96 THE POPULAR SCIENCE MONTHLY 



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 cov- 

 ered 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 mucu3 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 



