SUSCEPTIBILITY AND IMMUNITY. 245 



Again, cultivation within the body of a living animal may, in 

 certain cases, cause a diminution in the virulence of a pathogenic 

 microorganism. Thus Pasteur and Thuiller have shown that the 

 microbe of rouget when inoculated into a rabbit kills the animal, but 

 that its pathogenic virulence is nevertheless so modified that a cul- 

 ture made from the blood of a rabbit killed by it is a suitable " vac- 

 cine " for the pig. 



On the other hand, we have experimental evidence that the viru- 

 lence of attenuated cultures may be reestablished by passing them 

 through the bodies of susceptible animals. Thus a culture of the 

 bacillus of rouget, attenuated by having been passed through the 

 body of a rabbit, is restored to its original virulence by passing it 

 through the bodies of pigeons. And a culture of the anthrax bacillus 

 which will not kill an adult guinea-pig may be fatal to a very young 

 animal of the same species or to a mouse, and the bacillus cultivated 

 from the blood of such an animal will be found to have greatly in- 

 creased virulence. 



In Pasteur's inoculations against anthrax "attenuated" cultures 

 are employed which contain the living pathogenic germ as well as 

 the toxic products developed during its growth. Usually two inocu- 

 lations are made with cultures of different degrees of attenuation 

 that is to say, with cultures in which the toxic products are formed 

 in less amount than in virus of full power. The most attenuated 

 virus is first injected, and after some time the second vaccine, which 

 if injected first might have caused a considerable mortality. The 

 animal is thus protected from the pathogenic action of the most 

 virulent cultures. 



Now, it has been shown by recent experiments that a similar im- 

 munity may result from the injection into a susceptible animal of the 

 toxic products contained in a virulent culture, independently of the 

 living bacteria to which they owe their origin. Chauveau, in 1880, 

 ascertained that if pregnant ewes are protected against anthrax by 

 inoculation with an attenuated virus, their lambs, when born, also 

 give evidence of having acquired an immunity from the disease. As 

 the investigations of Davaine seemed to show that the anthrax 

 bacillus cannot pass through the placenta from the mother to the 

 foetus, the inference seemed justified that the acquired immunity of 

 the latter was due to some soluble substance which could pass the 

 placental barrier. More recent researches by Strauss and Chamber- 

 lain, Malvoz and Jacquet, and others, show that the placenta is not 

 such an impassable barrier for bacteria as was generally believed at 

 the time of Chauveau's experiments, so that these cannot be accepted 

 as establishing the inference referred to. But, as stated, we have 

 more recent experimental evidence which shows that immunity may 



