EFFECTS OF UUBERCULIN. 59 



have differed widely. In these cases, where the animals were so 

 slightly diseased that their condition could not be detected by a 

 physical examination, the results have differed so widely that 

 we are forced to the conclusion that we can never tell whether a 

 slightly diseased animal will react to the tuberculin test or not. 

 A reaction may be obtained to-day and again next month, or as 

 in the case of the cow Agnes, reaction may fail for nearly three 

 years and then there be a very decided reaction. 



We do not believe that tuberculin is poisonous for cows one 

 day and not another, but our experience might lend some color 

 to such a conclusion. That there is some law underlying the 

 whole matter we believe must be true, but we have not yet seen 

 any satisfactory statement of what the law is. 



The theory has been quite generally advanced that aside from 

 a possible curative action, one dose of tuberculin influences the 

 action of the next dose even though it is injected after a consid- 

 erable interval, the length of time being variously estimated. 

 In view of the facts that animals do not usually acquire a toler- 

 ance, either temporary or permanent, for vegetable poisons as 

 the result of one moderate dose; that some tuberculous animals 

 react repeatedly to tuberculin and others do not; and that the 

 intervals between reactions even in our limited experience varies 

 from two days to more than a year, we see no grounds for sup- 

 porting the theory that the failure of tuberculous animals to react 

 to tuberculin is due to an acquired tolerance of tuberculin. 



It is not a wild assumption that the growth of tubercle bacilli 

 in diseased cows keeps their systems charged with tuberculin 

 nearly up to and sometimes a little beyond the point where it 

 manifests its poisonous effects in elevation of temperature. 

 Such animals, charged to the danger point with tuberculin, read- 

 ily react to a slight addition. The cow that has been taken at 

 a disadvantage and has contracted tuberculosis and then 

 so far recovers as to hold the disease in check, may be compared 

 to the well cow so far as the action of tuberculin is concerned, 

 since her system is free, or comparatively free, from tuberculin 

 and no reaction follows the giving of the usual test dose. 



Our experience has suggested to us the possibility that the 

 failure of tuberculin to cause reaction in tuberculous cows at 



