TEXT-BOOK OF BACTERIOLOGY. 143 



The real efficacious substances are sometimes formed not only 

 by one definite species of bacteria, but by several in the same man- 

 ner. Thus the results obtained by Hueppe and Wood may be ex- 

 plained as also those of Roux, Chamberland, and others, who by 

 inoculating" with one kind of bacteria produced immunity from 

 several diseases, and who observed the frequent occurrence of re- 

 ciprocal inoculative protection by which one micro-organism se- 

 cured immunity against another, and vice versa. Some of the sub- 

 stances recognized as bacterial products are chemical bodies well 

 known and exactly defined as to their composition. 



As neurin is one of them, we can understand how Foa and 

 Bonome were enabled to employ it with success. The results ob- 

 tained by Wooldridge may also be explained in a similar manner. 

 At all events, we have, on the one hand, in protective inoculation 

 certain chemical substances, excretions of bacteria, as an active 

 principle, and on the other hand the acquired immunity i.e., a con- 

 dition which resembles that of the individuals to which nature has 

 granted immunity. How can such a cause produce such an effect ? 



Of the many attempts at explanation which have been sug- 

 gested, we can only enumerate a few of the more important ones. 

 There is one supposition which we can clearly refute by the light 

 of what has already been written, and which we only mention on 

 account of its historical interest. It is the so-called "fheory of ex- 

 haustion set up by Pasteur and Klebs, which supposes that on the 

 "first invasion" a number of substances are consumed in the body, 

 which form a necessary nutriment for the invading species of bac- 

 teria. As these substances are not afterward renewed, a second 

 attack becomes impossible; the exhausted soil has become unfruit- 

 ful. Being thus incompatible with the already-developed opinion 

 that it is not the bacteria themselves, but the chemical substances 

 that decide the question, this theory could no longer be maintained 

 after it had been discovered that soluble substances freed from all 

 living germs, which certainly could not produce such an exhaustion, 

 were, nevertheless, able to produce immunity. The observation 

 of Bitter, too, that the growth of the inoculated bacteria takes 

 place within a narrowly-limited portion of the organism, goes to 

 disprove this explanation, which supposes a general diffusion of 

 micro-organisms throughout the body and a complete exhaustion of 

 the available nutriment. 



The second explanation, the so-called hypothesis of retention, 

 forms a direct contrast to the one just mentioned. Its principal 

 champion is Chauveau. It supposes that the excretions of the 

 bacteria remain in the body after the first invasion and prevent 



