Although experiments show that a simple plethora with great distension of the capillaries of the abdominal organs occasions no considerable increase of transudation, a different result follows a hydræmic plethora[25] induced by the injection of immense quantities of salt water into the blood-current--often six times as much liquid as the animal had blood. Here, too, the arterial blood-pressure shows no permanent increase, nor does that within the large veins become perceptibly increased till enormous quantities of fluid are injected. The blood flows through the vessels with increased rapidity in consequence of the diminished friction of the diluted blood, and an increased transudation begins at once. The various glands, salivary and gastro-intestinal, kidneys and liver, secrete more copiously, and the flow of a dilute lymph from the thoracic duct becomes greatly increased, while that from the cervical lymphatics becomes moderately accelerated. The lymph from the extremities, however, is no greater in quantity than that flowing from an animal in a perfectly normal condition. The localization of the increased transudation from the blood-vessels is further characterized by the abundant accumulation of watery fluid in all the abdominal organs and abdominal cavity, in the salivary glands and surrounding connective tissue, while elsewhere in the body the organs and tissues are almost invariably in the same condition with {69} regard to moisture as are those of a healthy animal under normal circumstances. [Footnote 25: Cohnheim and Lichtheim, _Virchow's Archiv_, 1877, lxix. 106.] The importance of these experiments with reference to the causes of the transudation of fluid from the blood is obvious. The pressure upon the walls of the blood-vessels cannot become sufficiently increased to be accompanied with augmented transudation until limits are reached which are beyond the possibilities of occurrence in the human body. When such limits are attained in animals, the increased pressure, however great it may be, does not suffice to produce a general transudation, but one limited to the vessels of those parts of the body whose normal function is connected with too abundant transudation of fluid. A simple hydræmic condition of brief duration has been proven, by experiment, insufficient to give rise to increased transudation, neither increased secretion nor increased flow of lymph taking place. The inference from these experiments is that an increased transudation is more dependent upon conditions of the filter than upon those of blood-pressure. The absence of any observable changes in the filter leads to the assumption of an increased permeability, of physiological occurrence in certain parts of the body, as the chief feature in the occurrence of increased transudations.