THE CAUSES OF EDEMA 339 



the blood, because they are more concentrated at the point of injection 

 than in the bh)()d, hence they may diffuse directly through the capil- 

 lary wall. Likewise we can understand the diffusion of water from 

 a hypotonic solution into the blood, but how a solution of the same 

 concentration as that of the blood can enter the blood is difficult to ex- 

 plain. Cohnstein and also Starling attribute this absorption to the 

 proteins of the blood in the following manner: After a fluid is in- 

 jected into the tissues or serous cavities there occurs a diffusion ex- 

 change between this fluid and the blood, until the concentration of the 

 crystalloids in each is equal; but the proteins of the blood cannot 

 diffuse, and as they exert a positive although very slight osmotic pres- 

 sure, this difference in osmotic pressure in favor of the blood causes 

 diffusion of the extravascular fluid into the blood. Roth has also ap- 

 plied this idea in a rather complicated manner to the absorption oc- 

 curring in metabolic processes (see Meltzer), but it must be admitted 

 that it is an unsatisfactory- solution of the problem. Fischer would 

 ascribe the passage of fluid to the relative affinity of the colloids of the 

 blood and of the tissues for the fluid, and this would be towards the 

 blood whenever the blood colloids had, from whatever possible cause, 

 a greater affinity for the fluid than the tissue colloids. 



Passage of the fluid from the tissues into the lymph stream was very 

 easy to understand in the light of the older conception of the lym- 

 phatic circulation, namely, that the lyinph-vessels were merely con- 

 tinuations of the interstitial spaces ; we could then assume that as 

 soon as the fluid left the blood-vessels it was practically within the 

 lymphatic system, and was crowded along the Ij-mphatic channels by 

 the vis a tergo, aided by the valves of the lymph-vessels and the intra- 

 thoracic vacuum. But it now seems, particularly through the studies 

 of MacCallum.^'^ that the lymphatic vessels form a closed system, not 

 in communication with the interstitial spaces. This being the case, 

 we have to explain the passage of the lymph through the walls of 

 the lymphatic vessels, and this is a problem which is not by any 

 means a simple one, and which has yet to be investigated. It is sig- 

 nificant that the thoracic lymph has a higher osmotic pressure than 

 the blood of the same animal (Luckhardt),-" so that the lymph which 

 enters the duct must do so against the osmotic pressure. 



THE CAUSES OF EIEMA 



With the facts and hypotheses mentioned in the preceding para- 

 graphs in mind, we may consider their bearing on the production of 

 abnormally large accumulations of lymph in the tissues, that is, edema. 

 We can imagine any one of the following factors as causing or helping 

 to cause such a pathological accumulation: 



19 Johns Hopkins Hosp. Bull., 1903 (14), 105. 

 20Anier. Jour. Physiol., 1910 (25), 345. 



