62 PATHOGENIC BACTERIA. 



and also by cultivation in an infusion of the tissues of 

 an animal similar to the one to be inoculated. 



It must be understood that the term "immunity" is 

 a relative one, and that while u a white rat is immune 

 against anthrax in amounts sufficiently large to kill a 

 rabbit, it is perhaps not immune against a quantity 

 sufficiently large to kill an elephant." 



It is not to be expected that such intricate phenomena 

 as these which have been mentioned could be observed 

 and suffered to go unexplained. We have explanations, 

 but, unfortunately, they are as intricate as the phenomena, 

 and, though each may possess its grain of truth, not one 

 will satisfy the demands of the thoughtful student. In 

 brief review, the theories of immunity are the following : 



1. THE EXHAUSTION THEORY. This hypothesis was 

 advanced by Pasteur in 1880, and suggests that by its 

 growth in the body the micro-organism uses up some 

 substance essential to its life, and that when this sub- 

 stance is exhausted the microbe can no longer thrive. 

 The removal of the necessary material, if complete, will 

 cause permanent immunity. 



As Sternberg points out, were this theory true we must 

 have within us a material of small-pox, a material of 

 measles, a material of scarlet fever, etc., to be exhausted 

 by its appropriate organism. It would necessitate an 

 almost inconceivably complex body-chemistry and a 

 rather stable condition of the same. 



2. THE RETENTION THEORY. In the same year 

 Chauveau suggested that the growth of the bacteria 

 in the body might originate some substance prejudicial 

 to their further and future development. There seems 

 to be a large kernel of truth in this, but were it always 

 the case we would have added to our blood a material 

 of small-pox, a material of measles, a material of scarlet 

 fever, etc. , so that we would become saturated with the 

 excrementitious products of the bacteria, instead of hav- 

 ing so many substances subtracted from our chemistry. 



3. THE THEORY OF PHAGOCYTOSIS. In 1881, Carl 



