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os is,                     229

soon surrounded with a zone of newly-formed contracting1
fibrillar tissue, by which it is perfectly isolated. In such
isolated masses lime-salts are commonly deposited. Some-
times this process is perfected without the destruction of
the bacilli, but with their incarceration and inhibition.
Such a condition is called latent tuberculosis, and may at
any time be the starting-point of a new infection and lead
to a fatal termination.

In 1890, Koch announced some observations upon toxic
products of the tubercle bacillus and their relation to the
diagnosis and treatment of tuberculosis, which at once
aroused an enormous but, unfortunately, a transitory

These observations, however, are of capital importance.
Koch observed that when guinea-pigs are inoculated
with a mixture containing tubercle bacilli the wound
ordinarily heals readily, and soon all signs of local dis-
turbance other than enlargement of the lymphatic glands
of the neighborhood disappear. In about two weeks there
occurs at the point of inoculation a slight induration which
develops into a hard nodule, then ulcerates, and remains
until the death of the animal. If, however, in the course
of a short time the animals are reinoculated, the course
of the process is altogether changed, for, instead of heal-
ing, the wound and the tissue surrounding it assume
a dark color and become obviously necrotic, and ulti-
mately slough away, leaving an ulcer which rapidly and
permanently heals without enlargement of the lymph-

Having made this observation with injected cultures
of the living bacillus, Koch next observed that the same
change occurred when the secondary inoculation was
made with pure cultures of the dead bacilli.

It was also observed that if the material used for the
secondary injection was not too concentrated and not
too often repeated (only every six to forty-eight hours),
the animals thus treated improved in condition, and,
instead of dying of the tuberculosis induced by the