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228                 PATHOGENIC BACTERIA.

pied at the expense of the tissue, whose component cells
are pushed aside or else included in the nodule. In mil-
iary tuberculosis of the kidney it is not unusual to find a
tubercle including a whole glomerule, and resolving its
component thrombosed capillaries and epithelium into
necrotic fragments.

As almost all tissues contain a supporting tissue-frame-
work of connective-tissue fragments, some of these must
be embodied in the new growth. The fibres which pos-
sess little vitality are more resistant than cells, aild, after
all the cells of a tubercle have been destroyed, will be
distinctly visible among the granules, so that the tubercle
has a reticulated appearance.

As a rule, tubercles steadily increase in size by the in-
vasion of fresh tissue. The tubercle bacillus does not seem
to find the necrotic centres of the tubercles adapted to its.
growth, and completes its life-cycle with the tissue-cells.
It is unusual to find healthy-looking bacilli in the necrotic
areas, most of them being observed at the edges of the
tubercle, where the nutrition is good. From such edges,
the bacilli are occasionally picked up by leucocytes and
transported through the lymph-spaces, until the phago-
cyte falls a prey to its prisoner, dies, and sows the seed
of a new tubercle. However, for the spread of tubercle
bacilli from place to place phagocytes are not always
necessary, for the bacilli seem capable of transportation
by streams of lymph alone.

Notwithstanding the steady advance which takes place
in most observed cases of tuberculosis, and the thoroughly
comprehensible microscopic explanation of it, many cases
of tuberculosis make quite perfect recoveries.

The periphery of every tubercle is a zone of reaction,
with a marked tendency to granulation and organization.
If the vital condition is such that through inappro-
priate nutriment or through unusually active phago-
cytosis the activity of the bacilli is checked or their
death is brought about, this tendency to cicatrization is
allowed to progress unmolested, and the necrosed mass is