Of course all bacteria are not disease-bringing (pathogenic); and it is well for us that it is so, for the air, earth and water teem with bacteria of some sort. Many are quite harmless and are occupied only with getting rid of dead bodies by putrefactive fermentation.
But our present concern is with our invisible foes, and we must now try to find out how our bodies protect themselves against their presence and their poisonings.
We have three chief methods whereby we defend ourselves from our invisible foes, namely, the physical, the vital or protoplasmic and the chemical.
We possess as an outermost line of defence the intact skin and mucous membranes, the horny layer (keratin) of the skin and the mucus-covered layer on the internal surfaces being impenetrable by micro-organisms.
The living colony—the entire animal—is surrounded by armor, the body is armor-plated, the keratin of the skin is the armor-plating. Once a rift occurs in the armor, a crack, a split, a crevice, an abrasion, a cut or a puncture, it matters not which, then the entrance of our foes is a possibility, nay, a probability. These rifts need not, of course, be perceptible to the naked eye, they may be barely discernible under the microscope, but they are large enough to admit bacilli, and that is all that is needed; diminished resistance within the citadel ensures its conquest. The outer surface of the teeth, the enamel, the hardest tissue known, is indeed not able to be directly attacked by bacteria, but they force an entrance just underneath it and undermine it so that it is easily broken in.
Another physical means of defence is wetness; the wet mucous membranes of nose, throat and lungs retain the dust and bacteria which stick to them. Bacteria wetted are bacteria imprisoned; it is only when dry that they can be wafted about on their disease-bringing errands.
But the mucous membranes of the nose, throat and lungs are covered with cilia.
When we mention cilia, we pass to the second or vital means of defence. Cilia are whip-like prolongations of the cells lining the breathing passages, and they are continually lashing the mucus in which they are immersed with its dust towards the mouth and nostrils. In this way the bacteria caught in the mucus are removed from the body, and thus it is that mucus containing disease-germs should be burned and not allowed to dry, and so set free its burden of bacteria. In prolonged bronchitis, these cilia are known to be absent from the bronchial mucous membrane, thus depriving it of a valuable mode of defending itself from microorganic invasion.
The chief vital agents concerned in fighting our invisible foes are the white cells or leucocytes of the blood. These minute living things
