270 SUSCEPTIBILITY AND IMMUNITY. 
planation of immunity, however probable it may appear, can hardly 
be said to rest upon a substantial experimental foundation, and we 
must admit that the exact source and method of production of the 
antitoxins in the animal body, and their mode of action, are still 
undetermined; and, for the present, we must be satisfied with the 
knowledge that in some way these so-called antitoxins, which have 
been proved to be present in the blood-serum of immune animals, 
protect these animals from infection by pathogenic bacteria. And 
that when transferred to susceptible animals they confer upon them 
a temporary immunity; or if introduced after infection, may neutral- 
ize the pathogenic action of the toxins produced by specific “disease 
germs.” 
Finally, there is experimental evidence to show that immunity 
from the pathogenic action of certain bacteria may be produced by 
previous injections of cultures of other bacteria (sterilized or other- 
wise), and even by the injection of the blood-serum of normal indi- 
viduals or of other substances. 
Pasteur, in 1880, communicated to the French Academy of Sciences 
the results of experiments which led him to the conclusion that fowls 
which had an acquired immunity against chicken cholera also had 
an immunity against anthrax. Roux has reported that the blood- 
serum of a horse which has been immunized against tetanus neutral- 
izes the toxic power of cobra poison. But the contrary effect is not 
produced—7.e., the bleod-serum of an animal immunized against the 
cobra poison does not neutralize the tetanus toxalbumin. The state- 
ment is also made that the blood-serum of a rabbit which has been 
made immune against hydrophobia will protect a susceptible animal 
against the cobra venom in doses four or five times as large as the 
usually lethal dose. iso that rabbits which have been immunized 
against snake-poison are less susceptible to the toxic effects of abrin, 
and the reverse—?.e., antiabrin neutralizes, to some extent at least, 
the toxic action of snake-poison. 
The writer, in his “Report on the Etiology and Prevention of 
Yellow Fever ” (1890), gives, on pp. 196 and 197, experimental evi- 
dence which shows that the injection into the peritoneal cavity of 
rabbits of cultures of Bacillus pyocyaneus or of Bacillus gracilis pro- 
tected the animals from the fatal results of subsequent injections of 
my bacillus X, which was extremely fatal to rabbits when injected 
into the cavity of the abdomen in doses of 1 or 2¢.c. In referring 
to these experiments I say: “ The evidence favors the view that death 
results from peritonitis (and toxemia?) induced by intra-peritoneal 
injections, and that «a tolerance on the part of the peritoneum may 
