The transudation through the vessels takes place chiefly through those with the thinnest walls, the capillaries, although it is probable that a certain degree of transudation may also occur through the walls of the smallest veins. The causes which are instrumental in promoting the circulation of the blood--viz. the contraction and dilatation of the heart, the contraction of the arteries, the inspiratory action of the thorax, and muscular movements throughout the body--are also essential in producing the flow of lymph; and the existence of pressure upon the hæmic side of the filter is the first feature of importance in occasioning the transudation. The constant removal of the transudation from the outer side results from the pressure being less in this position. At the same time, an increase in the quantity of blood in the vessels is not necessarily productive of any considerable increase in the fluid transuded. Cohnheim calls attention to the experiments of Worm Müller, which show that a plethoric condition may readily be produced by the injection of quantities of blood into the circulation of animals, the amount of which cannot exceed twice the volume of the animal's blood without producing death. Although a temporary increase of the blood-pressure results, a return to the normal quickly follows. This is permitted by the propulsion of the excess of blood into the capillaries and veins, which become consequently distended, especially those of the abdominal organs. There is no increased transudation corresponding with the quantity of fluid introduced, nor is there any considerable distension of the blood-vessels of the skin, subcutaneous or intermuscular connective tissue. Such experiments show no permanent increase in the blood-pressure within the large veins if there is no obstruction to the admission of venous blood into the heart, presumably owing to their capacity for considerable distension.