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 
