272 PHYSIOLOGY OF ALIMENTATION. 



cipitates, etc., but the details of these experimental findings 

 must be sought in the original publications. 



A simple glance at the table given in the last paragraph 

 shows that we have to deal with all manner of chemical sub- 

 stances, from those relatively simple in composition to those 

 very complex, some of physiological importance and found 

 as normal constituents of the living cell, others entirely 

 foreign to the living organism. What physico-chemical 

 character have all these substances in common which 

 allows them to penetrate living cells more or less readily 

 and so modify the otherwise simple osmotic behavior of 

 cells in general? 



An explanation frequently given and long believed to be 

 the correct one is that the size of the molecules is the con- 

 dition which determines the entrance of the dissolved particles. 

 According to this conception the cell membranes may be 

 regarded as sieves which allow all molecules that do not 

 exceed a certain size to pass into the cell, while those larger 

 than this are held back. The deficiencies of such an explana- 

 tion are at once apparent when it is remembered that mem- 

 branes which readily give passage to such large atomic aggre- 

 gates as the alkaloids or sodium salicylate hold back the 

 much simpler amino acids and potassium sulphate. 



According to OVERTON all the substances enumerated above 

 enter cells because the membranes surrounding them behave 

 like films composed of a substance which in its properties as a 

 solvent is not unlike ether or the fatty oils. For this reason all 

 those substances which are more soluble in such ethereal or 

 oil-like substances' than' in watel* enter the cell and this the 

 more rapidly* the greater the solubility of the substance in 

 the ethereal or oil-like substances as compared with water; 

 in other words, the greater the distribution coefficient between 

 the film surrounding the cell and water. This distribution 

 coefficient is, at the same temperature, independent of the 

 concentration of the substance. This means that if a sub- 

 stance soluble in any two solvents is .offered these simulta- 



