THE FLOW OF LYMPH 6 1 



insufficient in themselves to account for various occurrences 

 in this connection, and ascribe to the capillary endothelium 

 some active secretory power governing, or at least influenc- 

 ing, the outward passage of the plasma. 



The Flow of Lymph. There is no organ corresponding to 

 the heart to keep the lymph current in motion. The main 

 causes for its direction from the extravascular spaces toward 

 the veins in the neck is the degree of pressure to which it is 

 subjected in those spaces as compared with the inferior, or 

 even "negative," pressure obtaining near the terminations of 

 the great ducts. It is known that at all times the venous 

 pressure in the subclavian veins is low and that it may even 

 fall below the atmospheric pressure, so that "suction" is ex- 

 erted upon the lymphatic ducts where they enter those ves- 

 sels. The lymph pressure in the extravascular spaces is esti- 

 mated to be one-half the capillary blood-pressure. Friction 

 and gravity (where the course of the vessels is upward) op- 

 pose the passage of the fluid. Consequently it accumulates 

 in the spaces and in the smaller lymphatics until the pressure 

 there becomes greater than the resistance of these forces, 

 when it passes onward. Since lymph is being continually 

 produced this superior pressure in the extravascular spaces 

 and small lymphatics is a fairly constant factor and keeps up 

 a correspondingly constant current. 



There are two factors which are accessory to this peri- 

 pheral pressure : ( i ) Thoracic aspiration by bringing about 

 negative pressure in the veins in and near the chest brings 

 about a like condition in the tributary lymphatic ducts ; fur- 

 thermore, the effect of aspiration makes itself felt directly 

 upon the thoracic duct since its greatest extent is in the tho- 

 rax. (2) The valves of the lymphatics act in a similar man- 

 ner to those of the veins and constitute a very necessary 

 factor in the lymphatic circulation. Although the lymph 

 flow resembles that of the venous blood, it is less regular and 

 more sluggish, but probably not so slow as might be sup- 

 posed. Properly colored solutions injected into the blood 



