THE PERMEABILITY OF MEMBRANES 145 



many of the properties of lipoids. At the same time, reasons are given for not 

 accepting the view that the cell membrane consists of lipoids alone, and still less 

 that it consists of protein alone. The various substances of which it is composed 

 exist in a complex colloidal intermixture in a more intimate connection than a 

 mere mosaic of lipoid and protein. 



It appears that, as a general rule, it may be stated that the cell membrane is 

 always permeable to substances soluble in lipoids, but whether this fact is 

 essentially due to the solubility itself, or to some other property, such as surface 

 tension or molecular dimensions, is uncertain. The apparent solubility of many 

 dyes and other substances in solutions of lipoids is not a true solution, but a surface 

 adsorption on the colloidal particles of the lipoid. These dyes are insoluble in the 

 lipoid itself. As regards substances insoluble in lipoids, the permeability of the 

 membrane is capable of variation, so that, while being usually impermeable to 

 salts, sugar, etc., it may sometimes become permeable to them. This latter fact 

 necessitates a complex structure. 



Various instances are given in the text which show that changes of permeability 

 do actually take place in functional processes. Tlie state of excitation of muscle, 

 narcosis, secretion, the passage of the nerve impulse from one neurone to another 

 or to a muscle cell, the fertilisation of the ovum and changes in the walls of the 

 blood vessels are referred to. 



Certain cases are known where substances produce profound changes in cell 

 processes without passing beyond the membrane. The action of alkali on the 

 oxidation process of sea urchin eggs and on the movements of medusae, and that 

 of potassium, sodium, and calcium ions on muscle are of such a kind. In other 

 cases, the substance, muscarine or pilocarpine, only produces its effect during 

 its passage through the membrane. \, 



In brief, the cell membrane is a local concentration of constituents of the 

 cell protoplasm due to their property of lowering surface energy of some kind. 

 Substances present in the external medium, if possessing the same property, 

 may also take part. The properties of the membrane are, therefore, not fixed, 

 but capable of modification according to the chemical processes taking place in 

 the cell, or they may be changed by influences on the outside. It is to be 

 regarded as a part of what we may, for the present, call the "living system" 

 of the cell. In its resting state, as usually investigated, it is impermeable both 

 to colloids and to the majority of crystalloids, but may become, temporarily, 

 permeable to all crystalloids and perhaps to some colloids. 



LITERATURE 



Hober (1911), Chapters VI., VII., and XIII. Overton (1907). Zangger (-1908). 



