LYMPH FORMATION 595 



which may be altered by food, excretion by the kidneys, &c. ; and 

 the composition of the tissue fluid, which is altered by metabolism 

 in the tissues. In both the latter cases the changes taking place 

 in the lymph are brought about by diffusion. 



The experimental evidence brought forward by Ludwig in 

 support of his theory was an attempt to show that there was a 

 direct relationship between capillary blood pressure and lymph 

 flow from a part. He and his pupils showed that lymph flow was 

 increased by the following means : by obstructing the venous 

 return, by overfilling the vascular system with either blood or 

 normal salt solution. Vaso-dilatation brought about by section of 

 vaso-constrictor nerves gave variable results, but active vaso-dila- 

 tation from stimulation of vaso- dilator nerves generally caused an 

 increased lymph flow. Most subsequent observers have confirmed 

 these results. But Lazarus-Barlow did not find that ligature of 

 the femoral vein in a dog increased the lymph flow, although the 

 pressure in the vein was from 50 to 75 mm. Hg for an hour. 

 Although, in the extremities at any rate, no very striking parallelism 

 between lymph flow and alterations of capillary blood pressure 

 either from the arterial or venous side has been found to exist, 

 there can be no doubt that variations in capillary blood pressure 

 are associated with an interchange of fluid between the capillaries 

 and tissue spaces. For, estimations of the corpuscular and haemo- 

 globin content of venous blood have shown that increased capillary 

 blood pressure is associated with an increased concentration of the 

 blood and a decrease of blood pressure with a decreased con- 

 centration. 



With regard to diffusion between blood and lymph, it had 

 been known long before Ludwig' s theory was published that dyes, 

 sugar, salts, &c., introduced into the blood or lymph soon appeared 

 in the other. 



Of these two factors, diffusion and filtration, no one, as we 

 shall see, doubts that diffusion may play a not inconsiderable part 

 in experimental and physiological lymph formation. But in re- 

 gard to filtration both Heidenhain and Hamburger have denied 

 that it is an important factor even in experimental lymph forma- 

 tion, and we must now consider their objections to Lud wig's view. 



View of Heidenhain. This view was published in 1891. Ex- 

 perimenting on the lymph flow from the thoracic duct in dogs, 

 Heidenhain came to the conclusion that it was impossible to explain 



