MEMBRANE PERMEABILITY AND DRUG STRUCTURE 55 



Stances is already known and that the molecular weight 

 of the drag is known. Table ix shows another interesting 

 case, where equation (6) is obeyed to a first approxi- 

 mation: this is the diffusion of anions through the walls 

 of the rumen of the sheep. 



TABLE VII 



THE PERMEABILITY OF CHITIN TO 

 DIFFERENT MOLECULES 



The two groups which we have just considered are 

 those in which the molecule penetrating the membrane 

 does not in fact leave the water in which it is dissolved. 

 In the first case the permeating molecule was carried 

 along by bulk flow of the water and in the second case 

 thermal diffusion caused the permeating molecules to 

 move through the water filling the pores in the membrane. 

 The next two groups which we have to consider differ 

 from the first two groups in that the permeating mole- 

 cules actually pass from the aqueous phase in which they 

 are dissolved into the non-aqueous phase constituting the 

 membrane. In such cases the main resistance to perme- 

 ation may lie either at the membrane-water interfaces or 



