274 PHYSIOLOGY OF ALIMEXTATION. 



substances in the cell? By making use of the so-called 

 "vital" staining methods, that is through use of stains which 

 will color living protoplasm, OVERTON has been able to show 

 that these fat-like bodies or as they are collectively called, 

 the LIPOIDS are cholesterin, lecithin, protagon, and cerebrin. 

 These substances are not, of course, true fats, but they re- 

 semble these in their property of dissolving more or less 

 readily the compounds which were found above to enter 

 living cells. The conception that living cells are surrounded 

 by lipoidal membranes seems, therefore, not to lack experi- 

 mental support, and osmotic pressure as a force determining 

 the movement of dissolved particles and of water becomes 

 a more clearly defined force in phenomena of absorption and 

 secretion when this selective power of solution of the surface 

 films of cells is taken into consideration. 



It must not be thought, of course, that these conceptions 

 of cell membranes and the movement of water and dissolved 

 substances through them, as outlined for cells in general and 

 applicable to the alimentary mucosa in particular, explain 

 more than a part of the phenomena of absorption and secre- 

 tion as illustrated by the gastro-intestinal tract. Even in 

 this statement we are not considering the absorption or 

 secretion of substances which before or during their passage 

 through the alimentary mucosa suffer a change, such as the 

 fats, proteins, and carbohydrates. All this will become more 

 apparent in the discussion of the absorption of the specific 

 elements of our food, wben a mass of isolated facts will be 

 found that still lack a unifying explanation. 



7. The Absorption of Salts. Recognized physical laws suf- 

 fice at present to explain only a part of the phenomena ob- 

 served in the absorption from the gastro-intestinal tract of 

 even such simple substances as the salts. From some points 

 of view the absorptive mucous membrane behaves like a dead 

 diffusion membrane on one side of which there is found the 

 blood, the composition of which may be looked upon as fairly 

 constant, on the other the salt solution undergoing absorption. 



