FORMATION OF LYMPH 335 



colloids. If no metabolic processes were going on in the tissues, wo 

 should have the diffusible substances leaving the vessel-walls (leaving 

 out, for the present, any question of secretory activity on the part of 

 the endothelium) until an osmotic equilibrium is established in the 

 tissues and in the blood. As a matter of fact, however, the blood pro- 

 teins are not absolutely non-diffusible, but small (juantities do pass 

 through the capillary walls, and so lymph under such a hypothetical 

 condition would consist of a mixture of the same osmotic concentration 

 as the blood plasma, with about the same proportion of crystalloids, 

 but a smaller proportion of proteins; this, it will l)e noticed, is just 

 about the composition of normal lymph. During life, however, the 

 cells of the tissues are causing metabolic changes in these lymphatic 

 constituents, and these changes consist chiefly in breaking down large 

 molecules of proteins, carbohydrates, and fats into much smaller 

 molecules. Now the osmotic pressure of a solution is dependent upon 

 the yiumher of molecules and ions it contains, hence by breaking down 

 these few large molecules with very little osmotic pressure into many 

 small molecules, the osmotic pressure in these cells and tissues be- 

 comes raised above that of the blood-vessels, and consequently water 

 flows out of the vessels because of the increased pressure. We see here 

 the probable explanation of the stimulating influence of metabolic 

 products upon the formation of lymph, noted by Hamburger, Heiden- 

 hain, and others. For suggesting and urging the importance of osmo- 

 tic pressure in the formation of lymph we are indebted particularly 

 to Heidenhain, v. Koranyi,^^ J. Loeb,^^ and Roth.'^ 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 atmos- 

 pheres, 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 in- 

 creased formation of metabolic products. Many of the lymphagogues 

 may act similarly by stimulating metabolic activity, with resulting in- 

 crease in the formation of osmotic pressure-raising products of metab- 

 olism in the organs; e. g., the increased lymph flow from the thoracic 

 duct that follows stimulation of hepatic activity by injection of pep- 

 tone (Heidenhain) or ammonium tartrate (Asher and Busch).'* As 

 we shall see later in considering edema, osmotic pressure may play an 

 important part in the pathological formation of lymph. It must be 

 admitted, however, that there are many difficulties in the way of 

 accepting unqualifiedly the original views as to the importance of 



" Zeit. f. klin. Med., 1897 (33), 1; 1898 (34), 1.- 



12 Pfliiger's Arch., 1898 (71), 457. 



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

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



