138 THE POPULAR SCIENCE MONTHLY 



acids may be lessened or counteracted by neutral salts like sodium or 

 calcium chloride. Such antagonistic actions between acids and salts, 

 while not shown by colloids in general, are peculiarly characteristic of 

 certain proteins. Thus the rate of swelling of gelatine (a typical 

 selenoprotein) in water is greatly increased by the addition of a little 

 acid; this effect is prevented by the addition of neutral salts, and the 

 basis of this form of anti-cytolytic action may possibly lie here — i. e., 

 the disruptive action of the acid on the proteins of the membrane is 

 checked or prevented by the salt. 1 Yet the plasma-membrane undoubt- 

 edly contains other constituents, and among these the substances be- 

 longing to the group of lipoids appear to be fundamentally important. 

 These substances, fat-like in their solubilities and colloidal in their 

 physico-chemical character, are always present in cells. Much light has 

 been thrown on their physiological significance by the investigations of 

 Overton and his successors, which have shown that ready permeability 

 to lipoid-solvents is highly characteristic of both animal and plant cells. 

 Alcohols, esters, ethers, hydrocarbons and similar compounds, all of 

 which are soluble in lipoids, enter living cells rapidly, in contrast to 

 neutral salts, sugars, amino-acids — the chief crystalloidal constituents 

 of protoplasm — which diffuse into resting cells (with unmodified 

 plasma-membrane) either imperceptibly or with extreme slowness. 

 Overton's results thus indicate that lipoids enter into the composition 

 of the plasma-membrane. This is to be expected. The structure prob- 

 ably consists of a mixture of all those protoplasmic constituents which 

 have marked effect in lowering the surface-tension of the cell-boundary. 

 Lipoids are conspicuous among this group of substances. That they 

 form part of the plasma-membrane is also indicated by the readiness 

 with which the permeability and other properties of this structure may 

 be altered by lipoid-modifying substances. Lipoid-solvents as a class, 

 when present in certain concentrations, have a specific action in in- 

 creasing, often irreversibly, the permeability of the plasma-membrane. 

 In lower concentrations many appear to decrease this permeability. 

 Their influence on irritability, which is probably a function of the con- 

 dition of this membrane, also indicates their importance as membrane- 

 constituents. Narcotic action is highly characteristic of lipoid-solvents, 

 and there is good evidence that this action depends on an alteration of 

 the plasma-membrane. I shall refer to this possibility later, in connec- 

 tion with the problem of the relation of membranes to stimulation. 

 All of these facts taken together indicate very clearly that the colloids 

 composing the semi-permeable surface-film of living cells consist of 

 1 This consideration, however, is not demonstrative. The precipitation of 

 lecithin by acid can be prevented by salts in concentrations which in themselves 

 do not precipitate, as Handowsky and Wagner have recently shown. Lecithin, 

 which seems always to be present in cells, probably forms an important part of 

 the plasma membrane, in which case changes in its physical condition would 

 influence the properties of the latter. 



