=Alimentary Canal.=--Probably more microörganisms enter the alimentary canal than gain access in any other way, these coming both from food and drink as well as air. Once within its confines, few of them are capable of prolonged existence. Welch states that the meconium of newborn infants is sterile, but that within twenty-four hours it usually contains abundant bacteria. That bacterial infection through this passage-way is a fertile source of non-surgical lesions is well known. The possibility of surgical infections being produced in the same way is both more remote and less demonstrable. Naturally, anaërobic organisms find here more favorable conditions, and even extremely acid or extremely alkaline conditions do not serve to destroy all such life. Pyogenic cocci are often present and are frequently found in peritoneal exudates. In the intestines of herbivorous animals the tetanus bacilli and those of malignant edema are regularly found. The fungus of actinomycosis also finds its way into the bowel along with ingested food. Under ordinary conditions the bile in its natural reservoirs is free from bacteria, but the colon bacilli and pyogenic cocci often invade these precincts. =Genito-urinary Tract.=--Even the healthy urethra may contain bacteria. While these may wander upward to an indefinite extent, it is believed that the urine contained within the bladder in a condition of perfect health is free from bacteria, and that if such gain entrance they do not long remain. The same is true of the female bladder and urethra. The vagina contains organisms of many species, some of which do not grow on ordinary culture media, but are to be recognized by the microscope. While it is generally acknowledged that the vaginal secretion is, as a rule, possessed of bactericidal properties, there is as yet no satisfactory nor comprehensive explanation of this fact, its normal acidity not being sufficient to account for the fact. =The Milk in the Lacteal Ducts.=--In a condition of perfect health milk secreted from the ideal mammary gland is sterile, but may easily become contaminated upon its exit from the nipple. Conversely, under many favoring conditions organisms may travel into the lacteal ducts from the skin without, and thus contaminate the milk. In all probability the breast corresponds in behavior to other glands whose ducts open upon the surface, and, while such openings invite entrance of bacteria, their migrations do not extend far from the surface unless some of the other conditions already mentioned predispose to further infection or extension. In summarizing the general topic of possible _sources_ and _paths of infection_ bacteria may enter and exert deleterious action: A. _From within the system_; and B. _From without_.