All those bacteria which are found in the intestines are, we may say, innocuous; they assist digestion and do no harm. But among such are often virulent bacilli against which, under normal circumstances, we are well guarded, as the epithelium of the intestines is so wonderfully arranged that so long as it is in a healthy condition it does not admit the passage of these microbes; but in the aged, or in those exhausted by debauchery or previous disease, or when there is an inflamed condition of the intestines, stagnation of hard masses of fæces for a very long time cause mechanical lesions of the epithelium when, conditions now having changed, nothing will prevent these microbes from entering the walls of the intestines and either cause disease, like typhoid or tuberculosis, or from passing through and entering into the blood. Besides these dangerous bacteria many other harmful substances pass from the stomach down into the intestines, whence they are taken up by the portal vein and brought to the liver. When the latter is in good condition so much the better for us, but when they arrive in too large quantities, or when the liver is more or less degenerated, as in old people, drunkards, gourmands, etc., then trouble arises. When the number of bacteria in the intestines is much greater than usual, certain dangers arise from such a condition, as thereby the immigration of bacilli into the bile-duct is facilitated causing inflammation of the gall-ducts and gall-bladder, and subsequently gall-stone disease. Further consequences of such a condition may be the closure of the bile-duct, and then no bile can reach the intestines. The presence of bile, however, is very important, for, according to current opinion, this exerts an influence on the checking of putrefaction in the intestines. Bile is a natural antiseptic of great efficacy, and has also a stimulating effect on the nerves of the intestines, promoting their peristaltic movements.