242 SUSCEPTIBILITY AND IMMUNITr. 



tion, upon the wild stock from which they originated. The acquired proper- 

 ties are transmitted indefinitely; and the same sap which on one twig nour- 

 ishes a sour crab apple, on another one of the same branch is elaborated into 

 a delicious pippin. 



" The tolerance to narcotics opium and tobacco and to corrosive poisons 

 arsenic which results from a gradual increase of dose, may be cited as an 

 example of acquired tolerance by living protoplasm to poisons which at the 

 outset would have been fatal in much smaller doses. 



"The immunity which an individual enjoys from any particular disease 

 must be looked upon as a power of resistance possessed by the cellular ele- 

 ments of those tissues of his body which would yield to the poison in the 

 case of an unprotected person." 



This theory of immunity, advanced by the author in 1881, has 

 received considerable support from investigations made since that 

 date, and especially from the experimental demonstration by Sal- 

 mon, Roux, and others that, as suggested in the paper from which I 

 have quoted, immunity may result from the introduction into the 

 body of a susceptible animal of the soluble products of bacterial 

 growth filtered cultures. 



The theory of vital resistance to the toxic products evolved by 

 pathogenic bacteria is also supported by numerous experiments 

 which show that natural or acquired immunity may be overcome 

 when these toxic products are introduced in excess, or when the vital 

 resisting power of the animal has been reduced by various agencies. 



Thus Bouchard has shown that very small doses of a pure culture 

 of the Bacillus pyocyanus are fatal to rabbits, when at the same time 

 a considerable quantity of a filtered culture of the same bacillus is 

 injected into a vein. The animal could have withstood the filtered 

 culture alone or the bacilli injected beneath its skin ; but when its 

 vital resisting power (paralysis of phagocytes ?) has been partially 

 overcome by the filtered culture injected into a vein the bacilli mul- 

 tiply abundantly and a fatal result follows. 



The same result may be obtained by injecting sterilized cultures 

 of a different microorganism. Thus Roger has shown that the rab- 

 bit, which has a natural immunity against symptomatic anthrax, 

 succumbs to infection when inoculated with a culture of the bacillus 

 of this disease, if at the same time it receives an injection of a ster- 

 ilized or non-sterilized culture of Bacillus prodigiosus. 



Monti has succeeded in killing animals with old and attenuated 

 cultures of the Streptococcus pyogenes or of Staphylococcus pyo- 

 genes aureus, by injecting at the same time a culture of Proteus vul- 

 garis. A similar result may be obtained by subjecting animals to 

 physical agencies which reduce the vital resisting power of the tis- 

 sues. Thus Nocard and Roux found by experiment that an attenu- 

 ated culture of the anthrax bacillus, which was not fatal to guinea- 

 pigs, killed these animals when injected into the muscles of the 

 thigh after they had been bruised by mechanical violence. Charrin 



