INFECTION AND IMMUNITY 361 



lished to certain mineral poisons, such as arsenic, and to 

 many alkaloids, such as morphine, strychnine, and nico- 

 tine. No other explanation is at hand than that this 

 depends upon the general principle that habituation to 

 the poison diminishes the tendency of the protoplasm 

 to become influenced by it. In these cases the toler- 

 ance is usually very limited, and any sudden increase in 

 the dosage may be followed by death. The phenomena 

 attending such tolerance are essentially dissimilar from 

 those following habituation to the microorganismal 

 toxins, which are chemically different, being colloidal 

 and protein in nature. 



It is not improbable that the different reactions of 

 the organism toward the protein poisons, the tox- 

 albumins and toxins, depend upon the closer resemblance 

 such compounds bear to substances concerned in the 

 nutrition of the cells. At all events, when an attempt 

 is made to habituate the organism to the microorgan- 

 ismal products, greater success attends, and it is found 

 that each administration is followed by a definite re- 

 action which results in a definite change in the physio- 

 logico-chemical relationships. 



Suppose, for example, an experiment be performed 

 with the venom of the cobra. If this poison, which is a 

 toxalbumin and of which a minute quantity is fatal 

 when injected beneath the skin or into the circulation, 

 is swallowed by a healthy warm-blooded animal, no 

 harm is done, presumably because it undergoes diges- 

 tion in the stomach and intestines and is thus rendered 

 harmless. When it is injected beneath the skin in doses 

 so small as not to produce death, the animal is made 

 ill, presents a definite train of symptoms, recovers, and 

 may then be injected with a much larger dose. After 

 a second illness or reaction, from which it is permitted 

 to recover, a still larger quantity may be administered 

 with similar effects, and so on until perhaps a thousand 

 times as much may be given without injury as would 

 have killed it as a first dose. 



