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.