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.

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