{42} The exudation is the most essential element of the swelling, and our knowledge of its origin and fate includes the most important features of the general pathology of the processes concerned. The inflammatory exudation is represented by the accumulation, outside the blood-vessels, of material previously within them. The prevailing views concerning the manner of origin of this exudation, and its relation to inflammatory processes, are essentially due to the rediscovery by Cohnheim of the forgotten observation of Addison, that white blood-corpuscles pass through the apparently intact walls of the blood-vessels. In the observation of the mesentery or other transparent part of a suitable animal, the changes taking place in inflammation are, at the outset, limited to the blood-vessels and their immediate vicinity. The vessels become dilated and the rapidity of the flow within them is soon diminished. In the veins particularly the white blood-corpuscles separate in considerable numbers from the general current and line the wall in constantly-increasing numbers, while the red corpuscles are borne along the middle of the stream. The white corpuscles stagnate, stick to the wall for a longer or shorter time, and often change their place, while the red corpuscles are in constant and progressive motion. In the capillaries a considerable number of white corpuscles are found in contact with the wall, but numbers of red corpuscles are associated with them. The formation of the exudation now begins by the passage of white corpuscles through the apparently intact wall of the veins and capillaries, especially of the former. Limited numbers, under ordinary circumstances, of red corpuscles also make their way through the walls of the capillaries. This is the phenomenon of emigration, and is associated with the amoeboid movements of the white corpuscles. With the passage outward of the white and red corpuscles there is also the effusion of liquid material. Both the liquid and solid constituents continually escape and spread in all directions beyond the wall, following the course of the least resistance. It is probable that this course is defined by the pre-existing spaces within the tissues of the part, the lymph-spaces. The exudation is more abundant in parts richly provided with blood-vessels and in those containing the larger spaces; it is diminished where the vessels are less numerous or the surrounding parts more resistant, with smaller and fewer lymph-spaces. The resulting swelling is the less when ready opportunities for the diffusion and removal of the exudation by lymphatics and veins are presented, and when the material appears upon surfaces over which it may flow away.