282 EDEMA 



Loeb, 1 and Roth. 2 Loeb shows very clearly the relative greatness 

 of the water-driving force of osmotic pressure as compared to 

 that of blood pressure, by his statement that the osmotic pressure 

 of a physiological salt solution is about 4.9 atmospheres, which 

 is twenty times as great as the blood pressure with which we have 

 to do in ordinary physiological experiments. In varying osmotic 

 conditions we may readily see an explanation for the increased 

 lymph flow that occurs during tissue activity ; namely, it is due 

 to the increased formation of metabolic products. Many of the 

 lymphagogues may act similarly by stimulating metabolic activ- 

 ity, with resulting increase in the formation of osmotic pressure- 

 raising products of metabolism in the organs ; e. #., the increased 

 lymph flow from the thoracic duct that follows stimulation 

 of hepatic activity by injection of peptone (Heidenhain) or 

 ammonium tartrate (Asher and Busch 3 ). As we shall see later 

 in considering edema, osmotic pressure plays an important part 

 in the pathological formation of lymph. 



Summary. We see from the above discussion that 

 numerous theories have been advanced to explain the normal 

 formation of lymph, and as their basis exist several diiferent 

 possible factors. Filtration, active secretion by the capillary 

 endothelium, attraction by the tissue-cells, osmosis in response 

 to formation of crystalloids outside the vessels ; all have been 

 shown to be possible causes of lymph formation. It is highly 

 probable that in a certain way all are involved, particularly if 

 we accept the view of the physical school that " secretion " and 

 " attraction " by the cells are merely the outcome of osmotic 

 forces ; the causes of lymph formation then reduce themselves 

 to two, filtration and diffusion. There has been, until recently, 

 no question but that lymph does escape from the vessels through 

 simple filtration, for the pressure inside the capillaries is pre- 

 sumably greater than outside, the capillary walls are not water- 

 tight, and they are not impermeable to the substances dissolved 

 in the plasma. 4 Likewise osmotic exchanges surely go on 

 between the vessels and the tissue-cells. The question that 

 remains is, do these two factors account for all of the lymph 

 formation, and are they sufficient by themselves to explain the 



^finger's Arch., 1898 (71), 457. 



2 Englemann's Arch., 1899, p. 416. 



3 Zeit. f. Biol., 1900 (40), 333. 



*Hill ("Kecent Advances in Physiology and Biochemistry," 1906, p. 618) 

 disputes the possibility of such a thing as nitration pressure, on the ground 

 that the structures within the capsule of an organ must all be alike under the 

 influence of the blood pressure. 



