§ XII.] Fowl-cholera. 143 



Individual birds, which had recovered from the disease, 

 were generally but not always found to be no longer susceptible 

 to virulent infection, to be secure against a fresh attack, and 

 on this Pasteur founded the ideas with regard to protective 

 inoculation and his method of employing it which he proceeded 

 to extend to anthrax also. 



When the fluid of a fresh culture in broth was separated by 

 filtration from the Micrococcus, which cannot be done by filtra- 

 tion through paper, for the Bacteria invariably pass with it through 

 the paper, but can be managed by means of filtration through 

 porous earthenware, the fluid was not in a condition to produce 

 the disease in the perfect form, not even when all the consti- 

 tuents contained in solution in 120 grammes of it were injected 

 into the blood of a fowl. But one characteristic symptom of 

 the' disease, the stupor, was produced; the birds were sleepy 

 and as if stupefied after infection, continuing in this state about 

 four hours, and then returning to their normal state of health. 



The observation, if established, shows that in this case a nar- 

 cotic poison separable from the Bacterium was actually disengaged 

 from it, and this is the reason why the fowl-cholera is especially 

 instructive in judging of the effects of parasites of this kind in 

 the production of disease. That the effect of the poison in these 

 experiments was comparatively slight and transitory, is to be 

 explained by the small amount of it in the fluid, and by the fact 

 that like other poisons it is either decomposed in the infected 

 fowl or is withdrawn from it with the normal secretions. The 

 case is different when the poisonous organism itself is present in 

 the fowl, apart from the circumstance that the conditions are 

 then probably more favourable for the formation of the poison. 

 While the poison is being perhaps constantly decomposed within 

 the fowl or is being removed with the normal secretions, it is 

 constantly being produced by the parasite, and what has been 

 removed is replaced ; thus the symptoms of the disease neces- 

 sarily become more permanent and more severe, and ultimately 

 also more complicated. Further complications also arising from 



