XIL] Anthrax. 137 



becomes harmless if the destruction by the blood-cells takes 

 place more rapidly and to a greater extent than the growth and 

 multiplication of the Bacillus, the converse being also true. 



If these ideas are correct, and a normally virulent Bacillus 

 is harmless after protective 

 inoculation and in the absence 

 of this is still virulent, we are 

 driven to the further assump- 

 tion that the protective inocu- 

 lation produces this result by 

 giving the blood-cells the 

 power which they did not be- 

 fore possess of absorbing and 



destroying virulent Bacilli. We have no distinct investigations 

 into this point for our guidance, but if we once more accept 

 the views set forth above we must almost necessarily assume 

 that by the actual absorption of less virulent Bacilli the 

 blood-cells of an animal successively acquire the power of 

 absorbing and destroying the more virulent, which would not 

 have been taken up without this preparation. 



The security of an animal from infection by a dangerous 

 parasite which has found its way into its blood, or the suscepti- 

 bility to it, would accordingly depend on the reaction of the 

 blood-cells on the parasite ; and it would be possible to alter 

 their power of reaction by gradually accustoming them as it 

 were to a succession of more and more virulent individuals. 

 In this way we obtain a partial explanation of the effects of 

 protective inoculation with material of increasing virulence. 



But now there is the further question, why are virulent Bacilli 

 scarcely ever taken up by the blood-cells of an unprepared 

 animal, while the attenuated ones are readily absorbed ? Since 



Fig. 19. a blood-cell of a frog in the act of engulphing a rod of Bacillus 

 Anthracis, observed in the living state in a drop of aqueous humour, b the 

 same a few minutes later; the shape of the cell is changed and the bacillus 

 is completely engulphed. After Metschnikoff. Highly magnified. 



