PHYSIOLOGICAL PATHOLOGY. 
131 
virulency is undergoing constant change. We believe that for 
one of the species, other things being equal, the virulency is in 
inverse proportion to the number of days of incubation, and that 
generally the proportions of the inoculated virus are as nearly 
equal as possible when a similar mode of inoculation has been 
practiced. In young animals the duration of incubation is usually 
somewhat less than in adults. 
As we entirely ignore the conditions that the rabid virus of 
the dog would exhibit in the human organism after successive 
passages from man to man, we have been obliged to experiment 
with rabies passing from monkey to monkey. At a later date 
I hope to lay before you the results of this study, which are very 
interesting though incomplete. 
I have already said that I have in my laboratory several dogs 
which are refractory to rabies, by all modes of inoculation. I 
may add, to-day, that they are also refractory to all descriptions 
of rabid virus. At the time of my last report upon rabies, how¬ 
ever, we were obliged, on account of the imperfect condition of 
our observations at that time, to leave unanswered the question 
whether these dogs were naturally refractory to rabies, or only by 
some peculiar conditions resulting from the operations to which 
they had been previously subjected. 
I now believe we can give a more accurate answer to this 
question, though it must still be with some reserve. I believe my¬ 
self authorized to say that our dogs were not refractory to rabies 
be their natural constitution. We have, indeed, found the quite 
practicable way to obtain refractory dogs in as large number as one 
can wish. Still, in consideration of the possibly great duration of 
rabies, which at times throws some doubt upon the experiments 
of control, I beg the Academy to accept for some time this asser¬ 
tion, and allow me to simply state at present that the refractory 
condition is obtained by a system of inoculation of virus of 
various degrees. We have at present twenty-three dogs which 
may without danger receive any virulent inoculation. 
To be able to render dogs refractory to rabies would not only 
be a solution of the question of the prophylaxy of this affection 
in that animal, but also in man, as man never contracts rabies ex- 
