EXPERIMENTS ON ANIMALS. 
103 
transmitted through cows before it could be introduced without 
danger into the system of man. This led Pasteur to think that 
the rabies virus might be attenuated by passing it through the 
bodies of other animals. Many attempts were made, but in the 
majority of cases, instead of decreasing, the poison gained in viru¬ 
lence. Fortunately, however, in the monkey an animal was 
found which possessed the peculiarity of attenuating the energy 
of this virus. Successive transmissions from monkey to monkey 
produced a virus with a gradually increasing period of incubation 
on again being transmitted to other animals, though if continued 
again through a series of animals more susceptible than monkeys 
to the virus, the original virulence was regained. The application 
of these facts yielded a method of vaccinating as a protection 
against rabies. 'The starting point was one of the rabbits which 
had been inoculated from monkeys to such a degree that hypo¬ 
dermic or intravenous injection did not cause death. The suc¬ 
ceeding protective inoculations with the extract from the brains 
of the rabbits which had been the subjects of successive trans¬ 
missions of infection from rabbit to rabbit, proceeding from the 
first infected. 
To demonstrate the truth of the protection against rabies con¬ 
ferred on dogs by this system of inoculation, Pasteur submitted 
to a commission appointed by the French Minister of Instruction, 
nineteen dogs thus rendered insusceptible to rabies, while the 
commission selected nineteen other trial dogs not thus protected. 
The commission report that in the case of the nineteen trial dogs, 
of six that were bitten, rabies occurred in three; of seven that 
were inoculated in a vein it occurred in five ; and of five that 
were inoculated by trephining it occurred in all, while not a single 
sign of rabies has shown itself in any of the nineteen protected 
dogs, though they were treated in the same manner as the trial 
dogs. The commission are at present engaged in experiments as 
to the insusceptibility to rabies of twenty dogs vaccinated by 
themselves, but their report has not yet been made public. 
Thus we have seen that in clearing up the processes of dis¬ 
ease, in devising means of prevention and in limiting the spread 
of contagious diseases, experiment on animals is absolutely indis- 
