The membranes studied have been the mesentery of the living and dead animal, and the parietal pericardium and pleura, post mortem. The membranes of dogs, cats, rabbits, and two human pericardia have been studied. All these membranes are essentially sheets of connective tissue, bearing blood vessels, lymphatics, and nerves, and in some instances fat cells, and lined on each surface by a single layer of pavement mesothelial cells. Intercellular fibers form the major bulk of the lean membranes. These are predominantly collagenous except in the pleural region used, in which elastin fibers are present in large proportion.

By using buffers alternately more acid or more alkaline than the reversal point, the direction of liquid flow across any given membrane site could be reversed an indefinite number of times. The time interval required is only that taken by the requisite manipulation in changing buffers and making the runs.

The mean values of the reversal points for the several membranes when bathed in hypotonic, unbalanced buffer and in isotonic, physiologically balanced buffers showed only small and inconstant differences. The fat and lean pericardia similarly showed small difference in the positions of the mean reversal points.

The apparent reversal points for the mesenteries of living animals proved to be lower than those for the mesenteries post mortem. This low value in the animals with functioning circulation is interpreted as essentially due to admixture with the buffers of buffer salts from the blood. Clear differences between the reversal points of the membranes in the first compared with later hours or days post mortem were not detected, with the possible exception of a small shift toward the acid range of the fat pericardium reversal points estimated several days post mortem. The reversal points with cat membranes were somewhat lower (more acid) than those of the dog.

The approximate mean reversal points found with the citrate-phosphate buffers used were as follows: For mesenteries of living animals pH = 4.4; for mesenteries, post mortem, pH = 4.8; for cat pleuræ, post mortem, pH = 4.3; for dog pleuræ, post mortem, pH = 5.0; for lean and fat pericardia, post mortem, pH = 5.1. The mean reversal point of the two human pericardia was about pH = 5.0. Reversal points determined with buffers containing only monovalent anions are somewhat higher (less acid), as will be shown later.

The bearing of these data on the question of the chemical composition of the surfaces of the fat cells of the serous membranes is discussed. Briefly, it is believed that proteins are probably present in important amount in these cell surfaces.