46 TUBERCULOSIS 



Whether or not all of these acid-fast organisms have been evolved 

 from a common ancestor is a fruitful field for theoretic discussion. 

 No one has, as yet, converted one of these into a pathogenic organism, 

 though this has been tried by frequent passage through animals. They 

 may produce local reactions which closely resemble those developed by 

 inoculation with one of the pathogenic types, but they do not cause 

 general tuberculosis. That their cellular structure is similar, chem- 

 ically at least, to that of the virulent types is shown by the following: 

 (1) their similar behavior toward stains, (2) their local effects, (3) 

 animals sensitized to one of these saprophytic forms are partially sensi- 

 tized to the virulent types. 



Many attempts have been made to convert one virulent type into 

 another. The human and avian types have been carried through many 

 cows and the human and bovine varieties have been carried through 

 chickens. The results of this work have been by no means uniform. 

 Some investigators have convinced themselves that transformation 

 from one type to another is easily and quickly secured, while another 

 man, equally competent, repeats the work and is sure that such a 

 transformation never results. The conditions under which such work 

 must be done render mistakes easily possible and to obtain satisfactory 

 and convincing proof seems quite impossible. 



For instance, one man attempts to carry the human type succes- 

 sively through many calves in order to transform it into the bovine 

 variety. At last he inoculates a calf and later finds in that animal a 

 typical bovine organism. The experimenter claims that he has dem- 

 onstrated a transformation, but his critic says the tuberculosis found 

 in the last calf is not a result of the inoculation, but is a true bovine 

 tuberculosis from a wholly different source. 



One of the most convincing experiments in favor of a transforma- 

 tion in type is that of Nocard. He placed a culture of the human 

 type in a collodion sack and introduced the sack into the abdominal 

 cavity of a chicken; after some weeks he transferred the sack to the 

 abdominal cavity of a second chicken and after another interval to a 

 third. After this he found that the type had been changed to avian. 



This has been repeated by two other investigators, one of whom 

 confirms and the other denies Nocard's claim. The solution of the 

 question concerning the relation of the saprophytic acid-fast bacilli to 



