RABIES. 663 



wound rapidly or deeply enough to ensure complete 

 destruction of the virus, Pasteur and others were there- 

 fore led to study the disease experimentally in animals, 

 with the hope of finding some method of immunization 

 or even cure through bacteriological methods; these 

 investigations finally resulted in the discovery of 

 methods of preventive inoculation applicable to man. 



Immunization against rabies may be effected in sev- 

 eral different ways. Pasteur's treatment is based upon 

 the fact that rabic virus may be attenuated or intensified 

 for any animal at will. He first observed that the tis- 

 sues and fluids taken from rabid animals varied con- 

 siderably in their virulence. Then he showed that the 

 virus taken from similar positions say the cerebro- 

 spinal fluid had always the same action in the same 

 species; but that fluid taken from an animal of different 

 species was weaker or stronger as the case might be. 

 Thus the cerebro-spinal fluid of a series of dogs is of 

 constant strength, and inoculations made from dog to dog 

 regularly produce death from rabies, the animals passing 

 through an incubation period fairly constant in length, 

 and through a series of similar symptoms up to death 

 at the same term. If, however, a series of monkeys 

 be inoculated the virus gradually becomes attenuated, 

 and this attenuation becomes more and more marked in 

 successive inoculations until eventually, after the disease 

 has run a longer and longer course in the successive 

 animals, there comes a time at which the virus is no 

 longer sufficiently active to cause death. If this atten- 

 uated fluid be now passed through a series of rabbits, 

 dogs, or guinea-pigs it comes back to such a strength 

 that it will kill, though slowly; then, however, its vir- 

 ulence gradually increases until the original intensity is 



