Fes., 1893. PASTEUR’S METHOD OF INOCULATION. 101 
found that if the cultivations succeeded each other at intervals not 
greater than twenty-four hours, each successive cultivation retained 
the power of producing disease with as great energy as did the first. 
If, however, a longer interval was allowed to elapse a remarkable 
change took place: the disease was produced in a milder form, and, 
more remarkable still, the animal inoculated with one of the later 
cultivations was protected from the more deadly disease. Moreover, 
by prolonging the period between the successive cultivations the 
power of producing disease became less and less, until it was entirely 
lost. 
In attenuating—as he terms it—the virus of splenic fever, again, 
Pasteur used heat and exposure to the air, while in other cases he has 
used the still more remarkable method of making his successive 
cultivations in the body of some animal. 
This is the method originally pursued with the microbe of hydro- 
phobia. A monkey was inoculated with the virus froma mad dog, and 
the spinal cord of this monkey was afterwards used to inoculate a second, 
which in its turn furnished matter to inoculate a third, and so on; 
and Pasteur found that the matter from the spinal cord of the first 
monkey produced a milder disease than the original virus ; that from 
the second monkey a still milder one, and so on; and by continuing the 
process long enough he could obtain a virus of any degree of mildness 
desired. 
As with the virus attenuated otherwise, inoculation with this 
gave immunity from the severer forms of the disease. Closely 
connected with this is the fact that if the virus, attenuated by 
passing through a series of monkeys, be passed in the same way 
through a series of rabbits, it regains all its former virulence and 
reproduces the disease in its original form; and Pasteur states 
that he is able to revive the power of the attenuated virus of 
splenic fever by passing it through a series of guinea-pigs, beginning 
with one just born and gradually increasing the age; and that of 
fowl-cholera by passing it in like manner through canaries, black- 
birds, &c. 
A later method of attenuating the virus of rabies was the simple 
exposure of the spinal cord of a rabid rabbit to the influence of dry 
air ina flask. At the end of about fifteen days it was found that the 
spinal cord thus exposed had almost entirely lost its virulence. And 
in preventive inoculation a series of such cords was used, beginning 
with the oldest and least virulent, and proceeding to the newest and 
most virulent. 
In the case of swine fever, again, Pasteur attenuated the virus 
by passing it through a series of rabbits. 
Various other methods of attenuation have been used. M. 
Chauveau! has shown that it may be effected by compressed oxygen. 
With a moderate degree of compression he found the growth of the 
1 Comptes Rendus, vol, xcviii., pp. 1232—35. 
