Next comes the phenomenon whose clear recognition and description is inseparably connected with Cohnheim’s name. This is known under different names as _migration_ or _diapedesis of the leukocytes_. The program is about as follows: A little protrusion of the vascular wall, a marked alteration in the shape of a leukocyte, which yet adheres to this point of its lumen, and then the curious fact so often seen under the microscope--the gradual passage of this cell through the vascular wall, from its inner to its outer side, by what is generally known as its _ameboid movement_. This migration of the leukocyte is not confined to its mere escape from the restriction of the vessel lumen, but goes on to an indeterminate extent after it has detached itself from the outer surface of the vessel. This seems to occur by virtue of the same ameboid characteristic which it exhibited in passing through between the cells of the vessel itself. If this occurs at one point, it occurs at innumerable points, in consequence of which a large number of leukocytes escape into the tissues of the part involved. This diapedesis occurs most markedly from the smaller veins, to a less extent from the capillaries. The cells which escape from the latter are usually accompanied by red cells, the consequence being that the exudate which necessarily occurs at the same time is more or less tinged with the coloring matter of the blood, and is known as a hemorrhagic exudate. The above phenomenon, described in so few words, is in its minutiæ a really complex one, depending on a variety of causes not easily appreciated; but it is at least positive and well known, because it can be observed at will in the mesentery or web or tongue of certain animals which can be confined upon the stage of the microscope. The _phenomena of inflammation_, therefore, comprise, first, _hyperemia_, and then _escape_ from the bloodvessels of the _corpuscular_ and _fluid_ elements of the blood. The former may be due, as already seen, to various irritations of a non-specific character; while, as we shall learn, the latter practically never take place save when the irritation has been, as pathologists say, specific or infectious. The phenomena of true inflammation comprise practically the roles played by the three elements which conspire to produce those changes--namely, the _tissues_, the _blood_, and the _specific irritants_ which are the primary cause of the entire lesion. Each of these should be considered separately.