THE PRESIDENT'S ADDRESS. 
141 
nidus is found, has been checked to a great extent by ascertaining 
what varieties will best resist the disease. Thus by a process of 
natural selection, aided by the efforts of man, the disease may be 
stamped out. Again, the study by Pasteur of the nature of the 
grape disease, viz., the pliyloxera, has lead to the proper means 
being taken to eradicate it. But perhaps the most brilliant 
example is the method of treatment adopted by Pasteur to prevent 
the development of hydrophobia in those who have unfortunately 
been bitten by rabid animals. 
Although up to the present time the microbe of hydrophobia 
has not been isolated, there is, as Pasteur says, as little doubt that 
it exists as that stars exist in the heavens which we have not yet 
seen. The treatment, then, is based on the principle before 
alluded to ; viz., that of attenuating the virus, and so preparing it, 
that when the patient is inoculated with it in successively 
increased doses the hydrophobia itself does not take place. 
Pasteur found, in the course of his experiments, that the saliva 
of the dog does not always give rise to rabies, but that the more 
virulent matter was to be found in the brain and spinal cord. 
When the poison is injected into the blood the spinal marrow 
is the part first attacked, the virus locating itself and multiplying 
there before spreading to other parts. Pasteur has found that the 
virus of rabies, if carried from the dog to the monkey, and 
subsequently from monkey to monkey, grows weaker at each 
passage ; and if carried back to the dog, rabit, or guinea-pig, still 
remains attenuated. But, on the other hand, successive passages 
from rabbit to rabbit, or from guinea-pig to guinea-pig, increase 
the virulence of the virus. 
This exalted virulence comes to a fixed minimum in the rabbit. 
If now transferred to the dog it remains exalted, and shows itself 
to be much more intensely virulent than the virus of ordinary 
street rabies. So great is this acquired virulence that the new 
virus injected into the blood-system of a dog unfailingly gives 
rise to mortal madness. 
A logical application of the results just indicated gives us the 
means of easily rendering dogs refractory to rabies ; for we can 
now prepare and keep at our disposal a series of attenuated 
viruses of different strength — some not mortal, preserving the 
animal economy against the ill effects of more active ones, and 
these latter against the effects of mortal ones. 
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