THE EVIDENCE. 71 



we have seen them on the margins of, and in wounds of the 

 mucous membranes, without in any wise interfering with the 

 healing process. 



It would be a great mistake, however, to suppose, from 

 these facts, that disease-causing germs are never present in 

 such situations. They may, at any time, mingle with the 

 harmless varieties, and leaving these, enter the neighboring 

 parts or circulation, and produce their legitimate results, 

 namely, sickness of some specific kind. 



Disease-producing organisms vary very much in their 

 power of gaining entrance into healthy blood or tissue. The 

 bacillus septicus and the cocco-bacterium of gangrene, the 

 most virulent organisms known, are totally unable to enter 

 normal structures. They must enter through some breach 

 in the surface, or await the weakening effect of other disease, 

 to let them in by way of the mucous membranes. But once 

 within the tissues, the strongest must yield to them. Others 

 of a less virulent nature can, of themselves, make the attack 

 successfully without any breach of tissue or weakening effect 

 of other disease. Such are the spirillum of relapsing fever, 

 the organism of diphtheria, and undoubtedly, also, most of 

 those of the contagious diseases. Still others may gain ad- 

 mission without a breach of continuity, but much more 

 readily enter through some such breach, as, for instance, the 

 bacillus of erysipelas. 



It will thus be seen that a man, perfect in all his parts, may 

 go among some of the more virulent of these organisms with- 

 out fear. He may enter the dissecting room and work for 

 weeks, and even years, among the septic germs, dipping his 

 hands freely into the tissues and fluids, so long as his skin is 

 perfect. But a slight scratch upon a finger may let in the 

 septic germs that will speedily cause his death. 



