Ill PASTEUR AND HYDROPHOBIA 143 



spinal cord of a rabid animal, Pasteur has found that 

 the virulence of the poison is increased. The incuba- 

 tion period becomes shorter, being at first about fifteen 

 days. After being transmitted from rabbit to rabbit 

 through a series of twenty-five individuals, the period 

 of incubation becomes reduced to eight days, and the 

 virulence of the poison is proportionately increased. 

 After a further transmission through twenty-five 

 individuals, the incubation period is reduced to seven 

 days, and after forty more transmissions Pasteur finds 

 an indication of a further shortening of the incubation 

 period, and a proportionate increase of virulence in the 

 spinal cord of the rabbit extracted after death and 

 used for inoculating other animals. Thus Pasteur 

 found it possible to have at his disposal simultaneously 

 rabid virus of different degrees of activity. 



It is curious that Pasteur found, on the other hand, 

 that the virus from a rabid dog, when transmitted 

 from individual to individual through a series of 

 monkeys, gradually lost its activity, so that after pass- 

 ing through twenty (?) monkeys it became incapable 

 of producing rabies in dogs. Thus a portion of the 

 spinal cord of such a monkey, itself dead of rabies, 

 when pounded in broth and injected beneath the skin 

 of a dog, failed to produce rabies, and even when ap- 

 plied to the dog's brain after trephining failed to pro- 

 duce rabies. 



Pasteur makes the very important statement that 

 the dogs thus treated with the virus which had been 

 weakened by cultivation in monkeys, although they 

 did not develop any symptoms of rabies, were ren- 



