ALIMENTARY TRACT AS AN ABSORPTIVE SYSTEM. 269 



shows no change in volume. Since all of these solutions 

 must then be isosmotic with the cell contents they must also 

 be isosmotic with each other. The red blood-corpuscles (and 

 other cells) have been employed in this way for the determi- 

 nation of osmotic pressure. But this is true only when 

 the cell has a true semipermeable membrane, and such 

 exist practically nowhere in living organisms. As the num- 

 ber of substances studied with reference to their power of 

 changing the volume of red blood-corpuscles (or causing them 

 to give up their red coloring-matter or " plasmolyzing " 

 various plant-cells, etc.) increased it was soon found that 

 there exist a large number of substances which affect 

 the cells either at no concentration at all or only when 

 present in amounts greatly exceeding the ordinary limits 

 at which a change in volume might be expected. Atten- 

 tion was called to this fact when it was pointed out that the 

 cells of the alimentary mucosa swell or shrink only when \ 

 exposed to great and sudden changes in osmotic pressure | 

 through the fluids surrounding them. These substances* 

 which in solution failed to bring about changes in the volumes 

 of cells in proportion to their osmotic pressure existed for a 

 long time as unexplained exceptions until OVERTON and 

 MEYER made a systematic study of them and so advanced 

 most markedly our knowledge of the fundamental character 

 of absorption and secretion. 1 



The power of a solution to abstract water from a cell (that 

 is, to shrink or "plasmolyze" it) is dependent upon the semi- 

 permeability of the membrane surrounding the cell to the 

 substance dissolved in the solution. If the substance is 



1 OVERTON: Vierteljahresschrift d. naturforsch. Gesellsch. in Zurich , 

 1895, XL, p. 1, and 1899, XLIV, p. 88; Zeitschr. f. physik. Chemie, 

 1897, XXII, p. 189. MEYER: Arch. f. exp. Path. u. Pharm., 1899, 

 XLII, p. 109, and 1901, XLVI, p. 338. This account is largely taken 

 from HOBER'S excellent Physikalische Chemie der Zelle und der Gewebe, 

 Leipzig, 1902, p. 101. See also SPIRO: Physikalische und physiolo- 

 gische Selection. Strassburg, 1897. 





