter is, of course, conceded. But we here regard only the animal body, guided by the brain as its engineer. The day begins, as with the locomotive, by taking a store of fuel and water, namely, food and drink. Food is not, however, burned in the body in a confined receptacle, like coal in the fire box of an engine, but is digested, assimilated and distributed through the body by means of the circulating blood. And while some of it goes to repair bodily waste, becoming tissue, other portions are oxidized or burned to produce heat. Non-digestible parts of the food pass away from the body as refuse, like ashes from the fire box of the engine. That the body fat and muscular tissue are also burned, producing heat, is literally true. A hibernating animal keeps his body warm all winter by burning up his autumnal store of body fat. Even a well-fed body is constantly wearing away, or burning away, and hence requires constant repair. Thus we see two distinct functions for food, which should be carefully distinguished. In the first place, as already indicated, food repairs waste and builds up the body. It makes blood, bone and muscular tissue. Herein we see a departure from the parallel with the steam engine. A locomotive is a machine which runs in a way determined by its builder. But it cannot grow nor repair wear and tear. It requires a whole machine shop plus skilful mechanics to do that. The body, on the other hand, not only runs like a complex mechanism when supplied with energy, but also builds itself up and repairs waste. We express this by saying that it possesses vital force or life, but in just what vital force consists is a matter of speculation and controversy. The raw material which is employed in this work of repairing and building up is found in the food. But not all food can be so utilized. Only those materials which contain nitrogen, the so-called proteids, as lean meat, the casein of milk and gluten of wheat, can be made use of in this most important work of growth and repair.