I need not go further into this subject as the facts which I have cited suffice for my present purpose. The complete atrophy of the large intestines in the case of the woman referred to in chap. iv. proves not only that this portion of the alimentary canal is not indispensable to healthy life, but that life may be maintained in the absence of the flora of the large intestines. And this really is the centre of the problem. The useless bacterial flora may give rise to serious or fatal maladies. Wounds of the abdomen are really serious only when they penetrate the large intestines and so allow the entrance of bacteria from that region to the peritoneal cavity. In such an event, the microbes rapidly multiply in the organism and produce a grave and frequently mortal illness. So long as the microbes remain within the intestines very few of them get into the circulation, and with these few the organism is able to cope. While most of the microbes are confined within the walls of the alimentary canal, the soluble excretions produced by them pass through into the lymph and blood. Quite a number of different facts establish this. Thus, for instance, it has been known for long that the urine of human beings and of animals contains a series of substances such as derivatives of phenol, indol, creosol, skatol, and so forth. In certain diseases the amount of these substances greatly increases. The stagnation of the contents of the intestines increases the amount of phenol and indol. Such facts and many others make it probable that these substances are the products of the bacterial flora of the intestines. They are absorbed by the intestinal wall, pass into the general circulation, and may give rise to various symptoms of a more or less serious nature. Baumann, who has done much work on the subject, has brought together a series of arguments supporting the bacterial origin of the presence in the urine of the substances in question. Ewald, working from another point of view has obtained strong confirmation of Baumann’s suggestions. He had the opportunity of making observations on a female patient, in whom, on account of a strangulated hernia, an intestinal fistula was established. Throughout the time during which the large intestines were inactive, the urine contained neither phenol nor indol. But as soon as the fistula was closed and communication with the large intestine had been re-established, phenol and indol reappeared in the excreta. Ewald formed the opinion, therefore, that these substances were products of the large intestine. I need not weary the reader with more of the facts serving to show that the bacterial flora of the large intestines is the source of many poisons harmful to the body. It is among such substances that we must look for the slow poisons which, in the absence of syphilis or alcoholism, produce the arterial sclerosis of old age.