THE FORMATION OF LYMPH 



121 



water but not to the gum. The gum attracts water to its 

 side of the membrane, but does not itself traverse the 

 membrane. 



Substituting the wall of the capillaries for the paper used 

 in the preceding ex[)eriment we have the conditions 

 necessary for a possibly diffusive 

 interchange between the blood on 

 the one side of that wall and the 

 fluid in the tissues on the other. 

 But here we may say at once that 

 diffusion will not account at all 

 completely for the formation of 

 lymph. In support of this state- 

 ment it may suffice to point out 

 that lymph contains a considerable 

 amount of proteins, and these are 

 characteristically non-dift'usible.^ 



On the other hand the blood- 

 pressure in the capillaries, though 

 much less than in the arteries, is 

 not inconsiderable, and is exerted 

 against the walls of these vessels. 

 Can we then account for the for- 

 mation of lymph as the result of 

 filtration ? Here again we may at 

 once say that the passage of fluid 

 through the walls of the capillaries 

 under the influence of pressure has 



a great deal to do with the formation of lymph. We 

 are justified in this view by the fact, that,- as a general 

 rule, increase of blood-pressure in the capillaries leads to 

 an increased flow of lymph from the parts they supply. 

 But we must not conclude therefore that the process is 

 entirely due to filtration. The process of osmosis also 



Fig. 36. — To Illustrate a 

 Simple Experiment on 

 Diffusion. 



t.t. thistle tube ; p.p. 

 paichiuent paptjr ; s. solu- 

 tion of gum ; b. beaker ; 

 w. water in beaker. 



1 Substances such as the proteins of blood, also gelatin, which will not 

 diffuse, are known as colloids, in couti-.idistiuction to crystalline sub- 

 stances or crystalloids, which diffuse readily. 



