If, now, the web be injured by pricking it or placing some irritating substance upon it, a change takes place in the circulation.  The arteries and the veins become dilated and the flow of blood more rapid, so rapid, indeed, that it is difficult to distinguish the single corpuscles.  In a short while the rapidity of flow in the dilated vessels diminishes, becoming slower than the normal, and the separation between the red and white corpuscles is not so evident.  In the slowly moving stream the white corpuscles move much more slowly than do the red, and hence accumulate in the vessels lining the inner surface and later become attached to this and cease to move forward.  The attached corpuscles then begin to move as does an amoeba, sending out projections, some one of which penetrates the wall, and following this the corpuscles creep through.  Red corpuscles also pass out of the vessels, this taking place in the capillaries; the white corpuscles, on the other hand, pass through the small veins.  Not only do the white corpuscles pass through the vessels, but the blood fluid also passes out.  The corpuscles which have passed into the tissue around the vessels are carried away by the outstreaming fluid, and the web becomes swollen from the increased amount of fluid which it contains.  The injured area of the web is more sensitive than a corresponding uninjured area and the foot is more quickly moved if it be touched.  If the injury has been very slight, observation of the area on the following day will show no change beyond a slight dilatation of the vessels and a great accumulation of cells in the tissue.

Everyone has experienced the effect of such changes as have been described in this simple experiment.  An inflamed part on the surface of the body is redder than the normal, swollen, hot and painful.  The usual red tinge of the skin is due to the red blood contained in the vessels, and the color is intensified when, owing to the dilatation, the vessels contain more blood.  The inflamed area feels hot, and if the temperature be taken it may be two or three degrees warmer than a corresponding area.  The increased heat is due to the richer circulation.  Heat is produced in the interior of the body chiefly in the muscles and great glands, and the increased afflux of blood brings more heat to the surface.  A certain degree of swelling of the tissue is due to the dilatation of the vessels; but this is a negligible factor as compared with the effect of the presence of the fluid and cells of the exudate.[1] The fluid distends the tissue spaces, and it may pass from the tissue and accumulate on surfaces or in the large cavities within the body.  The greatly increased discharge from the nose in a “cold in the head” is due to the exudation formed in the acutely inflamed tissue, and which readily passes through the thin epithelial covering.  Various degrees of inflammation of the skin may be produced by the action of the sun, the injury being due not to the heat but to the actinic