inoculated with a virus of full
strength can be protected by subsequent inoculations of attenuated virus
repeated in doses of increasing strength.

Innumerable attempts have been made to discover the causative agent, and
investigators have announced the finding of many of the lower forms of
animal and vegetable life as the pathogenic factor. Among the recently
described causes, certain protozoanlike bodies found in the ganglionic
cells in 1903 by Negri, and termed Negri bodies, are of a very
suggestive nature. Negri claims that these bodies are not only specific
for rabies, but that they are protozoa and the cause of the disease. His
work has been corroborated by investigators in all parts of the
scientific world. An examination of the vitality of these bodies will
show a striking resemblance to the vitality of an emulsion of the
virulent tissue. Thus, they have been found to be quite resistant to
external agencies, such as putrefaction, drying, etc., and are about the
last portion of the nerve cell to survive the advance of decomposition.
They are also found in more than 96 per cent of the cases of rabies
examined, but have not been proved to exist in other diseases.

Valenti states, as his strongest evidence of the protozoan nature of the
bodies, that the virus of rabies is neutralized in test tubes by quinin,
while no other alkaloid has this property. As a result of the work
performed in the New York City Board of Health laboratory, Park claims
that Negri bodies are found in animals before the beginning of visible
symptoms, and evidence is given that they may be found early enough to
account for the infectiousness of the central nervous system. These
bodies are now almost universally considered as diagnostic of rabies,
and in the pathological laboratory of the Bureau of Animal Industry
their detection in the nerve cells of the brain suffices for a diagnosis
of rabies without animal inoculations. In case these granular bodies are
not found in a suspected animal, the plexiform ganglion is next
examined, and should negative results still be obtained, the inoculation
of rabbits is then made as a last resort. It is indeed rare that
positive results are obtained from the latter method after the first two
methods have been negative, but it has occurred occasionally in cases in
which the animal had been killed in the early stages of the disease.

_Symptoms._--From the moment of inoculation by the bite of a rabid dog
or oth