Naturally, we will ask what all these other organs are for if they do not share in our conscious life. Why can we not get along with just those that we use for getting food, for avoiding harm, for play, and for the other activities which they carry on? The answer to this question is found in the fact emphasized above that continuous power development is necessary to continued life. By themselves the muscles, nerves, and sense organs cannot carry on power development; they require the aid of a great many other organs to do this. Just how these other organs work will be described later; at present it will be enough to recall that every muscle, every nerve, and every sense organ is actually made up of a great many of the tiny construction units--the cells about which we were talking a few pages back--and that every one of these cells must be developing power all the time if it is to go on living. In order to be able to do this they must, every one, be able to oxidize fuel continuously, and this means that they must receive constant supplies, both of the fuel itself and of the oxygen with which it combines. Some system of delivering these materials must be in operation, and in case the materials have to be prepared for use beforehand this must be provided for also. The heart, the lungs, the stomach, and the various other organs that are useful but not conspicuous, are concerned in these necessary jobs. In an automobile factory we have a similar situation; the men that stand at the machines actually make the parts that go into the finished automobiles, but unless other men are hard at work preparing the castings, and bringing them to where the machine operators can get them, not many automobiles will be turned out. So in the body, unless the various organs are carrying on their work of preparing and delivering materials to the muscles, the nerves, and the sense organs, these latter cannot perform their tasks of getting the food for the whole body and of securing the body against harm, nor can they carry on the pleasant, but not absolutely necessary, activities of play and recreation. CHAPTER III THE SOURCES OF FOOD