MEMBRANE PERMEABILITY AND DRUG STRUCTURE 59 



With those cells which have a lipoid membrane deter- 

 mining their permeability, it commonly happens that as 

 the structure of the penetrating molecule is changed so 

 as to make it penetrate more slowly, a transference is 

 made from group four to group three. Another case 



TABLE XIII 



VALUES OF PM'^V^ FOR FATTY ACIDS P ASSING^THROUGH CELLS 



OF THE SHEEP RUMEN 



which may be of practical importance is that of trans- 

 ference from group three to group two. This is illus- 

 trated by the results shown for the diffusion of sub- 

 stances through the sheep rumen. The sheep rumen 

 membrane consists of cells between which lies an inter- 

 cellular cement. Molecules such as the free fatty acids, 

 which can penetrate the cells rapidly, permeate the mem- 

 brane mainly by passing through the cells, though, of 

 course, some free fatty acid also passes through the 

 pores. The fatty acids passing through the cells encounter 

 the main resistance to permeation in the interior of the 

 cell membranes they pass through, and thus fall into 

 group three. But the fatty acid anions permeate the cell 

 membranes much less readily. Consequently passage 



