162 ATLAS OF BACTERIOLOGY. 



certain poisons) diminish the immunity to a consider- 

 able extent, so that a large number of organisms 

 which are weakened in this manner succumb to a 

 subsequent infection. 



Hence in every newly isolated variety of bacteria 

 whose pathogenic action we desire to prove it is 

 necessary to experiment upon various animals if the 

 experiments on those first selected proved negative. 

 The principal animals for experimentation are : the 

 white domestic mouse, white rat, guinea pig, rab- 

 bit, chicken, pigeon, and, for special purposes, the 

 monkey. More rarely we employ the gray domestic 

 mouse and rat, field mice, dogs, cats, cows, sheep, 

 pigs, and horses. The most convenient animal, but 

 one requiring good care, is the guinea-pig, charac- 

 terized by suitable size, mildness, and modest con- 

 sumption of food. Animal plagues are studied and 

 explained much more readily than human diseases, 

 because the animals are at our disposal for experi- 

 mentation. In difficult cases various experiments in 

 infection have also been made upon man. 



The causes of congenital immunity (resistance) 

 reside in protective arrangements of the organism 

 which I cannot here consider in detail. It may be 

 said, however, that the views formulated by Buchner 

 as a compromise between the various opposing 

 theories are in tolerable accord with all the facts. 

 In an invasion of pathogenic germs into the resisting 

 organism a part is destroyed by substances (alexins) 

 already present in the serum (and derived from leu- 

 cocytes) ; another part is destroyed by substances 

 which are produced from leucocytes (or other tis- 

 sues) under the influence of the bacteria. A part of 



