310 CLASSICS OF MODERN SCIENCE 

 of that same quantity. All those rabbits died of rabies, the incuba- 

 tion having been eight days, nine and ten days for the third and 

 fourth, twelve and sixteen days for the last ones. 



Those variations in the length of incubation were not the result of 

 any weakening or diminution of the intrinsic virulence of the virus 

 brought on possibly by its dilution, for the incubation of eight days 

 was at once recovered when the nervous matter of all those rabbits 

 was inoculated into new animals. 



Those examples show that, whenever rabies follows upon bites or 

 hypodermic inoculations, the differences in respect of length of incu- 

 bation must be chiefly ascribed to the variations, at times within 

 considerable limits, of the ever-undeterminate proportions of the 

 inoculated viruses which reach the central nervous system. 



If, therefore, we desire to make use of the length of incubation as 

 a measure of the intensity of the virulence, it will be indispensable to 

 have recourse to inoculation on the surface of the brain, after trephin- 

 ing, a process the action of which is absolutely certain, coupled with 

 the use of a larger quantity of virus than what is strictly sufficient to 

 give rise to rabies. By those means the irregularities in the length 

 of incubation for the same virus tend to disappear completely, because 

 we always have the maximum effect which that virus can produce; 

 that maximum coincides with a minimum length of incubation. 



We have thus, finally, become possessed of a method enabling us 

 to investigate the possible existence of different degrees of virulence, 

 and to compare them with one another. The whole secret of the 

 method, I repeat, consists in inoculating on the brain, after trephin- 

 ing, a quantity of virus which, although small in itself, is still greater 

 than what is simply necessary to reproduce rabies. We thus disen- 

 gage the incubation from all disturbing influences and render its dura- 

 tion dependent exclusively on the activity of the particular virus used, 

 that activity being in each case estimated by the minimum incubation 

 determined by it. 



This method was applied in the first instance to the study of canine 

 madness, and in particular to the question of knowing whether dog- 

 madness was always one and the same, with perhaps the slight varia- 

 tions which might be due to the differences of race in diverse dogs. 



We accordingly got hold of a number of dogs affected with ordi- 

 nary street rabies, at all times of the year, at all seasons of the same 



