THE CAUSES OF EDEMA 287 



the impression is growing that uncomplicated rise in blood 

 pressure is not sufficient by itself to produce edema. Some of 

 the reasons for belittling this factor will be brought out in the 

 subsequent discussion. 



3. Decreased Extravascular Pressure. This factor 

 is particularly prominent in the so-called " edema ex vacuo" 

 which occurs after the absorption of an area of tissue which is 

 so located that the surrounding tissues cannot contract or fall 

 in to fill the gap, e. g., brain softening, serous atrophy of fat. 

 A still better example, however, is the edema that follows local 

 decrease in atmospheric pressure in " cupping." In these 

 instances the edema depends partly upon increased transudation, 

 and partly on the retention of the fluid in the tissues, because 

 it cannot well leave them against the atmospheric pressure. 

 The idea advanced by Landerer that decreased elasticity of the 

 tissues was a possible cause of edema has been disproved by 

 Bonniger, 1 who found but little alteration in the elasticity of 

 tissues the seat of edema. Edema ex vacuo is again an illus- 

 tration of edema due to purely mechanical causes, but it is of 

 little practical importance. 



4. Increased Permeability of the Capillary Walls. 

 The importance of this factor in the production of edema was 

 first demonstrated by Cohnheim and Lichtheim, who found that 

 the production of an enormous increase in the amount of fluid 

 in the blood (hydrernic plethora) by injecting large quantities of 

 salt solution, caused an edema of the viscera and serous cavities, 

 but not any subcutaneous edema until the skin had been irritated 

 by some means, such as hot water, iodin, etc. By this irrita- 

 tion the capillary walls are injured, and an excessive escape of 

 the blood fluids follows. Magnus also showed that poisoning 

 with arsenic, which injures the vessels, favored the experimental 

 production of edema by transfusion. Starling, as noted before, 

 observed that the permeability of the capillaries varies normally 

 in different organs and tissues, which determines quantitative 

 and qualitative differences in the lymph normally flowing 

 from various vascular areas. Heidenhain's " lymphagogues 

 of the first class," which are all poisonous substances, prob- 

 ably act by increasing the permeability of the capillaries, and 

 in this way they produce local urticaria, which is often 

 observed as a result of poisoning by these same lymphagogues, 

 e. g,, shellfish and strawberry poisoning. Just what changes 

 are produced in the capillary walls that renders them more per- 

 meable we do not know. Possibly in some instances it is a 



1 Zeit. exp. Path. u. Ther., 1905 (1), 163. 



