FORMATION OF LYMPH 281 



become relaxed by any cause, such as poisons, high blood pres- 

 sure, poor nourishment, etc., the pores are enlarged, and in- 

 creased escape of fluids results. It must be borne in mind, 

 however, that not all histologists admit that capillary walls con- 

 tain pores. 



Osmotic Pressure. Still another important factor in 

 causing fluid to leave the vessels is osmotic pressure. Heiden- 

 hain refers to this cause the transudation produced by crystal- 

 loid lymphagogues, although in a rather unsatisfactory manner. 

 As a result of the more recent studies of physical chemistry, 

 and its application to biological processes, we have learned to 

 appreciate the importance of osmotic pressure in cell activi- 

 ties (see Introductory Chapter), and in the question of 

 lymph formation it occupies a particularly important place. 

 We may consider it as follows : In the blood we have certain 

 proportions of readily diffusible crystalloids and of non- 

 diffusible colloids. If no metabolic processes were going on in 

 the tissues, we should have the diffusible substances leaving the 

 vessel-walls (leaving out, for the present, any question of 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 proteids are not absolutely non-dif- 

 fusible, but small quantities 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 proteids ; this, it will be 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 proteids, carbohydrates, and 

 fats into much smaller molecules. Now the osmotic pressure 

 of a solution is dependent upon the number of molecules and 

 ions it contains, hence by breaking down these few large mole- 

 cules with very little osmotic pressure into many small mole- 

 cules, the osmotic pressure in these cells and tissues becomes 

 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, Heidenhain, and others. For suggesting and urging 

 the importance of osmotic pressure in the formation of lymph 

 we are indebted particularly to Heidenhain, v. Koranyi, 1 J. 

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



