HYDROPHOBIA. 321 



Having found, then, that the virus could be intensified or 

 modified, and that different animals were affected in a 

 diiferent degree by the same virus, Pasteur set himself to 

 work out the question whether it was possible so to alter the 

 resistance of an individual that a virulent hydrophobic 

 material would have little or no eifect on the tissues. The 

 only way in which this could be done appeared to be by the 

 introduction of an attenuated virus into the animal that was 

 to be rendered immune, as in the case of anthrax, and so to 

 accustom the tissues to the presence of the specific poison, 

 rendering them better able to resist a stronger poison ; he 

 thought, in fact, that by successive inoculations of stronger 

 and stronger poisons he would be able gradually to " accli- 

 matize" the tissues to the presence of even the strongest 

 virus, and so enable them to deal with it successfully, probably 

 by converting it into innocuous proteid materials, so rendering 

 it harmless to the delicate and highly organized cells of the 

 nervous system. In his earlier experiments he obtained an 

 attenuated virus by inoculating a monkey, from which he 

 took material from the central nervous system with which 

 to inoculate a series of rabbits ; each rabbit supplied a 

 slightly more powerful virus with which to inoculate another 

 one of the series. He thus obtained inoculation material of 

 all degrees of virulence. With material from each rabbit 

 in the series he inoculated twenty dogs, each one receiving 

 a stronger and stronger dose each time it was inoculated. 

 Out of the twenty dogs so treated only about three-quarters 

 were protected against virulent hydrophobia ; but this for a 

 first series of experiments was a most extraordinary success, 

 and so satisfied and delighted Pasteur that he was encouraged 

 to continue his research ; eventually the results he obtained 

 were even more remarkable. 



Having observed that the cords of rabbits that had been 

 dead for some time contained a less virulent poison than the 

 cords of freshly-killed animals — this being.especially the case 

 in dry weather — he adopted a method based on this observa- 

 tion, by means of which he was able from the same cord to 

 obtain inoculating materials of very different degrees of 

 virulence, this varying according to the length of time that 

 had intervened between the death of the animal and the use 

 of its cord for protective injection. He proceeded as follows : 

 Having sterilized a glass flask plugged with cotton wool by 



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