ON SOME APPLICATIONS OF THE THEORY 

 OF OSMOTIC PRESSURES TO PHYSIO- 

 LOGICAL PROBLEMS. 



PART II. 



IN my previous article I gave some account of a research 

 by Heidenhain in which this observer, after drawing 

 certain deductions from the theory of osmotic pressures, shows 

 that the phenomena of absorption from the intestinal canal are 

 irreconcilable with these deductions, and are therefore not 

 susceptible of a mechanical explanation, but must be as- 

 cribed to the active intervention of cells. Since analogous 

 problems to those discussed by Heidenhain are continually 

 coming before us in physiology, it is important that we 

 should have a clear idea of the factors which are involved 

 in the passage of water or dissolved substances across 

 membranes. I therefore propose to reproduce Heiden- 

 hain's statements, and then to consider how far they are 

 true for the special cases which occur in the body. 



These statements are as follows : — 



i. If two watery solutions with the same osmotic pres- 

 sure are separated by a membrane through which diffusion 

 can take place, no change in volume occurs on either side 

 of the membrane. 



2. If the solutions on either side of the membrane are of 

 unequal osmotic pressure, water passes from the side where 

 the pressure is less to the side where the osmotic pressure 

 is greater. 



3. The osmotic pressure of a solution is equal to the sum 

 of the partial pressures of the various dissolved substances. 



4. If the solutions on the two sides of the membrane 

 have the same total osmotic pressure but unequal partial 

 pressures of their various constituents, each constituent of 

 the solution passes from the side where it has the higher 

 partial pressure to the other side. No change in the volume 

 of water on the two sides takes place. 



Of these four statements only one (No. 3) is absolutely 



