A number of explanations have been offered to show how a serous effusion is removed by Nature. It seems important to take these into consideration here, not because it is always best to imitate Nature in removing pathologic conditions, but in order that we may have a rational basis for our therapeutic methods. The oldest explanation is that of absorption of the fluid. It is evident that absorption may take place either by blood vessels or by lymphatics, but as a matter of fact absorption by blood vessels was usually referred to when the term absorption was used. Absorption by the blood vessels takes place through the veins; increased intra-pleural pressure increases it, but when this pressure is too great the veins themselves become compressed and absorption ceases. S. West1claims that this method of absorption plays a very small part in the removal of fluid from the pleural