144 PASTEUR AND HYDROPHOBIA III 



dered refractory to subsequent inoculations with 

 strong virus that is, were " protected." 



Thus we note a contrast between the effect obtained 

 by inoculating an animal with a virus weakened by 

 cultivation, and those resulting from using a minute 

 quantity of the virus. The latter proceeding does not 

 result in protection, but the former does. 



The fresh spinal cord of an animal that has died 

 of rabies is apparently full of the rabid virus, and it 

 will, if kept so as to prohibit putrefaction, retain for 

 some days its rabies-producing property. Neverthe- 

 less it gradually, without any putrefactive change, 

 loses, according to Pasteur's observations, its virulence, 

 which finally disappears altogether. So that it is 

 possible to obtain cord of a very low degree of viru- 

 lence, and all intermediate stages leading up to the 

 most active, by the simple process of suspending a 

 series of cords at definite intervals of time in glass 

 jars containing dry air. 



There are thus two ways of bringing the virus of 

 rabies taken from a dog into a condition of diminished 

 activity the one by cultivation in monkeys or some 

 other animal, the other by exposing the spinal cord to 

 dry air whilst preventing it from putrefying. 



It was found by Pasteur that dogs inoculated with 

 the virus weakened by cultivation in monkeys were 

 protected from the effects of subsequent inoculation 

 with strong virus. Hence he proceeded to experiment 

 in the direction so indicated. He inoculated dogs 

 with a very weak virus taken from a rabbit that is, 

 a virus having a long incubation period and at the 



