120 THE BLOOD AND THE LYMPH 



EDEMA 



With such an imperfect knowledge concerning the physiology of 

 lymph formation, it is not surprising that the causes of excessive accu- 

 mulation of fluid in and between the tissue elements should he little un- 

 derstood. All of the conditions which have been mentioned as capable 

 of causing an increased secretion of lymph — such as increase in capillary 

 pressure, hydremic plethora, action of poisons on the endothelium — are 

 likely to cause edema if the lymphatics of the part are simultaneously 

 obstructed. To produce in animals edema of the subcutaneous tissues 

 like that observed clinically, it is, however, necessary that'the vascular 

 disturbance be accompanied either by local damage to the capillary 

 endothelium, such as is produced by arsenic or uranium; or by a gen- 

 eral toxemic condition, such as is set up by nephritis. "When large 

 amounts of saline solution are injected intravenously, extensive ex- 

 travasation of fluid may occur into the liver, peritoneum and intestinal 

 lumen, without any subcutaneous edema. 



Clinical edemas are of at least three types: 



1. The inflammatory edemas, in which the fluid permeates the cells of 

 the inflamed area and does, not shift to other parts of the body under 

 the influence of gravity. 



2. The nephritic edemas, in which the fluid is more or less loose in the 

 subcutaneous tissues and readily changes its position, and which is 

 accompanied by excess of water in the blood with a corresponding in- 

 crease of sodium chloride ; the percentage concentration of sodium chlo- 

 ride in the blood remains unchanged, but that the other constituents 

 diminished. 



3. Cardiac edemas, which are also hypostatic, but are unaccompanied 

 by changes in the relative amount of water and sodium chloride in the 

 blood. 



The second and third varieties of edema may of course be more or 

 less present together, for the kidneys are likely to become secondarily 

 affected during venous stasis. 



The salt retention in nephritic edema is very significant. As ex- 

 plained elsewhere, it is revealed by comparing the daily output of so- 

 dium chloride by the urine with the concentration of this salt in the 

 blood. Less salt is eliminated than would be the case in a normal in- 

 dividual Avith the same percentage of salt in the blood. Tn many cases 

 also edema can be diminished by withholding salt from the food. Widal 

 and Javal have conclusively shown the relationship of retention of water 

 in the body, as judged by variations in body weight, to the hydremic 

 condition, as judged by the refractive index of the blood serum, and 



