206 LOUIS PASTEUR 



under the anesthetic, the dog behaved in a per- 

 fectly normal manner, but in about two weeks it 

 developed unmistakable symptoms of hydrophobia 

 and died. By similar experiments on dogs and 

 rabbits Pasteur showed that hydrophobia could be 

 produced, in practically every case, by the direct 

 inoculation of the virus into the nervous system. 

 Not only is the transfer of the disease by this 

 method practically certain, but its period of incu- 

 bation is shorter than when it is introduced by 

 means of saliva. Moreover, Pasteur found that in 

 rabbits the incubation period became shorter and 

 shorter with successive inoculations from brain to 

 brain. The virus had apparently been increased in 

 virulence as it is sometimes known to do in other 

 diseases. After a time, however, when the incuba- 

 tion period was shortened to a little less than seven 

 days, it could be reduced no more. The virus had 

 reached its maximum potency; it had become, as 

 it is now called, a fixed virus. Here, at last, the 

 disease, instead of being of uncertain transfer and 

 indefinite incubation period, was made a thing 

 which could be definitely controlled, and whose 

 time of appearance in an inoculated rabbit could 

 be accurately predicted. 



Efforts to cultivate the virus obtained from nerv- 



