INFECTION, IMMUNITY, AND RECOVERY. 119 
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. 
