234 SUSCEPTIBILITY AND IMMUNITY. 



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 

 foatus, 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 we have more recent 

 experimental evidence which shows that immunity may result from 

 the introduction into the bodies of susceptible animals of the toxic 

 substances produced by certain pathogenic bacteria. The first satis- 

 factory experimental evidence of this important fact was obtained by 

 Salmon and Smith in 1886, who succeeded in making pigeons im- 

 mune from the pathogenic effects of cultures of the bacillus of hog 

 cholera by inoculating them with sterilized cultures of this bacillus. 

 In 1888 Roux reported similar results obtained by injecting into sus- 

 ceptible animals sterilized cultures of the anthrax bacillus. As 

 already stated, Behring and Kitasato have quite recently reported 

 their success in establishing immunity against virulent cultures of 

 the bacillus of tetanus and the diphtheria bacillus by inoculating 

 susceptible animals with filtered, germ-free cultures of these patho- 

 genic bacteria. 



