EDEMA 183 



sponsible for kidney disease or nephrotoxins. Volhard accepts this ex- 

 planation as the primary cause of edema, but he finds it necessary to add 

 to it the conception of disturbance of the reabsorption of fluids from the 

 tissues by the capillaries. 



On the basis of the theoretical considerations outlined above, we may 

 accept an altered permeability of the capillary walls as a cause of edema, 

 if the permeability becomes altered to such an extent that there is actually 

 leakage of molecules of a kind which otherwise would have been held 

 back by the capillary wall. We should then expect to find the protein 

 content of subcutaneous effusions approaching that of the blood serum, 

 or at least exceeding that of the normal tissu'e fluids. The difficulty of 

 collection has interfered with all efforts to secure normal tissue fluids for 

 comparison. Eppinger(d), however, has injected salt solution subcu- 

 taneously, with and without gelatin, and has found that three per cent of 

 gelatin in physiological salt solution greatly hinders the appearance of salt 

 in the urine after such injections. On the basis of this indirect evidence 

 he accepts as an important factor in the production of edema a qualitative 

 increase in the permeability of the capillary walls. 



If we accept increased permeability of the capillary walls in the 

 quantitative sense, for the passage of substances in one direction, as this 

 explanation has been accepted by some recent writers (Volhard), we must 

 assume a specific function on the part of these cells, similar to^the func- 

 tion of cells having to do with secretion, since permeability in one direction 

 only is not recognized in non-living membranes. The situation is analo- 

 gous to that in the lungs, as was disclosed in the recent discussion on the 

 oxygenation of blood. Certain investigators believed that alveolar epi- 

 thelium actually secretes oxygen into the blood, while others believed that 

 the whole phenomenon is readily explainable on simple physico-chemical 

 grounds. 



Certain other phases of the effect of hydrostatic pressure seem to be 

 of importance in the occurrence of edema, especially as they relate to the 

 flow of lymph through the lymph channels. In the edema of heart failure 

 the influence of gravity seems to be of particular importance. This in- 

 fluence is readily observed on inspection of the patient, The localization of 

 edema depends on a combination of all the other forces concerned and the 

 effect of gravity. Edema occurs first in regions where the effect of gravity 

 tends to hinder venous and lymphatic return, and occurs late or not at 

 all in regions where the effect of gravity tends to facilitate this return. 

 From these considerations it follows that hydrostatic pressure, and par- 

 ticularly the influence of gravity, is a factor in the equilibrium at all 

 times, and may prove to be the factor which determines edema. 



Diffusion Pressure. Diffusion pressure is the force which results 

 in the movement of molecules or ions in a solution, from regions of 

 greater to regions of lesser concentrations. In a solution we may differen- 



