The science of life : fully illustrated in tone and line and including many diagrams

THE BODY IS A MACHINE

development and mind into an agreement with known physical and chemical laws similar to that which holds good for the primary round of life. But here we will content ourselves by pointing out that these phenomena do not occur unless that fundamental round is sustained. A brain must have food and oxygen or it cannot think, and an embryo, if it is to grow, must have them too. There is no observable lifeprocess which is independent of a supply of physical energy. When that ceases, growth, movement, and all signs of consciousness cease also—so that in studying the physical mechanism of the body, even if we are not studying development and mind themselves, we are studying conditions upon which they rest and from which they are apparently inseparable.

We have spoken in this discussion of Mr. Everyman and Mr. Everymouse, and of the animals that are most nearly related to them. But our conclusions are not by any means limited to mammals. It can be shown that

they have a very much wider application than that. There is hardly a living creature that does not depend in the last analysis upon oxidation for the energy by which it lives. All animals and all green plants are energized in this way. Plants differ from animals in being able to make their own fuel instead of eating it; they can do what human invention has so far failed to do upon a paying scale—they utilize the radiation of the sun directly for making substances on which their engines can run. There are indeed one or two microscopic creatures that have other energy-sources—yeast, for example, can use a chemical reaction that does not involve oxygen. But these are exceptions ; we shall tell of them later. For the present it is enough to remember that all animals (including mice and men) are combustion engines of an intricate and curious kind, which live by oxidizing their food. We will now go on to the lay-out of these engines and some interesting details of their structure and working.