62 MEMBRANE PERMEABILITY AND DRUG ACTION 



or groups the drugs in which we are interested belong. 

 The reservations mentioned above are of two kinds. 

 If a substance happens to fall into one of the types which 

 are secreted, we usually cannot calculate the effect of a 

 change in structure. Then also there are small patches 



TABLE XIV 



THE PERMEABILITY OF VARIOUS CELL MEMBRANES TO CERTAIN 

 SUBSTANCES, COMPARED WITH THE PERMEABILITY CALCULA- 

 TED FOR AN EQUIVALENT THICKNESS OF W^ATER, AND OF OIL 

 HAVING A VISCOSITY lOO TIMES THAT OF WATER 



on the surfaces of at least some cells which appear to be 

 specially adapted to permit rapid permeation by a given 

 molecular species without permitting a similar advantage 

 to other molecular species. These patches may, of course, 

 be part of the cellular secretory mechanisms, but until 

 we know more about them it is wiser to assume that they 

 may be distinct from active secretory processes. The 

 fact that these reservations must be made should not be 



