nness of
surface which the attached corpuscles produce.

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 v