er rabbits caused their death with rabies. He found also that
saliva from rabid dogs almost always caused the disease. The incubation
period varied within wide limits, and very often the animals lived. He
then used the blood of rabid dogs for inoculation, but these blood
inoculations always failed to produce the disease. Pasteur was convinced
after careful study of rabid animals during the many months necessary to
complete his experiments, that rabies was a disease of the nervous system,
and that the poison (virus) was transmitted from the wound to the brain by
the way of the nerve trunks. Then to prove his theory Pasteur removed a
portion of the brain of a dog that had died of rabies. A part of this was
rubbed up in sterile water and used to inoculate other animals; and
subcutaneous inoculations with this material almost always produced death.

After this Pasteur tried a new method and injected directly into the
nervous system, either into the nerve trunk or directly into the brain,
after trephining, and all such injections produced rabies in the injected
animal and death. He also found that rabbits inoculated in the brain
always died in the same length of time. When he injected into the nerve
trunk the inoculation period was longer, depending upon the distance from
the brain. Two problems now remained for Pasteur to solve, and these were,
how could he obtain the definite virulence and how could he reduce the
virulence regularly and gradually, so that it could be used by inoculation
safely as a vaccine to produce immunity to rabies in healthy animals, and
also to prevent the development of rabies in animals bitten by rabid
animals. He first tried successive inoculations. These inoculations were
made, after trephining, directly to the brain, and he used a portion of
the brain as a virus each time. He inoculated rabbit number one with a
portion of brain taken from a rabid dog, and this rabbit died on the
fifteenth day. He then inoculated rabbit number two with a portion of the
brain of rabbit number one; from the brain of rabbit number two the virus
was supplied for inoculating rabbit number three, and thus the brain of
each inoculated rabbit was taken, after its death, for material to
inoculate the next rabbit in the series. This experimentation showed him
that each rabbit in the series died a little sooner, showing that the
virus was becoming more virulent, till no increase in activity of the
poison was shown after the f