124 L. J. MULLINS 



OOUOD1 



Fig. 1. Macromolecular cylinders, representing structural elements of the cell 

 membrane, are shown in hexagonal packing. The spacing between the molecules has 

 been greatly expanded to show the membrane pore that is formed between any three 

 cylinders, and molecules are shown passing through one pore. The length of the 

 cylinders is the membrane thickness, and the diameter a function that sets the pore 

 size. 



(Curtis and Cole, 1938) observed during activity. Finally, a lipid film membrane 

 cannot show the specificity of interaction with certain molecules that is ob- 

 served, while a pore membrane, as will be shown later, is able to discriminate 

 between molecules of identical lipid solubility. 



If one assumes that the evidence favors a membrane with pores, two general 

 types may be considered: (a) the random polymer network, a structure 

 of fibrous molecules that are cross-linked so as to yield pores of widely varying 

 size and shape, and (b) the oriented macromolecular liquid membrane, a struc- 

 ture in which the pores are formed by the inability of macromolecules, in their 

 most efficient packing, to fill all the space in the liquid. The simplest example 

 of (b) is that of macromolecular cylinders in regular hexagonal packing; the 

 pores exist at the junction between any three cylinders and are characterized 

 by a regularity of size but, as will be shown later, can show a considerable degree 

 of specificity in their interaction with molecules of various sorts. Fig. 1 shows 

 the suggested arrangement for a membrane structure. 



Ion Transfers Across the Membrane 



Because much of our present understanding of the action of the membrane 

 has come from studies of the transfer of ions across it, certain structural infor- 

 mation can presumably be derived from such work. In order to apply the results 

 of ion transfer studies it is necessary to know something about the properties, 

 and more especially the sizes of the ions concerned in nervous excitation. The 

 suggestions that the permeability difference of the resting membrane to Na + 



