252 SUSCEPTIBILITY AND IMMUNITY. 
body of an immune animal, we must admit that the animal has ac- 
quired a tolerance to the pathogenic action of these toxic substances, © 
for their presence no longer gives rise to any morbid phenomena. 
And this being the case, we are not restricted to the explanation 
that immunity depends upon a restraining influence exercised upon 
the microbe when subsequently introduced. 
The Vital Resistance Theory.—Another explanation offers itself, 
viz., that immunity depends upon an acquired tolerance to the 
toxic products of pathogenic bacteria. This is a view which the 
writer has advocated in various published papers since 1881. Ina 
paper contributed to the -Lmerican Journal of the Medical Sci- 
ences in April, 1881, it is presented in the following language: 
‘‘The view that I am endeavoring to elucidate is that, during a non- 
fatal attack of one of the specific diseases, the cellular elements implicated 
which do not succumb to the destructive influence of the poison acquire a 
tolerance to this poison which is transmissible to their progeny, and which 
is the reason of the exemption which the individual enjoys from future 
attacks of the same disease.” ' 
In my chapter on “‘ Bacteria in Infectious Diseases,” in ‘‘ Bac- 
teria,” published in the spring of 1884, but placed in the hands of the 
publishers in 1883, I say: 
“Tt may be that the true explanation of the immunity afforded by a mild 
attack of an infectious germ disease is to be found in an acquired tolerance to 
the action of a chemical poison produced by the microdrganism, and conse- 
quent ability to bring the resources of nature to bear to restrict invasion by 
the parasite.” 
The ‘‘ resources of nature” are referred to in the same chapter as 
follows : 
‘‘The hypothesis of Pasteur would account for the fact that one individual 
suffers a severe attack and another a mild attack of an infectious disease, 
after being subjected to the influence of the poison under identical cireum- 
stances, by the supposition that the pabulum required for the development: 
of this particular poison is more abundant in the body of one individual 
than in the other. The explanation which stems to us more satisfactory is 
that the vital resistance offered by the cellular elements in the bodies of 
these two individuals was not the same for this poison. It is well known 
that in conditions of lowered vitality resulting from starvation, profuse 
discharges, or any other cause, the power to resist disease poisons is greatly 
diminished, and, consequently, that the susceptibility of the same individual 
differs at different times. 
‘From our point of view, the blood, as it is found within the vessels of a 
living animal, is not simply a culture fluid maintained at a fixed tempera- 
ture, but under these circumstances is a tissue, the histological elements of 
which present a certain vital resistance to pathogenic organisms which may 
be introduced into the circulation. 
“Tf we add a small quantity of a culture fluid containing the bacteria of 
putrefaction to the blood of an animal, withdrawn from the circulation into 
a proper receptacle and maintained in a culture oven at blood heat, we will 
find that these bacteria multiply abundantly, and evidence of putrefactive 
1«« What is the Explanation of the Protection from Subsequent Attacks, result- 
ing from an Attack of Certain Diseases, ete?” American Journal of the Medical 
Sciences, April, 1881, p. 376. 
