In order for the blood that circulates through the body to pass on to the body cells the materials which they need, it is evident that the blood itself must have some source from which to obtain the materials. Our present task is to examine this source. In Chapter IV we talked about food and its uses; here we are concerned with the way in which the foodstuffs are taken into the body and prepared for use. We know that the material which we take in is part solid and part liquid, giving rise to the familiar distinction between food and drink. For our present purposes this distinction has no importance and will not be made. What we do have to note is that of the complex mixture of materials which makes up any ordinary meal, some of the substances are ready to be taken up at once by the blood and to be distributed around the body; others have to go through a preliminary course of preparation. All of the dietary accessories, so far as we know, pass from the digestive organs into the blood without change. All or nearly all of the energy-yielding foods, on the other hand, must have a preliminary preparation to which we give the name of digestion; the operation of digestion being to break down the complex foodstuffs that are eaten into simpler materials of which the body can make use. Digestion is made up of a series of chemical changes by which the large molecules of the original foodstuffs are broken into smaller and simpler molecules. This breaking down of large molecules into smaller is a very common kind of chemical process. A feature of it is that under ordinary circumstances the breaking down of the large molecule goes on very slowly, but if the right conditions are provided the breaking down proceeds rapidly. The particular condition which is necessary is the presence in the solution where the large molecules are breaking down of something which will hurry up the process. There are a good many kinds of substances which have this ability to hasten molecular decomposition; those that do it in carrying on digestion are given the name of _enzymes_. We do not know just how the enzymes act; we merely know that when they are put along with the substance which is to be decomposed, it goes to pieces very much more rapidly than if none of them was present. The enzymes themselves are not used up in the process, so if time enough is allowed, a very small amount of enzyme can bring about the decomposition of a very large amount of material. In our bodies the whole digestive process consists of a succession of decompositions of complex materials into simpler ones under the speeding-up influence of enzymes. We have a number of different kinds of foodstuffs to be decomposed and a corresponding number of enzymes.