Various facts about the functioning of the intestine which have seemed to many people mysterious are easily explained on the basis of what we have next to talk about, which is the presence both in the small and in the large intestine of flourishing colonies of bacteria. It has perhaps not occurred to most of us that we are in the position of involuntary hosts to enormous numbers of these microscopic plants, but that is exactly what we are. There is no possibility of preventing this, since the combination of warmth, moisture, and abundant food, which is afforded within the intestine, is the most favorable possible situation for many kinds of bacteria, and no effort on our part would enable us to be free from them. Fortunately, in the ordinary course of events, we are not affected one way or the other by their presence. In the small intestine their chief diet consists of the sugars which are produced as the result of starch digestion or are eaten directly. In connection with nourishing themselves from these sugars, they bring about what is known as alcoholic fermentation; that is, some of the sugar is converted into alcohol with a by-product of carbon dioxide. Exactly the same thing happens when yeast develops in a mass of dough; in the latter case the bubbles of carbon dioxide are what we are after, since they are what makes the bread light, the alcohol being driven off by the heat of cooking. In the alcoholic fermentation that goes on in our small intestines the carbon dioxide is a waste product and is passed on out of the body, but the alcohol is absorbed and used as fuel. Little has been said thus far about the physiological effects of alcohol. At this point all we care to say is that alcohol can be oxidized by the body tissues with the liberation of energy and so is a perfectly good fuel food. In the minute amounts in which it is absorbed from the small intestine it is utilized completely and there is not the slightest reason to suppose that it has any ill effects whatever. The objections to the use of alcohol, which have led to its abolition as a beverage in this country, depend on certain definite pernicious drug effects which it shows when consumed in any but the smallest amounts, and which destroy the usefulness of what would otherwise be a valuable article of human diet. Fermentation within the small intestine ordinarily goes on entirely unperceived by us. Certain foods, or the invasion of the intestinal tract by certain species of bacteria, may change the fermentation in such a way that irritating substances are produced which cause the movements of the small intestine to be very greatly increased; its contents are swept on through in very much less than the usual time and we have the condition known as diarrhea.