118 THE BLOOD AND THE LYMPH 



EXPERIMENTAL INVESTIGATIONS 



It has proved a most difficult problem to gain any exact kiiOAvledge 

 of the production of lymph by experimental means. Starling, some years 

 ago, in repeating many of the experiments of older physiologists in the 

 light of the newer facts of physical chemistry, added much that is of 

 interest, and it is chiefly with his work that we will concern ourselves 

 here. 



The unequal lymph supply of different regions of the body is strik- 

 ingly demonstrated by comparing the lymph flow from the lymphatics 

 of the leg with that from the thoracic duct. No lymph flows from the 

 former unless the muscles are thrown into activity or the blood is pre- 

 vented from leaving the limb by ligaturing all the veins. Changes in the 

 arterial blood pressure do not affect the flow. On the other hand, a great 

 increase in the flow from the thoracic duct can readily be induced by 

 disturbances in the blood supply. Obstruction of the portal vein, for 

 example, immediately increases the lymph flow four or five times because of 

 venous congestion in the intestinal capillaries, whilst a still greater 

 increase perhaps tenfold is induced by obstruction to the inferior vena 

 cava, which raises the capillary pressure in both the liver and the intes- 

 tines. After ligation of the hepatic lymphatics (at the hepatic pedicle), 

 obstruction of the vena cava no longer causes the outflow of lymph to 

 increase, indicating that the lymph in the last mentioned experiment 

 must have come from the hepatic lymphatics. 



These results, so far as they go, could be satisfactorily explained on 

 the basis that lymph formation is a filtration process, that is, a process 

 dependent upon difference in mechanical pressure between the blood 

 capillaries and the tissue spaces. The lymphatics would then serve as 

 channels to return this fluid to the blood vessels through the thoracic 

 duct. The difference in the magnitude of the increased lymph flow from 

 increase in capillary pressure in different regions would be dependent 

 on the permeability of the filter, the capillaries of the limbs being much 

 less permeable than those of the intestine, and particularly of the liver. 

 Another fact in conformity with this view concerns the composition of 

 the lymph from the two regions, that from the limb lymphatics being 

 poor in protein, whereas that from the thoracic duct does not fall far 

 behind the blood plasma in this regard. 



Although filtration may explain the considerable increase in lymph 

 flow produced by extreme changes in capillary pressure, it by no means 

 suffices to explain lymph formation under less abnormal conditions. 

 When a muscle or a gland is at rest, it produces practically no lymph, 



