268 SUSCEPTIBILITY AND IMMUNITY. 
mixture into a susceptible animal, then we must admit that this 
neutralizing effect occurs outside of the body of the animal, as has 
been generally assumed. 
The experiments of Vaillard are also opposed to Buchner’s view. 
He reports that in a rabbit immunized against tetanus, “a volume of 
blood equal to the total amount which circulates in its body may be 
withdrawn without diminishing, in an appreciable manner, the anti- 
toxic power of its serum. Therefore the antitoxin must be repro- 
duced as fast as it is withdrawn.” The author from whom we have 
just quoted (Roux) also reports the results of experiments which 
show that the antitoxic value of the serum of a rabbit immunized 
against tetanus does not bear a direct relation to the quantity of the 
tetanus toxin introduced, but depends also upon the method adopted. 
When a few large doses are given the result is far less favorable than 
that obtained by giving the same amount in repeated small doses. 
The serum of an animal immunized by thirty-three small doses was 
found to neutralize, in vitro, 150 parts of toxin, while that of an 
animal which received the same amount in nine doses only neutral- 
ized 25 parts of the same toxin. On the other hand we have experi- 
ments which indicate that the supposed neutralization of a toxin by 
an antitoxin in vittro is not really a chemical neutralization. Thus 
Buchner found in his experiments with the tetanus toxin and anti- 
toxin, in a dry powder, that when mixed in a certain proportion and 
injected into white mice no tetanicsymptoms were induced. Butthe 
same mixture gave rise to distinct tetanic symptoms in guinea-pigs, 
showing that the inference that the toxin had been neutralized in 
vitro, based upon the experiment on mice, would have been a mis- 
take. And certain observations made by Roux and Vaillard seem 
to give support to the view that neutralization does not occur im 
vitro, but that the result depends upon some physiological reaction 
induced by the antitoxin within the body of the livinganimal. These 
bacteriologists found that when the antitoxin was apparently in ex- 
cess, tetanic symptoms could be induced in susceptible animals if 
they had been in any way exhausted prior to the injection of the 
mixture of toxin and antitoxin; and that the same result followed 
when their resisting power had been reduced by injecting into them 
at the same time filtered cultures of other bacteria. 
In this connection the results reported by Calmette, Phisalix, 
and Bertrand are of interest. These investigators found that when 
the antitoxin of snake-poison was mixed with this venom in a pro- 
portion which neutralized its toxic properties, as shown by experi- 
mental inoculations, and the mixture then heated to 70° C., by which 
