IMMUNITY. 343 



cubation. If the period of incubation be long 

 the disease may require days before showing 

 itself openly ; perhaps in this intermediate 

 period it may be possible to bring about a suc- 

 cessful protective inoculation, even although a 

 successful infection had previously occurred. 

 Such was Pasteur's train of thought, and he 

 sought to accomplish the desired end by 

 making use of the much-discussed method of 

 inoculation against rabies. If we do not neg- 

 lect the local factor in protective inoculation 

 already several times mentioned and the way 

 in which immunity spreads from the place of 

 infection to the blood and central nervous sys- 

 tem, it must be unconditionally admitted that 

 Pasteur's method of treating rabies has cer- 

 tainly achieved success in cases in which no 

 favorable issue could have been hoped for from 

 the use of other means. Partial success has 

 been attained also in the method, described 

 on p. 305, of inoculation with the metabolic 

 products of bacteria, carried out on the same 

 theory. That is to say, the incorporation of 

 the protective substances is resorted to only 

 after the disease has broken out. 



In the same group of conceptions belongs 

 the use of proteids for curative purposes. The 

 first experiments were made in 1884-86, by 

 Ferran and Zaeslein, who administered dead 



