IMMUNITY. 315 



habituation to poison the body has acquired 

 merely the power of " paralyzing " the specific 

 poison, although the parasite may perhaps 

 still be able to grow in the t body ; the latter 

 event, however, is no longer of moment because 

 any poison formed is rendered innocuous. In 

 both natural and acquired immunity the body 

 disposes of a quantity of poison equal to that 

 formed in the course of a natural or artificial 

 infection, while by gaining an habituation to 

 poison, the body becomes accustomed to very 

 much larger quantities. The body, then, may 

 acquire the power-to cope with poisons, or the 

 power naturally existing may be heightened, in 

 a manner differing, at least quantitatively, from 

 that shown in natural resistance and acquired 

 immunity : for example, according to Behring, 

 an animal may not be tolerant of tetanus toxin 

 although displaying immunity to the disease, 

 or, conversely, the animal may be susceptible 

 to the toxin at a time when its blood-serum is 

 to the very highest degree antagonistic to it. 

 In hydrophobia both the poisonous action and 

 the immunizing power are associated with the 

 central nervous system and inoculation with 

 material from this source can produce immunity 

 even when the material no longer acts poison- 

 ously. By inoculation with " metabolic pro- 

 ducts " the body acquires immunity to the dis- 



