180 FRANKLIN C. McLEAN 



in the movement of water or in an electric current. A similar relation 

 applies to the movement of lymph, whether we consider the movement of 

 water alone or include that of its dissolved substances. 



If the tissues are regarded as a heterogeneous system in the physico- 

 chemical sense, the liquid phases of the system, each composed of water 

 with dissolved and suspended substances, are represented by the con- 

 tents of the red blood cells, the blood plasma, the tissue fluids in the 

 intercellular spaces, the intracellular fluids, and the contents of the lymph 

 vessels. These phases are separated from one another by more or less 

 permeable membranes, all of them certainly permeable to water and to 

 certain dissolved substances. The whole system is under a certain 

 pressure, and at the temperature of the body. Both the pressure and tem- 

 perature, for any locality selected, are approximately constant and uni- 

 form. The various phases tend to maintain a correlated state or equi- 

 librium, and any force which tends to disturb this state is met by a shift in 

 equilibrium. Equilibrium, under the conditions of the body, is maintained 

 by a continual shift of volume between the phases, and it is this process 

 which is concerned in fluid exchange and in lymph production. 



The mechanism of the entrance of fluid into the lymphatic circulation 

 is not at all clear, especially since the weight of evidence at present is to 

 the effect that the endings of the lymph vessels in the tissues are closed 

 (MacCallum(6)), and are therefore not directly continuous with the tissue 

 spaces. Cohnheim believed that lymph is an overflow from the tissue 

 spaces into the lymphatics. In the absence of further evidence, it must be 

 assumed that fluid passes into the lymphatics as a result of the same 

 forces that control its passage through the capillary walls, and that it still 

 remains in equilibrium with the fluid in the tissue spaces until it is carried 

 away from intimate contact with them. 



The flow of lymph in the lymph channels is apparently influenced in 

 part by hydrostatic pressure, especially in the larger channels, but the flow 

 of lymph in the smaller lymphatics seems to be due to mechanical pressure 

 from outside, chiefly as the result of muscular activity. Since the lym- 

 phatics are supplied with valves at short intervals, the result of such pres- 

 sure is the movement of fluids in one direction only. 



The Nature of Edema. Under abnormal conditions, the equilibrium 

 which maintains the tissue at a fairly definite volume is disturbed in 

 such a manner as to result in an increased amount of fluid in the tissue 

 spaces or cells themselves that is to say, in edema. From the standpoint 

 of the theoretical considerations already mentioned a change responsible 

 for such a disturbance may originate in any part of the system just de- 

 scribed ; that is to say, in the blood, intercellular fluids, the cells, the lymph 

 vessels, or any of the membranes through which exchange of fluid takes 

 place. It is evident that the condition must be maintained by the con- 

 tinuous action of the disturbance which brought it about. Otherwise there 



