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VirrMRI X-\R^' PATHOLOGY. 



1. The Exhaustion Theory.— This theory was championed by 

 Tasteur. who proposed it about ISSO. It is based upon the 

 supposition tliat there are certain substances in the animal 1)ody 

 that are fond for micro-parasites and that these substances are 

 not reoenerated. Hence when they have l)een consumed the 

 micro-parasites cease to develop and the animal becomes im- 

 mune. ■] Iii.s tlieory is not tenable because immunity can be 

 produced bv bacterial products and by dead bacteria neither of 



which consume sul>stances from tlie tissues of an animal immun- 

 ized. 



2. The Retention Theory. — In the study of bacteriology it 

 has been found tliat Ijacteria, like most other organisms, can 

 not develop in the presence of a large quantitv of their own 

 excrements. This thcor)- presu]iposes that bacterial products 

 remain in a body after it has been infected and that these 

 products prevent the future development of like bacteria. This 

 theorv does not explain the production of an immunity with 

 toxines and is not sujiported l)v an^' scientists at the present 

 time. The theor^• was proposed by Chauveau. 



3. The Phagocytosis Thcorx. — ^This theory was proposed 

 independently bv Sternberg and Metchnikoff about lf>.Sl. The 

 theorv was the outgrowth of the experimental study of the 

 action of leucocytes upon bacteria and 3feast, in which it was 



