312 CLASSICS OF MODERN SCIENCE 

 inoculation. With the bulb of one of those rabbits two more were 

 inoculated, of which one took rabies on the fifteenth day, the other 

 on the twenty-third day. 



We may notice, once for all, that when rabies is transferred from 

 one animal to another of a different species, the period of incubation 

 is always very irregular at first in the individuals of the second species 

 if the virus had not yet become fixed in its maximum virulence for 

 the first species. We have just seen an example of that phenomenon, 

 since one of the rabbits had an incubation of fifteen days, the other 

 of twenty-three days, both having received the same virus and all 

 other circumstances remaining apparently the same for them. 



The bulb of the first one of those last rabbits which died was in- 

 jected into two more rabbits, still after trephining. One of them took 

 rabies on the tenth day, the other on the fourteenth day. The bulb of 

 the first one that died was again injected into a couple of new rabbits, 

 which developed the disease in ten days and twelve days respectively. 

 A fifth time two new animals were inoculated from the first one that 

 died, and they both took the disease on the eleventh day after inocula- 

 tion : similarly, a sixth passage was made, and gave an incubation of 

 eleven days, twelve days for the seventh passage, ten and eleven for 

 the eighth, ten days for the ninth and tenth passages, nine days for 

 the eleventh, eight and nine days for the twelfth, and so on, with dif- 

 ferences of twenty-four hours at the most, until we got to the twenty- 

 first passage, when rabies declared itself in eight days, and subse- 

 quently to that always in eight days up to the fiftieth passage, which 

 was only effected a few days ago. That long experimental series 

 which is still going on was begun on November 15, 1882, and will be 

 kept up for the purpose of preserving in our rabies virus that maxi- 

 mum virulence which it has come to now for some considerable time, 

 as it is easy to calculate. 



Allow me to call your attention to the ease and safety of the opera- 

 tions for trephining and then inoculating the virus. Throughout the 

 last twenty months we have been able without a single interruption in 

 the course of the series to carry the one initial virus through a succes- 

 sion of rabbits which were all trephined and inoculated every twelfth 

 day or so. 



Guinea-pigs reach more rapidly the maximum virulence of which 

 they are susceptible. The period of incubation is in them also variable 



