Tuberculosis. 467 



slaughter, and that no such experimental evidence has been had, 

 or can be had, of any of the deadly diseases of man. Infection 

 by exposure and accidental inoculations can be had in abundance, 

 just as they can in tuberculosis, but never under the rigid pre- 

 cautions which would exclude the possibility of extraneous infec- 

 tion. 



The subject has assumed such importance that I may be excused 

 for introducing a portion of my paper read before the New York 

 State Medical Society in 1900. 



I . This Variability is Common to Microbes Generally. Certain 

 bacilli, like those of anthrax, grow in the living body as rods 

 only, but become long filaments in given artificial media. They 

 produce no spores in the living tissue, but do .so readily in the 

 carcass or soil. Transferred from ox to ox they are generally 

 fatal, but if grown for several generations in Guinea-pigs, and 

 then transferred to cattle, the resulting disease is slight (Burdon- 

 Sanderson, Duguid, Greenfield). Rabies passed from dog to 

 dog is almost constantly fatal, but if passed through the ape and 

 then back to the dog it is comparatively harmless (Pasteur). In 

 both these cases the inoculated animals become immune from the 

 more virulent germs, showing that they have passed through the 

 actual disease in an unusually mild form. The later system of 

 Pasteur is founded on this same general truth, as are also the 

 methods of lessening the pathogenesis of germs by subjecting 

 them to compressed oxygen, to graduated heating, to an altered 

 chemical condition of the culture medium, to anti.sepsis, etc. For 

 a time such weakened cultures often retain their lessened patho- 

 genesis, even through a succession of cultures in a susceptible 

 animal body, acting as if the germ were indeed a distinct species. 

 But it might well have been considered that a microbe which had 

 changed its aptitudes in a given environment could presumably 

 revert to its original habits under the incentive of a suitable 

 medium. And this is precisely what does take place. Pasteur 

 has shown that the less potent rabic virus becomes more potent 

 when passed several times through the body of a rabbit, and that 

 the weakened anthrax germ acquires greater force when passed 

 through a series of small birds or newly-born mammals. 



To come to tuberculosis, Trudeau tells us that a culture of ba- 

 cillus tuberculosis from man inoculated on the rabbit, and then 



