At first the leukocytes adhere but slightly to the walls of the blood-vessel, assuming a pear shape, the enlarged ends pointing in the direction of the current. In the course of five or six hours all the small veins of the involved area may show a mass of leukocytes along their walls. These in time become closely attached, pass through the vessel walls, and, finally, may become pus cells.

As a rule, the greater number of leukocytes that escape are of the polymorphonuclear variety. They project a small mass of protoplasm through the vessel wall. This mass becomes gradually larger until the cell lies outside in the surrounding tissues. This process is known as emigration.

Diapedesis refers to the escape of red cells from vessels whose walls show no lesions. •

At the same time that the cells escape there is an exudation or outflow of lymph through the vessel walls. This increased amount of lymph renders the tissue edematous and gives room for free ameboid movement of the leukocytes. Many are actively phagocytic, many die, while some get back into the lymph-vessels and return to the general circulation.

Fig. 23. - Acute Inflammation (Mallory).

Emigration and accumulation of polymorphonuclear leukocytes in the subcutaneous tissues of a rabbit's ear as the result of rubbing the surface with dilute croton oil.

As to the emigration of the leukocytes there are various theories, but the reasons are not perfectly understood. The phenomena can hardly be due to nervous influences, as the changes occur too slowly. It is also impossible to bring about an inflammatory reaction by stimulating either the vasoconstrictors or the vasodilators. When the latter is done, there is an exudation of plasma, but not of cells. According to Cohnheim, there is an increased permeability of the bloodvessel wall due to structural changes.

Probably the chief reason is that the ameboid motion of the leukocytes is very much stimulated.

It may also be the result of positive chemotaxis, the attraction that certain substances exert upon motile cells. Dead tissues and the products of bacterial growth are positively chemotactic, and their influence may be exerted upon the leukocytes while they are still within the blood-vessel.

Besides the polymorphonuclear leukocyte the round mononuclear form may also escape, giving rise to the round-cell infiltration that is found in subacute or chronic inflammation, particularly in tuberculosis and syphilis.

As a result of the disturbances of the circulation there are certain changes in the inflamed part as a whole that are frequently spoken of as the cardinal symptoms of inflammation:

Pain, or dolor, due probably to the pressure exerted upon the terminal nerve-filaments. Also to the action of toxins, acids, enzymes, etc., upon the nerve-endings.

Swelling, or tumor, due to the increased amount of blood present and to the exudate within the tissues.

Redness, or rubor, due also to the hyperemia. The increase of blood to the involved part brings more leukocytes, diluents, and antibodies, facilitates the removal of harmful substances, and possibly in some instances, affords increased nutrition to the cells in that area.

Heat, or calor, the result of two causes, one that more blood is brought to the part, the other that the blood moves more slowly and heat accumulates.

Altered junction, or functio laeso, may be added to the first four.