286 Chapter IX 



immunity against micro-organisms. It is impossible to deny that 

 the superactivity of the phagocytes which is always found in this 

 immunity, although readily observed, cannot be demonstrated in a 

 rigorous fashion outside the fluids which bathe the cells. There are, 

 however, very important analogies which may be invoked in favour 

 of this thesis. We have already cited in our fifth chapter Delezenne's 

 experiments on the digestion of gelatine by the leucocytes of the 

 dog, which show in the most demonstrative fashion that these cells 

 accustom themselves to bring about this digestion more and more 

 quickly and this quite independently of any humoral influence. 



For some time past there has been no doubt as to the funda- 

 mental fact that the phagocytes in immunised animals seize and 

 destroy living micro-organisms. Several attempts have been made 

 to show that such destruction of these bacteria takes place solely 

 by the body fluids, and that the phagocytes intervene only as 

 "scavengers^' to carry ofi" the dead bodies of the micro-organisms. 

 The numerous observations, described in the preceding chapter, 

 absolve us from again entering into a discussion of this question. 

 Moreover, the majority of these opponents now recognise that micro- 

 organisms are ingested in a living state by the phagocytes of immu- 

 nised animals. Some, however, have expressed the opinion that 

 these living micro-organisms, before becoming the prey of the phago- 

 cytes, must undergo some preliminary attenuation of virulence 

 through the action of the body fluids. Hence the theory of the 

 attenuating power of the fluids of the body, maintained especially by 

 Bouchard and his pupils. During the course of our exposition of the 

 facts concerning acquired immunity, we have several times had 

 occasion to speak of the virulence of micro-organisms in the im- 

 munised animal. Here, therefore, we may confine ourselves to a brief 

 summary of the observations collected on this point. 



Having observed tliat the anthrax bacillus, when developed in the 

 blood of immunised sheep, was incapable of giving fatal anthrax to 

 [301] rabbits, I expressed^ the opinion that under these conditions its 

 virulence had become attenuated. Later, analogous changes were 

 shown by Charrin- in the Bacillus pyocyaneus when cultivated in the 

 serum of immunised animals. Bouchard^, generalising on these data, 

 arrived at the following theory of vaccination. " The inoculation of 



^ A7m. de VInst Pasteur, Paris, 1887, 1. 1, p. 42. 



^ Compt rend. Soc. de bioL, Paris, 1889-1891. 



3 " Essai d'une theorie de rinfection." Berlin, 1890. 



