266 PHYSIOLOGY OF ALIMENTATION. 



ditions, that is, as in osmotic equilibrium with the liquids 

 with which it is bathed, then a and c represent respectively 

 the effect upon this cell of immersion in a solution more 

 dilute or more concentrated than that in which it is immersed 

 normally. If the cell is placed in a solution which has a 

 lower osmotic pressure than the cell contents, then water 

 will pass into the cell until the osmotic pressure on both sides 

 of the (physiologically) semipermeable membrane is the same. 

 The cell will in consequence increase in size (if some external 

 obstacle does not prevent it) and may swell so much that 

 the cell-wall is ruptured and the cell contents are allowed 

 to escape. The reverse must occur when the cell is immersed 

 in a fluid having a higher osmotic pressure than the cell 

 contents. Under these circumstances water passes through 

 the semipermeable membrane surrounding the cell in the 

 direction from within outwards until osmotic equilibrium 

 is once more restored. As the water passes out of the cell, 

 this shrinks, and the amount of the shrinkage is determined 

 by the amount of water given off by the cell. This in turn 

 is determined by the amount of the difference between the 

 osmotic pressure of the cell contents and that of the sur- 

 rounding liquid. 



True semipermeable membranes are rarely found sur- 

 rounding any cells that have been accurately studied, and 

 so it will not seem strange that physiological experiment 

 has shown that the epithelial cells of the alimentary tract 

 obey the ordinary laws governing the osmotic absorption 

 of water only when exposed to great and sudden changes 

 by being flooded with solutions of very high or very low 

 osmotic pressures. The reason for "this is readily intelligible 

 when it is remembered how very permeable the alimentary 

 mucosa must be to allow the passage of the host of soluble 

 substances which daily pass through it into the blood or 

 lymph or from these circulating fluids out into the lumen 

 of the digestive tube. Whenever a membrane allows the 

 passage of a substance dissolved in a liquid found on either 



