INFECTION, IMMUNITY, AND RECOVERY. H9 



terious to the bacteria. As to the origin of these 

 substances, we may conceive that they may be either 

 regularly produced by the body cells, or by the fluids, 

 or by both, or that they may only be produced or at 

 least increased when bacteria invade the body. When 

 formed they may remain unaltered in the fluids or be 

 quickly eliminated or destroyed. It is probable that 

 more than one of these suppositions is actually true. 



The bactericidal effect upon most bacteria of the body 

 fluids, noted by Nuttall in 1888, is now undisputed, 

 and is shown by the fact that bacteria when injected 

 into the blood usually soon die, and this destruction 

 may be so rapid that in a few hours none remain alive. 

 Even when bacteria survive and produce infection 

 there is for a time a decrease in the number living, 

 but this is soon followed by a progressive increase. 

 This fact can be observed not only by injecting bac- 

 teria into the blood and peritoneal cavity, but also when 

 the bacteria are placed in the animal body after being 

 enclosed in capsules. The bacteria are killed even if 

 they have previously grown outside the body in blood- 

 serum. Bacteria have also been injected into a vein 

 carefully ligated above and below, and here, without 

 coagulation, the blood exerts bactericidal properties. 

 The general germicidal effect of the blood-serum can 

 also be watched outside of the body. Here mixed 

 with it some species of bacteria die quickly, some 

 slowly, and some lose only a portion of their number, 

 those remaining alive after a time rapidly increasing. 

 The number of bacteria introduced is of great impor- 

 tance, for the serum with its contained substances seems 

 capable of destroying only a certain number, and after 

 that loses its bactericidal properties. 



