BACTERIUM XEROSIS 477 



be so adjusted as to cause only temporary inconvenience 

 and not endanger life, and this dose be injected repeatedly, 

 gradually increasing in size as the animal is able to bear 

 it, after a while a marked tolerance is established, so that 

 the animal may be given many times the amount of the 

 toxin that would otherwise prove fatal — i. e., many times 

 the lethal dose for an animal that had not acquired such a 

 tolerance. 



If blood be now drawn from the animal that has become 

 habituated, so to speak, to the diphtheria toxin, and the 

 serum collected from it, we discover several important 

 facts, viz.: 



That this serum when mixed with the previously deter- 

 mined lethal dose of the toxin in a test-tube will either 

 neutralize its toxicity or greatly reduce it, according to the 

 amount of serum used. 



That if we inject into an animal the determined fatal 

 dose of the toxin, and immediately afterward inject a quan- 

 tity of the serum, either the animal will not die or the death 

 will be more or less delayed, according to the amount of 

 serum employed. 



That if a susceptible animal be inoculated with a living 

 culture of virulent bacterium diphtherise, its life may be 

 saved, or its death postponed, by the subsequent injection 

 of the serum; the result depending upon the amount of 

 serum used and the lapse of time between inoculation with 

 the bacteria and injection of the serum. 



And, finally, that although this serum has such a marked 

 effect upon the toxins of bacterium diphtherise in a test- 

 tube or in the animal, and so striking an influence upon 

 the course of infection with the living organisms in the 

 animal, it has little or no effect upon the living bacteria 



