LOUIS PASTEUR 309 



It is possible, nevertheless, to gauge with sufficient accuracy the 

 degree of intensity of our virus by means of the time of incubation, 

 on condition that v^^e make use exclusively of the intra-cranial mode 

 of inoculation ; and secondly, that we do away with one of the great 

 disturbing influences inherent to the results of inoculation made by 

 bites, under the skin or in the veins, by injecting the right proportion 

 of material. 



The duration of incubation, as a matter of fact, may depend largely 

 on the quantity of efficient virus — that is to say, on the quantity of 

 virus which reaches the nervous system without diminution or modifi- 

 cation. Although the quantity of virus capable of giving rabies may 

 be, so to speak, infinitely small, as seen in the common fact of the 

 disease developing itself after rabid bites which, as a rule, introduce 

 into the system a barely appreciable weight of virus, it is easy to 

 double the length of incubation by simply changing the proportion 

 of those very small quantities of inoculated matter. I may quote 

 the following examples ii — 



On May 10, 1882, we injected into the popliteal vein of a dog ten 

 drops of a liquid prepared by crushing a portion of the bulb of a 

 dog, which had died of ordinary canine madness, in three or four 

 times its volume of sterilised broth. 



Into a second dog we injected %ooth of that quantity, into a 

 third %ooth. Rabies showed itself In the first dog on the eighteenth 

 day after the injection, on the thirty-fifth day in the second dog, 

 whilst the third one did not take the disease at all, which means that, 

 for the last animal, with the particular mode of Inoculation employed, 

 the quantity of virus Injected was not sufficient to give rabies. And 

 yet that dog, like all dogs, was susceptible of taking the disease, for 

 it actually took it twenty-two days after a second inoculation, per- 

 formed on September 3, 1882. 



I now take another example bearing on rabbits, and by a different 

 mode of inoculation. This time, after trephining, the bulb of a 

 rabbit w^hich had died of rabies after inoculation of an extremely 

 powerful virus is triturated and mixed with two or three times its 

 volume of sterilised broth. The mixture is allowed to stand a little, 

 and then two drops of the supernatant liquid are injected after trephin- 

 ing into a first rabbit, Into a second rabbit one-fourth of that quantity, 

 and in succession Into other rabbits, Meth, %4th, H28th, and yi52nd 



