This important question has been investigated experimentally by Grober1 in Stintzing's clinic in Jena. Of the various factors that have been emphasized as promoting absorption from the pleural cavities, Grober does not attach any importance to the aspiration on part of the thoracic duct nor to the vis a tergo of the blood pressure and of the tissue fluids. The purely physical forces of osmosis and capillary attraction are of some importance undoubtedly. Whenever accumulations of fluid in the pleural cavities are of less concentration than the blood serum and the lymph, the walls of the vessels in the pleuræ will act as semipermeable membranes and permit currents of fluid to pass into their contents, according to the general laws of osmotic pressure.
Thus Castaigne2 finds that during the first week of exudative pleuritis the pleural fluid is more concentrated than the serum of the blood. Aspiration would
THE RESORPTIVE POWERS OF THE PLEURA.. JAMA. 1901;XXXVII(13):837. doi:10.1001/jama.1901.02470390039005