THE FLOW OF LYMPH 6 1 



selves to account for various occurrences in this connection, and 

 ascribe to the capillary endothelium some active secretory power 

 governing, or at least influencing, 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," pres- 

 sure 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 pres- 

 sure, so that "suction" is exerted upon the lymphatic ducts where 

 they enter those vessels. The lymph pressure in the extravas- 

 cular spaces is estimated to be one-half the capillary blood-pres- 

 sure. Friction and gravity (where the course of the vessels is 

 upward) oppose the passage of the fluid. Consequently it ac- 

 cumulates in the spaces and in the smaller lymphatics untU the 

 pressure there becomes greater than the resistance of these forces, 

 when it passes onward. Since lymph is being continually pro- 

 duced this superior pressure in the extravascular spaces and small 

 lymphatics is a fairly constant factor and keeps up a correspond- 

 ingly constant current. 



There are two factors which are accessory to this peripheral 

 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; furthermore, the 

 effect of aspiration makes itself felt directly upon the thoracic 

 duct since its greatest extent is in the thorax. (2) The valves of 

 the lymphatics act in a similar manner 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 supposed. Properly colored solutions injected into the blood 



