DIPHTHERIA ANTITOXIN. 423 



may be given many times the amount of the toxin that 

 would otherwise prove fatal /. 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 

 determined lethal dose of the toxin in a test-tube will 

 either neutralize its toxicity or greatly reduce it, accord- 

 ing 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 

 quantity 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 either in a test-tube or at the site of inoculation 

 in the living animal body. 



This serum with which we have been experimenting 

 is the so-called " diphtheria antitoxin " or " antidiph- 

 theritic serum." 



For practical purposes, it is obtained from horses, 



