DESCRIPTIONS OF PATHOGENIC BACTERIA. 147 



should be found in such numbers and in such 

 positions that their presence could explain the 

 characteristic symptoms of the disease ; (2) 

 They should be grown in pure culture outside 

 of the body ; (3) These pure cultures, when 

 inoculated into susceptible animals, should in- 

 duce the same disease. Proof on each of these 

 several heads could be furnished in the case of 

 many diseases, for example, in the case of 

 septicaemic diseases like anthrax and in tuber- 

 culosis. On the other hand, proofs on heads (2) 

 and (3) failed in relapsing fever, proof (i) was 

 not forthcoming for diphtheria and lockjaw, 

 nor were proofs (i) and (3) for cholera. 



It was necessary then to make a further ad- 

 vance. The fact that was first recognized, 

 although it did not at once receive the attention 

 it deserved, was the existence of a suscepti- 

 bility to disease. Only if an animal was pre- 

 disposed to " take " the disease in question 

 could the disease be imparted to it. If an ani- 

 mal was not susceptible, it was not made ill 

 or affected by the bacteria in any way. It was 

 next observed that certain bacteria did not 

 produce their deleterious effect mechanically 

 by their presence and increase in the organs, 

 but rather chemically through the formation 

 of poisons. (It is certainly remarkable that 

 Koch who had made the first of these observa- 



OF THE 



tJNIVERSITY 



