THE PERMEABILITY OF MEMBRANES 131 



It may be looked upon as glycero-phosphoric acid combined up with one 

 molecule each of oleic and palmitic acids, on the one hand, and with choline, a 

 base, with the constitution of a tertiary amine, on the other hand. It is to be 

 remembered, however, that other fatty acid radicals may take the place of oleyl or 

 palmityl, and other bases the place of choline. 



The physical properties of this substance are the most important in the 

 present connection, and they are somewhat remarkable. It is a soft, waxy, 

 substance, soluble (probably in colloidal form) in chloroform, benzene, oil, and 

 alcohol, rather less so in ether ; insoluble in cold acetone or ethyl acetate. Placed 

 in contact with water, it tends to disperse, assuming the so-called "myelin" 

 forms, like the pseudopodia of amoeboid organisms. If shaken up with water, it 

 forms a colloidal solution of the emulsoid type, in which the internal phase consists 

 of lecithin containing " imbibed " water. 



Although the physical properties are the most striking, the chemical com- 

 position suggests important functions of a chemical nature, but what these are is 

 at present very uncertain. 



When alcoholic solutions containing lecithin and glucose or certain proteins are 

 evaporated to dryness, it is found that ether takes up from the residue adsorption 

 compounds of lecithin with glucose or protein, substances normally insoluble in 

 ether. It was at one time supposed that these were definite chemical compounds, 

 but it has been shown that the proportion of the constituents varies with that in 

 the original mixture and is never definite. The cases of "jecorin," which 

 contains glucose, and of " vitellin," have been already discussed (page 66 above). 



The relationship of lecithin to water is of much importance as regards the 

 cell membrane. This membrane, with very rare exceptions, is freely permeable to 

 water. Now, the true fats are not so, while lecithin, as stated above, easily 

 swells up in water, and is therefore permeable to it. But, as Nathansohn (1904, 

 p. 640) points out, in this state it has lost its power of being a solvent for lipoid- 

 soluble substances only ; dry lecithin in solution in benzene dissolves the " basic " 

 dyes only, but moist lecithin in benzene is also a "solvent" for the sulphonic 

 acid dyes, to which the cell membrane is normally impermeable. Buhland 

 (1909, p. 34) prepared membranes of lecithin and cholesterol in the manner of 

 Pascucci (1905), and found that no dye, "basic" or "acid," diffused through 

 cholesterol at all. Neither did this happen through lecithin membranes, which 

 were completely impermeable until saturated with water : when this occurred, 

 as could be seen from the fall of the water column in the cell, both kinds of dyes 

 began to come through. 



Loewe (1912) has recently published important work on the physical chemistry 

 of these "lipoids." Kephalin is a substance closely related in its composition to 

 lecithin and present in considerable amount in brain. As already mentioned, 

 lecithin forms an obviously colloidal solution in water, and Loewe shows that 

 kephalin, even in chloroform or benzene, is also in the colloidal state. The 

 solutions show the Faraday-Tyndall phenomenon and an illuminated cone under 

 the ultra-microscope. In solution in chloroform, the raising of boiling point is 

 too small to be detected, showing that the solute is in large aggregates, a fact also 

 evident from vapour pressure measurements. Further, when swollen by the 

 action of water, it becomes insoluble in ether. The reader will probably remember 

 that, in the old Hoppe-Seyler method of extracting the lipoids from brain, it 

 was necessary to dehydrate first with alcohol in order to make them soluble in 

 ether. The meaning of this insolubility in ether will be apparent when it is 

 remembered that presence of water does not affect true solubility in ether, such 

 as that of picric acid, which is extracted by ether from its solution in water. 

 Kephalin, then, is not in true solution in these various so-called "solvents" for 

 lipoids. 



This fact raises considerable difficulty in the interpretation of Overton's 

 experiments with " lipoid-soluble " dyes and other substances. According to his 

 view, a substance obtains admission to the cell because it is soluble in lecithin and 

 similar substances. Take the case of methylene blue. This is insoluble, except to 

 a minute degree, in chloroform, but, if kephalin be present in the chloroform, the 



