284 EDEMA 



the blood-vessels is still unknown. We can easily understand the 

 entrance of injected poisons and coloring- matters from the tissues 

 into the blood, because they are more concentrated at the point 

 of injection than in the blood, hence they may diifuse directly 

 through the capillary 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 explain. Cohnstein and also 

 Starling attribute this absorption to the proteids of the blood 

 in the following manner : After a fluid is injected into the 

 tissues or serous cavities there occurs a diffusion exchange be- 

 tween this fluid and the blood, until the concentration of the 

 crystalloids in each is equal ; but the proteids of the blood can- 

 not diffuse, and as they exert a positive although very slight 

 osmotic pressure, this difference in osmotic pressure in favor of 

 the blood causes diffusion of the extravascular fluid into the 

 blood. Roth has also applied this idea in a rather complicated 

 manner to the absorption occurring in metabolic processes (see 

 Meltzer), but it must be admitted that it is an unsatisfactory 

 solution of the problem. 



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 lymphatic circulation, namely, that the lymph-vessels 

 were merely continuations 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 lymphatic channels by the vis a tergo, aided by the 

 valves of the lymph- vessels and the intrathoracic vacuum. But 

 it now seems, particularly through the studies of MacCallum, 1 

 that the lymphatic vessels form a closed system, not in com- 

 munication 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. 



THE CAUSES OF EDEMA 



With the facts and hypotheses mentioned in the preceding 

 paragraphs in mind, we may consider their bearing on the pro- 

 duction of abnormally large accumulations of lymph in the tis- 

 sues, that is, edema. We can imagine any one of the following 

 factors as causing or helping to cause such a pathological accum- 

 ulation : 



1 Johns Hopkins Hosp. Bull., 1903 (14), 1. 



