248 MICRO-ORGANISMS AND DISEASE. [CHAP. 



of animals there can be no reasonable doubt ; it is a fact 

 with which physiological chemistry is quite familiar. 



We arrive then, after all this, at the conclusion that owing 

 to the presence in the blood and tissues of particular chemical 

 substances, present only during life, and a result of the life of 

 the tissue, the organisms in a particular case cannot thrive and 

 produce the disease. And further, that for each particular 

 species of organism there is a particular chemical substance 

 required to exert this inhibitory power, for, as we have seen, 

 while the anthrax-bacillus is not capable of thriving in the 

 pig, it does well in the guinea-pig, while the bacillus of swine- 

 plague thrives well in the pig, it does not in the guinea-pig. 

 The incapability of non-pathogenic organisms to thrive in 

 healthy living tissues would on this theory be explained 

 by the assumption that these chemical substances present in 

 every healthy living tissue are inimical to all putrefactive 

 organisms. 



What we have said hitherto refers only to the healthy 

 living tissues. It is quite possible to imagine that owing to 

 the presence of a particular chemical substance in the healthy 

 living tissue, a pathogenic organism is not able to thrive in 

 a particular animal ; but under certain abnormal conditions, 

 when for instance owing to a diseased or altered state of the 

 tissue that substance is absent, the organism might be enabled 

 to exist and multiply and to produce the disease. Supposing 

 it to be true that a person so long as he is healthy and well 

 is able to successfully withstand the growth of a particular 

 pathogenic organism, he is then unsusceptible to the disease ; 

 but we can understand that if the alimentary canal or the 

 lungs be diseased, then the organism passing into the bowels 

 by ingestion or into the lungs by inspiration would find there 

 a tissue in which the necessary inhibitory substance, present 

 in the healthy state, might be absent, and the organism 



