IMMUNITY AND SUSCEPTIBILITY. 143 



emulsion of spinal cord, for which it was known to have 

 a strong affinity (tetanus acts chiefly upon the spiual 

 cord) that the mixture became harmless for mice and 

 even for guinea pigs. 



It not only was true of mixtures made without the 

 body, but was also operative when the two components 

 of the mixture were injected in different parts of the 

 body and at different times, the nervous tissue being in- 

 jected always before the toxin up to periods as long as 

 twenty-four hours. 



It was supposed that in these cases the protection af- 

 forded by the nervous matter depended upon the satura- 

 tion of the toxophoric atoms in the toxin by the cell sub- 

 stances, and it was deduced that acquired immunity to 

 poisons depends upon the ability of the cells to saturate 

 the toxophoric atoms of the respective toxins by their 

 combining lateral chains. As in these combinations the 

 combining substance is used up, more of it is produced, 

 the production being in excess of the requirements. It 

 is supposed by the promulgators of these views that anti- 

 toxin is the excess of such combining substance as has 

 escaped into the blood. 



From this survey of the subject it must be evident that 

 in investigating both natural and acquired immunity we 

 have unravelled a great mass of most interesting and im- 

 portant facts; and out of them have constructed a very fair 

 knowledge of the reactions and phenomena of a process 

 whose essence has so far escaped us. At present we only 

 know natural immunity as a remarkable spontaneous 

 power of resistance or endurance, and acquired immunity 

 as a similar power developing in animals not naturally 

 so endowed. The ability of the animal to endure the 

 toxins, which is the key to the whole situation, depends 

 upon some tolerance the 7iature of which has not yet been 

 satisfactorily determined. 



