THE PERMEABILITY OF MEMBRANES 135 



are taken up by the nervous system in greater proportion than they are by 

 other tissues, this must be understood to mean that lipoid constituents are in 

 greater proportion in this tissue. It does not necessarily mean that the proto- 

 plasmic substance of the nerve cell itself is exposed to a greater concentration of 

 narcotic than that of other cells is. 



Experiments by Osterhout (1911) on Spirogyra show that the cells of this 

 alga are permeable to both sodium chloride and to calcium chloride, as well as 

 to many other salts, when present alone the conclusion is drawn that the 

 membrane is not lipoid, since these salts are insoluble in such substances. The 

 remarkable fact was observed that a mixture of chlorides of sodium and calcium 

 renders the membrane impermeable to both. These results are regarded as 

 indicating a protein constitution. It appears to me, however, that caution must 

 be exercised in interpreting them and that they indicate rather that a pure 

 salt affects the membrane in such a way as to produce an abnormal permeability, 

 which is of a temporary nature, since, in the experiments referred to, the cells 

 were not permanently injured. From the fact that the natural cells were found 

 to be isotonic with O375 molar sodium chloride, it must be concluded that they 

 contain a considerable amount of osmotically-active crystalloids, which would 

 diffuse out into the nearly pure water in which the cells normally live, unless 

 the membrane were impermeable to salts. 



That a simple protein membrane is insufficient to account for such imperme- 

 ability is shown by the behaviour of some interesting protein membranes prepared 

 by Newton Harvey (1912). When chloroform is shaken with solutions of egg- 

 albumen, a membrane is formed on the surface of the drops by condensation of 

 the protein in the manner described by Ramsden (1904). If these globules are 

 allowed to stand in water, the chloroform diffuses out faster than water enters, 

 so that they shrink ; if lecithin be dissolved in the chloroform previously to the 

 shaking with the egg-white, water is taken up sufficiently rapidly to prevent 

 shrinking and, if left in an open vessel, the chloroform disappears entirely in 

 the course of an hour or two and there remains, inside the delicate protein 

 membranes, a colloidal solution of lecithin, partly in the form of granules which 

 are visible under the microscope. When a dilute solution of neutral red is 

 added to a suspension of these artificial cells in water, the lecithin granules 

 take up the dye by adsorption and become red, so that an opportunity is given 

 to test the permeability of the protein membrane as regards alkalies. It is 

 found, contrary to the behaviour of the living cell, which is impermeable to 

 sodium hydroxide but permeable to ammonium hydroxide, that the two alkalies 

 pass through the protein membrane at an equal rate. The membrane of the 

 living cell is therefore of quite a different composition from that which condenses, 

 on chloroform drops in a solution of egg-white. 



Another instructive experiment made by the same observer is to take a 

 solution of lecithin in benzene, instead of in chloroform, and to repeat the 

 above procedures. The benzene of the droplets cannot, of course, diffuse away 

 into water, but, if they be stained with neutral red and placed in ammonium 

 hydroxide of O'OOOl molar concentration, the change to yellow is almost 

 instantaneous, while even in O'l molar sodium hydroxide, it takes twenty 

 minutes to produce the change. Ammonium hydroxide, in fact, is readily soluble 

 in benzene-lecithin solution, while sodium hydroxide is not. But it is easy 

 to show that wet benzene itself behaves in the same way. 



Gelatine, stained with neutral red, is allowed to set in the bottom of an Erlenmeyer flask, 

 which is then filled with water and inverted in a vessel of water. By means of a bent tube, 

 benzene is passed up into the flask, where it forms a layer between the water and the gelatine. 

 Various alkalies and acids can be added to the water in the flask by the same tube, and it will 

 be found that benzene is permeable to ammonium hydroxide and to acetic acid, impermeable to 

 sodium hydroxide and to hydrochloric acid, in fact it behaves like the cell membrane. According 

 to these experiments, the cell membrane should be composed of benzene, which is absurd. 

 Newton Harvey's experiment, in fact, tells us nothing as to the properties of lecithin when 

 saturated with water ; according to Pascucci, as we saw above, a lecithin membrane is 

 attacked both by ammonium and sodium hydroxides. None of these experiments, indeed, 

 affords proof that the cell membrane is composed only of lipoid material. 



