LOUIS PASTEUR. 249 



This was a most important step forward, especially as 

 Salmon and Smith, two American investigators, were after- 

 wards able to show that in the case of hog cholera the 

 toxins derived from the specific bacillus associated with that 

 disease were, when injected subcutaneously in minute doses 

 or in an attenuated condition, capable of conferring an 

 immunity against the attacks of the micro-organism. We 

 have in these observations the foundation of the " antitoxin " 

 treatment of diphtheria and tetanus, although the full details 

 of the method could not at that time be worked out or even 

 foreseen. 



Working at the question of vaccination against swine 

 fever, Pasteur found that when the swine fever organism is 

 passed through several generations of pigeons it gradually 

 becomes more virulent not only lor the pigeon but also for 

 the pig. When, on the other hand, the virus is inoculated 

 from rabbit to rabbit, these animals die undoubtedly, but the 

 blood inoculated into a pig does not set up an acute illness in 

 that animal ; it recovers and is proof against a fresh attack 

 of swine fever. It must be noted however that, whether 

 this organism is cultivated in the pigeon when it is most viru- 

 lent, in the rabbit when it is least so, or in the pig when it is 

 as it were in an intermediate condition, there comes a stage 

 in the development of the organism, when, if kept to the one 

 species, its virulence remains stationary — it is in fact a virus 

 JixS, and will so remain until transferred to another species. 

 This most important observation was destined to bear 

 fruit in the great work on hydrophobia to which Pasteur 

 devoted the last years of his life. Difficulty after difficulty 

 was overcome. Inoculation with saliva failing, through 

 the production of mixed infections, and Galtier having failed 

 to produce rabies by the inoculation of blood from rabid 

 animals, inoculation with cerebro-spinal fluid and por- 

 tions of the central nervous system was tried ; this time 

 the inoculations were attended with success. Then 

 instead of making his inoculation under the skin merely, 

 Pasteur introduced particles of the brain or cord of an 

 affected animal under the covering of the brain of a 

 healthy animal, with the result that the disease most 



