468 Veterinary Medicine. 



cultivated for two years in vitro, becomes much less destructive 

 to Guinea-pigs, and that after six years of such artificial culture 

 all the Guinea-pigs inoculated with it live for many months, some 

 for two and a half years, and some even recover. The usual life 

 of the Guinea-pig after inoculation is seventeen days.' All of our 

 zymotic diseases have in a similar way cycles of malignancy and 

 benignancy. For a series of years measles, scarlatina, diphtheria, 

 smallpox, or grippe have an unwonted mildness, and, again, one 

 or another merges into a cycle of extreme and fatal malignancy. 

 Rinderpest on the steppes of Asia is comparatively harmless to 

 the native stock, but among outside cattle imported into the steppes 

 or attacked in their native lands it is habitually fatal. Texas 

 fever is mild among the indigenous cattle in the Gulf States, but 

 very deadly to Northern stock. Glanders is not at all fatal to 

 horses of the plains, the Rockies, or the Sierras ; but it becomes 

 redoubtable when these horses carry it to the Eastern seaboard, 

 and still more so in Western Europe. It is a common experience 

 to see a malady transformed through the effects of heredity or 

 acquired immunity, through environment or the temporary miti- 

 gation of virulence in the germ ; and again we see the same 

 disease, no longer restrained by such inhibitory conditions, burst- 

 ing forth as a malignant and deadly plague. We have, therefore, 

 no warrant for the hypothesis that a pathogenic germ which, 

 under given conditions of life, has lost in pathogenesis, but not 

 in vitality, should continue forever to exist as a harmless microbe. 

 2. Varying Malignancy of the Tubercle Bacillus in Man. 

 Nothing is more familiar to physicians than the slow progre.ss of 

 tuberculosis of the lymph-glands and bones, on the one hand, 

 and its frequent rapid progress in pulmonary, abdominal, or 

 encephalic organs on the other. It has on this account been 

 rather difficult to persuade many of the etiological identity of 

 scrofula and consumption. In experimental tuberculosis the 

 same truth constantly crops up. Arloing and his followers found 

 that the tubercle bacillus from the lymph-glands of man proved 

 less virulent and deadly than that from the human lungs {Lecons 

 sur la Tuberculose) . As early as 1880, Creighton drew attention 

 to this in his work on Bovine Tuberculosis in Man. 



' Johns Hopkins Hospital Reports, Bulletin 100. 



