ORIGIN AND COMPOSITION OF LYMPH 363 



If the lymph were derived from the blood by mere leakage or 

 filtration through the walls of the bloodvessels, the rate of leakage 

 should be greater, the greater the pressure of the blood. The rate of 

 flow of lymph in the Thoracic Duct, however, does not always decrease 

 when the arterial blood-pressure decreases, nor does it always increase 

 when the arterial pressure increases. Then, again, the injection of 

 strong salt solutions into the circulation might be expected to with- 

 draw fluid from the lymph-spaces by osmotic attraction, yet the lymph- 

 flow from the thoracic duct is actually increased by this procedure. 

 Finally certain specific substances, particularly crayfish extract, and 

 extracts of leeches or shell-fish, certain Proteoses and also the South 

 American arrowhead poison Curare cause a very great increase in the 

 flow of lymph, as Heidenhain supposed, by stimulating the secretory 

 activity of the vessel-walls through which the lymph issues into the 

 interstices of the tissues. 



Nevertheless Starling has conclusively demonstrated that the pro- 

 duction of lymph is, after all, a process of passive filtration. The 

 phenomena adduced by Heidenhain, convincing as at first sight they 

 appear to be, are nevertheless simply attributable to .the fact that the 

 Permeability of the bloodvessels for lymph varies very greatly in differ- 

 ent parts of the body. These differences in permeability lead to differ- 

 ences in the rate of filtration of lymph no less pronounced than the 

 difference in the rate of filtration of water through paper and through 

 unglazed porcelain. The most permeable vessels are the capillaries 

 in the Liver, while the capillaries in the skeletal muscles are almost 

 impermeable. We can render the capillaries in the leg-muscles per- 

 meable by heating them to 56 C., and in this way cause such extensive 

 transudation of lymph that a frog's leg, so treated, becomes rapidly 

 edematous. If the blood-pressure in the liver be raised or lowered 

 the lymph-flow is raised or lowered in like proportion, but the pressure 

 in the liver and that in the general arterial system do not always run 

 parallel, so that the departures from parallelism between arterial pres- 

 sure and lymph-flow observed by Heidenhain were not inconsistent 

 with the view that lymph is a transudate, mainly furnished by the 

 vessels of the liver. Strong salt or sugar solutions simply alter the 

 distribution of the interstitial fluids, causing a general imbibition of 

 fluid into the vascular system, and a Hydremic Plethora which results 

 in readjustment by more rapid filtration into the lymph-spaces in the 

 liver. If we previously withdraw from the vascular system enough 

 blood to equal the volume of fluid which is attracted into it by the 

 subsequent injection of salt or sugar, no plethora results, and no 

 increased flow of lymph ensues. 



The various Lymphagogues or lymph-producing substances alluded 

 to above cause an increased transudation by the injury they cause to 

 the walls of the blood-vessels, greatly increasing their Permeability, 

 and producing an effect analogous to that of heating to 56 C. 



The composition of lymph is very variable. In general it may be 



