166 



RABIES 



one, full of life, inquisitively rummaging about in 

 all directions, he exhibited the greatest delight, and 

 lavished most charming words upon it." 



Henceforth all uncertainty was at an end, and 

 the way was clear ahead : Pasteur had now to deal 

 with a virus that had a definite period of incubation, 

 and a suitable medium for development. The 

 central nervous system was to the virus of rabies 

 what the test-tube was to the virus of fowl-cholera 

 or anthrax. As he had controlled these diseases, 

 had turned them this way and that, attenuated and 

 intensified them, so he could control rabies. By 

 transmitting it through a series of rabbits, by sub- 

 dural inoculation of each rabbit with a minute 

 quantity of nerve-tissue from the rabbit that had 

 died before it, he was able to intensify the virus, to 

 shorten its period of incubation, to fix it at six days. 

 Thus he obtained a virus of exact strength, a 

 definite standard of virulence, virus fixe : the next 

 rabbit inoculated would have the disease in six 

 days, neither more nor less. 



As he was able to intensify the virus by trans- 

 mission, so he was able to attenuate it by gradual 

 drying of the tissues that contained it. The spinal 

 cord, taken from a rabbit that has died of rabies, 

 slowly loses virulence by simple drying. A cord 

 dried for four days is less virulent than one that 

 has been dried for three, and more virulent than 

 one dried for five. A cord dried for a fortnight has 

 lost all virulence : even a large dose of it will not 

 produce the disease. By this method of drying. 



