HANS H. USSING 



With these figures inserted in equation (3) the pore radius, r, works out as 134 x 

 io -8 cm. The same skin before the hormone treatment gave an equivalent pore 

 diameter of 9-25 x io~ 8 cm. 



Table II shows the molecular dimensions of a number of biologically interesting 

 substances, collected and in part also determined by Pappenheimer (1953). Thus 



Table II 



Dimensions of some biologically interesting molecules (after Pappenheimer, 1953) 



the frog-skin equivalent membrane has a pore size which is smaller than the dia- 

 meter of the inulin molecule but larger than the molecules of most crystalloids. 

 However, as pointed out by Pappenheimer in his very important paper, there is a 

 considerable restriction to diffusion of molecules only slightly smaller than the pore 

 diameter. These calculations show that the concept of diffusion and flow through 



38 



