298 PASTEUR: THE HISTORY OF A MIND 



re-survey these results and the conclusions which can be 

 drawn from them. In the case of rabies, where cultures 

 could be made only on the living creature, this method 

 was obligatory. He tried, theref<{re, the inoculation 

 of a rabbit by trepanning and saw that the virus, when 

 thus passed from rabbit to rabbit, was strengthened, 

 and that the duration of the incubation in the end was 

 no more than six days. In the monkey, on the contrary, 

 the virus becomes attenuated. This confirmed the 

 analogies between rabies and the virus diseases. 



But if the spinal cord of an animal that has died of 

 rabies can be considered as a pure culture of the virus, 

 why not try to attenuate the virus by allowing a portion 

 of this cord to become old in contact with pure air, as the 

 virus of anthrax is attenuated by exposing a pure culture 

 to pure air. Thus occurred wholly naturally this great 

 discovery that the spinal cord from a rabid animal, 

 exposed to the action of air, in an atmosphere free from 

 humidity, loses its activity on drying. After 14 days, 

 the virus is harmless in the strongest doses; between the 

 fresh cord and that 14 days old, there is a whole series of 

 degrees of attenuation. "A dog that receives this 

 rabic spinal cord 14 days old, then the following day 

 that 13 days old, then that 12 days old, and so on until 

 the fresh cord is used, does not contract rabies and has 

 become immune to it. Inoculated in the eye or the 

 brain with the strongest virus, it remains healthy. It 

 is, therefore, possible in 15 days to give to an animal 

 immunity against rabies. Now men, bitten by mad 

 dogs, ordinarily do not contract the disease until a month 

 or even more after the bite, and this period of incuba- 

 tion can be utilized for rendering the bitten person 

 immune. 



''Experiments made on dogs bitten and inoculated 

 were successful beyond all hope. One recalls how, with 



