EXPERIMENTAL PATHOLOGY. 
181 
not only accepted by science, but even admitted in practice. 
With such a direction for our investigations we can appreciate all 
the interest attached to the researches into the methods of attenu- 
i 
ation applied to new viruses. 
On this occasion I report a step of progress in that direction 
in relation to rabies. 
1. If we pass from the dog to the monkey, and again from 
monkey to monkey, we may observe that the virulency of the 
rabid virus diminished as it passed to each animal successively. 
When the virulency has been diminished by successive transmis¬ 
sions from monkey to monkey, if the virus is again carried back to 
the dog, the rabbit or the guinea pig, it still remains attenuated. 
In other words, the virulency does not return at once to that of 
the dog with street rabies. 
In these conditions the attenuation may be easily brought by 
a small number of passages from one monkey to another to such a 
point that it cannot communicate rabies to the dog by hypodermic 
injection. Inoculation by trephining, so positively reliable for the 
development of rabies, cannot even produce the slightest result, 
though it renders the animal refractory to the disease. 
2. The virulency of rabies virus increases when passing from 
rabbit to rabbit, or from one guinea pig to another. When the 
virulency is increased and fixed at its maximum in the rabbit, it 
passes with its increased power to the dog, and shows itself there 
stronger than that of the dog affected with street rabies. This 
virulency is such in those conditions that the virus which pos¬ 
sesses it when inoculated into the circulatory system of the dog 
gives him with certainty a fatal rabies. 
3. Though the rabid virulency in the passage from rabbit to 
rabbit, or from one guinea pig to anocher, several passages are 
necessary through the bodies of those animals to enable it to re¬ 
cuperate its former condition of strongest virulency after it has 
been first diminished in the monkey. And again, the virulency of 
the dug with street rabies , which, as I have just remarked, is not 
at its maximum strength, requires when carried on the rabbit, 
several passages through individuals of that species, before reach¬ 
ing its maximum strength. 
