+Infection.+ Bacteria gain admission to the living tissues under natural conditions, by penetrating any of the mucous membranes which they can reach, or by entering open wounds. It may be said in general that an intact epidermis is almost a complete protection against infection, and that an intact mucous membrane is a good protection. This difference in vulnerability between the mucous membrane and the skin is important, and is probably due to the cornifaction of the epithelial cells, and to their numerous layers, as well as to the protection afforded by the thick corium. The single layer of soft mucous cells is much more easily penetrated. Typhoid bacilli and other hostile germs have been actually observed in the urine, in the bile, in the intestinal secretions and in the saliva. The bacteria of typhoid fever and tuberculosis have been found in the milk of nursing mothers. The local phenomena of inflammation usually follow the introduction of living bacteria into the tissues, and general symptoms of poisoning follow later, when the bacteria, toxins, or ptomains, have entered the circulation. Some bacteria, however, excite no local reaction, but enter the circulation at once. The pyogenic variety, it should be noted, cause the production of pus. +Elimination.+ Bacteria can be eliminated from the blood in several ways; the kidneys, however, are the organs which carry the burden of most frequently relieving the body of them. Even the sweat glands are supposed to eliminate both bacterial toxins and bacteria. +Resistance Offered by Tissues.+ The tissues have considerable power of resistance under ordinary circumstances, although the exact sources of this power are not well under stood. _Phagocytosis_—the power of destruction and removal of bacteria supposedly possessed by the leucocytes emigrating from the blood vessels—explains it in part. It is also accounted for by the germicidal properties of the blood serum. The resistance of the tissues may in some cases be due to the absence from them of some particular element necessary to the growth of a particular microorganism. This refractoriness varies in every species of animal in its relation to every form of germ. Different individuals of one species also vary in their susceptibility, and even different parts of the body vary in the same individual. The lower animals offer a greater resistance to pyogenic bacteria than do human beings. Any cause that lowers the vitality by depressing the system, reduces the resistance to bacteria and is therefore apt to favor their growth. Exhausting diseases such as anemia, obesity, alcoholism, diabetes, fatigue, or even exposure to cold, are instances. Germ growth is also favored by the presence of dead, or injured tissues, of blood clots, of foreign substances, and above all, by the presence of some of the substances in which the germ has already been growing at the time of its inoculation, and containing some of its toxins.