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66                   PA THO GEN 1C BA CTERIA.

extraordinary is the fact that a few drops of blood from
the recovered mouse injected into another will protect it
from tetanus.

Immunity is the condition in which the body of an
animal resists the entrance of disease-producing germs,
or, having been compelled to allow them to enter, resists
their growth and pathogenesis. The resistance so mani-
fested is a distinct, potential vital phenomenon.

Susceptibility is the opposite condition, in which, in-
stead of resistance, there is a passive inertia which allows
the disease-producing organisms to develop without oppo-
sition. Susceptibility is accordingly the absence of im-

Immunity is either natural or acquired.
Natural Immunity.óBy this term is meant the natural
and constant resistance which certain healthy animals
exhibit toward certain diseases.

The white rat is peculiar in resisting anthrax.    It is
almost impossible to develop anthrax in a healthy white
irat, but Roger found that such an animal would easily
rsuccumb to the disease if compelled to turn a revolving
wheel until exhausted.   Susceptibility which follows such
an exhaustion of the vital powers cannot be regarded as
other than accidental, and makes no exception to the
statement  that the white rat  is immune  to  anthrax.
Animals such as man, sheep, cows, rabbits, and white
mice are susceptible to anthrax, while birds and reptiles
are generally immune.   The great difference in the morph-
ology between mammals and birds and reptiles, together
with the fact that their temperature, blood, and tissues
all differ, makes this immunity reasonably intelligible.
Morphological differences, however, will not suffice to
explain all cases, for the Caucasian nearly always suc-
cumbs to yellow fever, while the negro is rarely affected ;
and scarlatina, which is one of our commonest and most
dangerous diseases of childhood, is said to be unknown
among the Japanese.    Nor is this all, for, close as is their
resemblance in all respects except color, the house-mouse,