10 PASTEUR AND HYDROPHOBIA III 



a virus of short incubation the virus of the rabbit- 

 will vaccinate, if it does not kill, before the virus from 

 the dog will have produced hydrophobia. If the 

 object had been a prophylaxis against possible future 

 bites, the source of the virus used would have mattered 

 little ; but the purpose was to obtain therapeutic in- 

 oculation. It is for this reason, I think, that M. 

 Pasteur chose the medulla of the rabbit as his source 

 of vaccinal virus. He submitted this medulla (spinal 

 cord) to desiccation, because he had established ex- 

 perimentally that, whilst the fresh medulla inserted 

 beneath the skin kills often, the desiccated medulla 

 does not kill, or kills very rarely. Further, he made 

 out experimentally that the more of the medulla he 

 inserted, the less chance there was of killing and the 

 greater chance of conferring immunity. In conse- 

 quence, M. Pasteur found it necessary to multiply his 

 inoculations, to make several every day and for several 

 days. Guided, no doubt, by prudential reasons of a 

 purely theoretical character, he chose for the first 

 inoculations the oldest medullas, which are no longer 

 virulent, or are very nearly ceasing to be so, and he 

 then passed on to medullas of a little more dangerous 

 character, thinking thus to establish step by step 

 partial immunities, so as to render each subsequent 

 inoculation with a less desiccated medulla quite in- 

 offensive. I imagine that the method of anti-rabic 

 vaccination was thus built up little by little under 

 the influence of theoretical views and experimental 

 conclusions. The practice having been demonstrated 

 good by experience, its formula has remained, and 



