628 Tuberculosis. 



The negative character of experimental attempts does not, of course, 

 exclude the possibility of a modilication in the course of many genera- 

 tions under favorable natural conditions. This possibility has recently 

 been demonstrated by Eber who succeeded in producing in three in- 

 stances an acute fatal tuberculosis in cattle with bacilli of the human 

 type obtained from phthisical sputum. The bacilli, after tirst a passage 

 through guinea pigs followed by intraperitoneal injection in cattle, 

 produced only a circumscribed peritoneal tuberculosis, but cultures 

 obtained from this affection and which had all the characteristics of 

 bovine bacilli, produced death in from 21 to 34 days. 



Experimental investigations thus far carried out have 

 therefore not resulted in the discovery of fundamental differ- 

 ences between the causes of human tuberculosis and tul)ercu- 

 losis of the various domestic animals that w^ould justify any 

 modification of our former view^s as to the specific identity of 

 all tubercle bacilli. 



The differences in the form or structure of bacilli of various 

 origin are least constant because they usually disappear within 

 a very short time wdien growai on artificial media. 



Their cultural differences, however, are more constant. 

 These may remain unchanged through many generations, even 

 after passage through the bodies of other species of animals. 

 In this respect, how^ever, gradual variations are observed 

 which make it impossible at times to classify certain strains 

 of bacilli in their proper group. 



The remarkable difference in virulence of strains of bacilli 

 of different origin is still less adapted for the determination 

 of their specific character. Great variation in virulence, 

 especially with respect to different species of animals, with 

 otherwise great constancy of character, has also been observed 

 in other pathogenic bacteria (very marked in the bipolar 

 bacilli of hemorrhagic septicemia) wdiile their specific identity 

 has never been questioned on that ground. 



The inadmissibility of such a view^ is further emphasized 

 by the fact that variations ranging from the highest degree of 

 virulence (with respect to certain species) to complete aviru- 

 lence may be artificially produced by successive animal passages 

 or by artificial cultivation and, on the other hand, by the fact 

 that strains of the same type, obtained from different indi- 

 viduals of the same species may show variations in virulence 

 of no inconsiderable degree. 



The similar form and the similar staining characteristics 

 of the various strains of tubercle bacilli, the general re- 

 semblance of their cultures, the identity of the pathological 

 changes caused by them, the gradual diiferences observed be- 

 tween the different strains as well as the occurrence of transi- 

 tion forms, the similar effect of tuberculins prepared from 

 strains obtained from different species of animals, the fact 

 that cattle may be immunized with strains of bacilli obtained 

 from human beings or fowls and, finally, the fact that the 



