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230                 PATHOGENIC BACTERIA.

primary injection in from six to ten weeks,  continued
to live, sometimes (Pfuhl) as long as nineteen weeks.

Koch also discovered that a 50 per cent, glycerin
extract of cultures of the tubercle bacillus produced the
same effect as the dead cultures originally used, and
gave this substance, tuberculin, to the scientific world
for experimental purposes, in the hope that the prolon-
gation of life observed in the guinea-pig might be true
in the case of man.

The active substance of the "tuberculin" seems to be
an albuminous derivative insoltible in absolute alcohol.
It is not a toxalbumin.

The action of the tuberculin upon the animal organ-
ism is peculiar, but readily understandable. It does not
exert the slightest influence upon the tubercle bacillus,
but acts upon the living tuberculous tissue. In the
description of the tissue-changes already given it has
been shown that the tubercle bacillus effects the coagu-
lation-necrosis of the cells, but does not derive its nutri-
ment from the dead tissue. As the cells die and are
incorporated in the necrotic mass, the bacilli find the
conditions of life unfavorable, and likewise seem to die.
The active bacilli, therefore, are always found at the mar-
gins of the tuberculous tissues, where the cells are fairly
active. The necrosis is due to bacillary poisons. When
tuberculin is injected into the organism the result is to
double the amount of poisonous influence upon the cells
surrounding the bacilli, to destroy their vitality, to re-
move the favorable conditions of growth from the organ-
ism, and to leave it for a time checkmated.

Virchow, who well understood the action of the tuber-
culin, soon showed that as a diagnostic and therapeutic
agent in man its use was attended with great danger.
The destroyed tissue was absorbed, and with it the bacilli
were likewise absorbed and transported to new areas,
where a rapid invasion occurred. Old tuberculous lesions
which had been encapsulated were softened, broken
down, and became sources of dangerous infection to the