ALIMENTARY TRACT AS AN ABSORPTIVE SYSTEM. 265 



and a sugar solution holds for any pure solvent and any solu- 

 tion. Furthermore, it holds for any two solutions which 

 have not the same osmotic pressure (that is to say, have not 

 the same number of particles dissolved in the unit volume 

 of solvent). The solvent moves always from the region cf 

 lower concentration to that of higher concentration (that is, 

 from the region of lower osmotic pressure to the region of 

 higher) when the two are separated by a semipermeable 

 membrane, and this movement continues until the osmotic 

 pressure on both sides is the same. It is clear, therefore, 

 that if the osmotic cell is filled with water instead of with a 

 sugar solution and the surrounding vessel contains a sugar 

 solution, or, to put it more generally, if the osmotic cell con- 

 tains a solution of a lower concentration than the surrounding 

 vessel, the water will move out of the cell into the solution in 

 the outer vessel until the osmotic pressure on both sides of 

 the membrane is again the same. 



Do similar conditions exist in the case of the body cells? 

 To a certain extent they do. It was pointed out above that 

 many cells which have been studied seem to be surrounded 

 by membranes which have to be considered as approxi- 



b 

 FIG. 30. 



mating true semipermeable ones more or less perfectly. 

 Certain cells even seem to possess membranes which are truly 

 semipermeable for a large number of substances. Fig. 30 

 illustrates the behavior of any cell possessing a truly semi- 

 permeable membrane towards solutions of various kinds. 

 If we consider b the cell under normal physiological con- 



