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BIOLOGY OF BACTERIA.                    63

3.   Actual penetration of the vessel-wall by the growth
of the microorganism.

4.   Entrance into the vessels via the lymphatics, either
passively or in leucocytes.

Seeing that the channels by which bacteria can enter
the body are so numerous, and that there is scarce a
moment when some part of us is not in contact with
them, how is it that we are not constantly subject to
disease? The consideration of this question, together
with the closely related questions why we should be
subject to certain diseases only, and to these diseases
at certain times only, must be reserved for another chap-
ter, in which the subjects Immunity and Susceptibility can
be taken up at length. Before passing on to it, however,
some attention must be paid to the subject of the

Klimi nation of Bacteria from the Body-—There is every
reason to think that non-pathogenic bacteria entering the
body ordinarily, or being experimentally injected into it,
follow the same course as inert, non-vital particles; con-
cerning which, the experiments of Siebel have shown
that they accumulate in the finest capillaries, especially
in the lung, liver, spleen, and bone-marrow, and are
slowly transferred to the surrounding tissues, either to be
collected in the connective-tissues, carried to the lym-
phatic nodes, or to be subsequently excreted with the
bile, succus enterieus, etc., or to be discharged from the
surface of the mucous membranes, pulmonary alveoli,
tonsils, etc. They also escape from suppurating wounds
to which they may be carried by leucocytes. They are
not excreted by the kidneys.

The experiments of Wyssokowitsch are in accord with
the results of SiebePs work, and show that the kidney
rarely eliminates bacteria, Cavazzani found that the
kidney had the power to retain bacteria in the blood,
unless the epithelium was injured.

The principal avenues of escape for the bacteria are,
therefore, for the non-pathogenic forms, the mucous mem-
branes, the bile, and the sweat. For the pathogenic