IMMUNITY. 19 



development of the pathogenic micro-organism, we infer that 

 diminished virulence is due to a diminished production of these 

 toxic substances. 



An important step was made in the progress of our knowl- 

 edge in this field of research when it was shown that animals 

 may be made immune against certain infectious diseases by 

 inoculating them with filtered cultures, containing the toxic 

 substances just referred to, but free from the living bacteria 

 to which they owe their origin. The first satisfactory experi- 

 mental evidence of this important fact was obtained by Salmon 

 and Smith in 1886. These bacteriologists succeeded in pro- 

 ducing an immunity in pigeons against the pathogenic effects 

 of the bacillus of hog cholera, which is very fatal to these 

 birds, by inoculating them with sterilized cultures of the 

 bacillus mentioned. Similar results were reported by Roux in 

 1888, from the injection into susceptible animals of sterilized 

 cultures of the anthrax bacillus, and also of the bacillus of 

 symptomatic anthrax. More recently (1890) Behring and 

 Kitasato have shown that animals may be made immune 

 against the pathogenic action of the bacillus of tetanus or the 

 bacillus of diphtheria by the injection of filtered, germ-free 

 cultures of these bacilli. Similar results have been obtained 

 by G. and F. Klemperer (1891), in experiments upon rabbits, 

 with filtered cultures of the micrococcus of croupous pneu- 

 monia. 



In Pasteur's protective inoculations against hydrophobia it 

 is probable that the immunity which is developed after infec- 

 tion by the bite of a rabid animal is due to the toxin (toxal- 

 bumin }) of this disease present in the emulsion of spinal cord 

 which is used in these inoculations. 



There is also some evidence to show that a certain degree 

 of immunity against tuberculosis may be produced in guinea 

 pigs by injections of the toxic substances developed during the 

 growth of the tubercle bacillus, — Koch's tuberculin. 



