IMMUNITY. 319 



arising from skin infection or from wounds, 

 succumb if the virus be inoculated directly 

 into the brain. According to experiments by 

 Hueppe and Salus, if fowls be inoculated in a 

 pectoral muscle with the bacteria of chicken 

 cholera, or pigeons be inoculated in the same 

 way with the germ of human cholera, the birds 

 can not afterwards be successfully infected on 

 the same side of the body, although an infec- 

 tion of the pectoral muscle on the other side 

 is still effective ; then ensues a stage where 

 even this mode of inoculation no longer suc- 

 ceeds, but where direct inoculation into the 

 blood current still suffices to cause death. 

 Guinea-pigs which have been securely immu- 

 nized against cholera infection by way of the 

 peritoneum succumb, just as readily as if noth- 

 ing had happened, to infection through the 

 stomach. 



Protective inoculation requires time, for the 

 simple reason that the protective substances 

 must act as liberating or exciting stimuli upon 

 definite tissue-cells of the body. This kind 

 of immunity is acquired only through the in- 

 tervention of the body-cells ; active immunity 

 is an immunity of cells or tissues. The same 

 thing holds good of the habituation to or 

 tolerance of poisons. " Active " immunity of 

 the sort obtained through the influence of the 



