38 STUDIES IN ADVANCED PHYSIOLOGY. 



and pure, while the bacteria which are found in such great 

 numbers in the soil act as a biological filter, destroying all 

 the impurities so that the water at a reasonable depth is 

 entirely free from organic particles. The very plants are 

 dependent upon the bacteria of the soil for their nitrog- 

 enous food. Botanists assure us that were it not for the 

 nitrification caused by the bacteria in the ground the entire 

 plant world would be reduced to a stunted growth. The 

 very processes of digestion in the human body are helped 

 along by bacteria which live normally in the human intes- 

 tine, and some of the most extensive commercial enterprises 

 are dependent upon fermentations brought about by the aid 

 of these small organisms. 



HOW GERMS PRODUCE DISEASE. 



The question naturally arises, just in what manner these 

 bacteria produce their respective diseases in the body. Is 

 it due to the depredations of the bacteria themselves, or 

 some product which they produce while in the body? No 

 one will question the fact that much harm results from the 

 actual destruction of tissues literally eaten up by these 

 germs. The gradual disintegration of the consumptive 

 lung is more than sufficient evidence, but it is now gener- 

 ally conceded that the injurious effects of these bacteria are 

 due to poisons which they elaborate in the body rather 

 than to their own immediate presence. These poisons 

 directly attack the tissues, and to their effect are due the 

 symptoms of the disease. It has been possible to extract 

 these poisons from animals afflicted with certain diseases 

 and then to inject this poison, free from the bacteria which 

 produced it, into healthy animals, there to reproduce all the 

 symptoms of the disease in question. It is probable that 

 these poisons are similar to the alkaloids of the organic 

 chemist. 



THEORIES OF IMMUNITY. 



The explanation of the fact that the first attack of many 

 infectious diseases renders the person immune from a 



