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Page 22
The exudate from the blood vessels in various ways assists in repair.
An injurious substance in the tissue may be so diluted by the fluid
that its action is minimized. A small crystal of salt is irritating to
the eye, but a much greater amount of the same substance in dilute
solution causes no irritation. The poisonous substances produced by
bacteria are diluted and washed away from the part by the exudate. Not
only is there a greater amount of tissue fluid in the inflamed part,
but the circulation of this is also increased, as is shown by
comparing the outflow in the lymphatic vessels with the normal. The
fluid exudate which has come from the blood and differs but slightly
from the blood fluid exerts not only the purely physical action of
removing and diluting injurious substances, but in many cases has a
remarkable power, exercised particularly on bacterial poisons, of
neutralizing poisons or so changing their character that they cease to
be injurious.
We have learned, chiefly from the work of Metschnikoff, that those
white corpuscles or leucocytes which migrate from the vessels in the
greatest numbers have marked phagocytic properties, that is, they can
devour other living things and thus destroy them just as do the
amoeb�. In inflammations produced by bacteria there is a very active
migration of these cells from the vessels; they accumulate in the
tissue and devour the bacteria. They may be present in such masses as
to form a dense wall around the bacteria, thus acting as a physical
bar to their further extension. The other form of amoeboid cell, which
Metschnikoff calls the macrophage, has more feeble phagocytic action
towards bacteria, and these are rarely found enclosed within them. It
is chiefly by means of their activity that other sorts of substances
are removed. They often contain dead cells or cell fragments, and when
h�morrhage takes place in a tissue they enclose and remove the
granules of blood pigment which result. They often join together,
forming connected masses, and surround such a foreign body as a hair,
or a thread which the surgeon places in a wound to close it. They may
destroy living cells, and do this seemingly when certain cells are in
too great numbers and superfluous in a part, their action tending to
restore the cell equilibrium. The foreign cells do even more than
this: they themselves may be devoured by the growing cells of the
tissue, seemingly being actuated by the same supreme idea of sacrifice
which led Buddha to give himself to the tigress.
The explanation of most of the changes which take place in
inflammation is obvious. It is a definite property of all living
things that repair takes place after injury, and certain of the
changes are only an accentuation of those which take place in the
usual life; but others, such as the formation of the exudate, are
unusual; not only is the outpouring of fluid greatly increased, but
its character is changed. In the normal transudation[2] the substances
on which the coagulation of the blood depends pass through the vessel
wall to a very slight extent, but the exudate may contain the
coagulable material in such amounts that it easily clots. The
interchange between the fluid outside the vessels and the blood fluid
takes place by means of filtration and osmosis. There is a greater
pressure in the vessels than in the fluid outside of them, and the
fluid filters through the wall as fluid filters through a thin
membrane outside of the body. Osmosis takes place when two fluids of
different osmotic pressure are separated by animal membrane.
Difference in osmotic pressure is due to differences in molecular
concentration, the greater the number of molecules the greater is the
pressure, and the greater rapidity of flow is from the fluid of less
pressure to the fluid of greater pressure. The molecular concentration
of tissue and blood fluid is constantly being equalized by the process
of osmosis. In the injured tissue the conditions are more favorable
for the fluid of the blood to pass from the vessels: by filtration,
because owing to the dilatation of the arteries there is increased
amount of blood and greater pressure within the vessels, and the
filtering membrane is also thinner because the same amount of membrane
(here the wall of the vessel) must cover the larger surface produced
by the dilatation. It is, moreover, very generally believed that there
are minute openings in the walls of the capillaries, and these would
become larger in the dilated vessel just as openings in a sheet of
rubber become larger when this is stretched. Osmosis towards the
tissue is favored because, owing to destructive processes the
molecular pressure in the injured area is increased; an injured tissue
has been shown to take up fluid more readily outside of the body than
a corresponding uninjured tissue. The slowing of the blood stream, in
spite of the dilatation of the vessels, is due to the greater friction
of the suspended corpuscles on the walls of the vessels. This is due
to the loss from the blood of the outstreaming fluid and the relative
increase in the number of corpuscles, added to by the unevenness of
surface which the attached corpuscles produce.
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