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 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.