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.