120 ELEMENTARY PHYSIOLOGY less. 



sider what are the causes which in the first place lead 

 to the presence of lymph in the lymph spaces of the 

 tissues. 



Three processes suggest themselves at once as possible 

 causes ; these are filtration, diffusion, and Osmosis. 

 With the first of these every one is more or less familiar, 

 and we need say no more than that it consists in the 

 passage of fluid and of substances in solution through a 

 porous membrane as the result of a difference of pressure 

 on the two sides of the membrane. Diffusion, on the 

 other hand, is, broadly s})eaking, independent of the 

 difference of pressure necessary to cause filtration. A 

 simple experiment shows at once the essential feature of 

 diffusion. If a small dish of parchment, similar to that in 

 which " ices" are frequently served, be floated on water, 

 and into the dish a solution of common table salt be 

 placed, by chemical analysis the salt will soon be found to 

 have passed into the water outside through the substance of 

 the dish, even though this contains no visible holes. There 

 will never be any appreciable difference of level between 

 the surface of the fluid within and without the dish. 



The difference between diffusion and osmosis will be 

 understood by comparing this experiment with the 

 following. Tie a piece of parchment paper tightly over 

 the wide end of an ordinary " thistle tube " as used by 

 chemists. Then fill the bulb and about one inch of the 

 tube with a strong solution of gum acacia and fix the tube 

 vertically, as in Fig. 36, in a beaker of water, so that the 

 surface of the solution in the tube is at the same level as 

 that of the water in the beaker. The water passes 

 gradually through the paper into the tube so that the 

 liquid rises in the narrow part of the tube and may ulti- 

 mately stand an inch or more above the surface of that 

 which is in the beaker. The essential difference between 

 the two experiments lies in the fact that in the former both 

 the water and the salt pass (diffuse) freely through the 

 membrane, in the second the membrane is permeable to the 



