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Page 23
The wonderful migration of the leucocytes, which seems to show a
conscious protective action on their part, takes place under the
action of conditions which influence the movement of cells. When an
actively moving amoeba is observed it is seen that the motion is not
the result of chance, for it is influenced by conditions external to
the organism; certain substances are found to attract the amoebae
towards them and other substances to repel them. These influences or
forces affecting the movements of organisms are known as
_tropisms_, and play a large part in nature; the attraction of
various organisms towards a source of light is known as
_heliotropism_, and there are many other instances of such
attraction. The leucocytes as free moving cells also come under the
influence of such tropisms. When a small capillary tube having one end
sealed is partially filled with the bacteria which produce abscess and
placed beneath the skin it quickly becomes filled with leucocytes,
these being attracted by the bacteria it contains. Dead cells exert a
similar attraction for the large phagocytes. Such attraction is called
_chemotropism_ and is supposed to be due in the cases mentioned,
to the action of chemical substances such as are given off by the
bacteria or the dead cells. The direction of motion is due to
stimulation of that part of the body of the leucocyte which is towards
the source of the stimulus. The presence in the injured part of
bacteria or of injured and dead cells exerts an attraction for the
leucocytes within the vessels causing their migration. When the centre
of the cornea is injured, this tissue having no vessels, all the
vascular phenomena take place in the white part of the eye immediately
around the cornea, this becoming red and congested. The migration of
leucocytes from the vessels takes place chiefly on the side towards
the cornea, and the migrated cells make their way along the devious
tracts of the communicating lymph spaces to the area of injury. The
objection may be raised that it is difficult to think of a chemical
substance produced in an injured area no larger than a millimeter,
diffusing through the cornea and reaching the vessels outside this in
such quantity and concentration as to affect their contents, nor has
there been any evidence presented that definite chemical substances
are produced in injured tissues; but there is no difficulty in view of
the possibilities. It is not necessary to assume that an actual
substance so diffuses itself, but the influence exerted may be thought
of as a force, possibly some form of molecular motion, which is set in
action at the area of injury and extends from this. No actual
substance passes along a nerve when it conveys an impulse.
We have left the injured area with an increased amount of fluid and
cells within it, with the blood vessels dilated and with both cells
and fluid streaming through their walls, and the cells belonging to
the area actively repairing damages and multiplying. The process will
continue as long as the cause which produces the injury continues to
act, and will gradually cease with the discontinuance of this action,
and this may be brought about in various ways. A foreign body may be
mechanically removed, as when a thorn is plucked out; or bacteria may
be destroyed by the leucocytes; or a poison, such as the sting of an
insect, may be diluted by the exudate until it be no longer injurious,
or it may be neutralized. Even without the removal of the cause the
power of adaptation will enable the life of the affected part to go
on, less perfectly perhaps, in the new environment. The excess of
fluid is removed by the outflow exceeding the inflow, or it may pass
to some one of the surfaces of the body, or in other cases an incision
favors its escape. The excess of cells is in part removed with the
fluid, in part they disappear by undergoing solution and in part they
are devoured by other cells. With the diminishing cell activity the
blood vessels resume their usual calibre, and when the newly formed
vessels become redundant they disappear by undergoing atrophy in the
same way as other tissues which have become useless.
When these changes take place rapidly the inflammation is said to be
acute, and chronic when they take place slowly. Chronic inflammation
is more complex than is the acute, and there is more variation in the
single conditions. The chronicity may be due to a number of
conditions, as the persistence of a cause, or to incompleteness of
repair which renders the part once affected more vulnerable, to such a
degree even that the ordinary conditions to which it is subjected
become injurious. A chronic inflammation may be little more than an
almost continuous series of acute inflammations, with repair
continuously less perfect. Chronic imflammations are a prerogative of
the old as compared with the young, of the weak rather than the
strong.
FOOTNOTES:
[1] The term exudation is used to designate the
passing of cells and fluid from the vessels in inflammation; the
material is the exudate.
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