(_b_) The intestines are frequently the seat of localized infections, as various “choleras” and “dysenteries” in men and many animals, anthrax, tuberculosis, Johne’s disease. Here doubtless enter the organisms causing “hemorrhagic septicemias” in many classes of animals, and numerous others. These various organisms must have passed through the stomach and the question at once arises, why did the HCl not destroy them? It must be remembered that the acid is present only during stomach digestion, and that liquids taken on an “empty stomach” pass through rapidly and any organisms present are not subjected to the action of the acid. Also spores generally resist the acid. Other organisms may pass through the stomach within masses of undigested food. The fact that digestion is going on in the stomach of ruminants practically all the time may explain the relative freedom of _adult_ animals of this class from “choleras” and “dysenteries.” MECHANISM OF ENTRANCE OF ORGANISMS. In the preceding chapters statements have been made that “bacteria enter” at various places or they “pass through” different mucous membranes, skin, etc. Strictly speaking such statements are incorrect--bacteria do not “enter” or “pass through” of themselves. It is true that some of the intestinal organisms are motile, but most of the bacteria which are pathogenic are non-motile. Even the motile ones can not make their way against fluids secreted or excreted on free surfaces. Bacteria cannot pass by diffusion through membranes since they are finite particles and not in solution. In the case of penetrating wounds bacteria may be carried mechanically into the tissues, but this is exceptional in most infections. Also after gaining lodgment they may gradually grow through by destroying tissue as they grow, but this is a minor factor. Evidently, there must be some mechanism by which they _are carried_ through. The known mechanisms for this in the body are ameboid cells, especially the phagocytes. It is most probable that these are the chief agents in getting bacteria into the tissues through various free surfaces. The phagocytes engulf bacteria, carry them into the tissues and either destroy them, are destroyed by them, or may disgorge or excrete them free in the tissues or in the blood. DISSEMINATION OF ORGANISMS. Dissemination of organisms within the tissues occurs either through the lymph channels or the bloodvessels or both. If through the lymph vessels only it is usually much more restricted in extent, or much more slowly disseminated, while blood dissemination is characterized by the number of organs involved simultaneously. PATHS OF ELIMINATION OF PATHOGENIC MICRÖORGANISMS.