124 PRINCIPLES OF GENERAL PHYSIOLOGY 



from water by means of a parchment paper membrane, and an electrical potential 

 difference established by placing electrodes, one inside, the other outside the 

 membrane, then it depends on the sign of the electrode compared with that of the 

 colloidal ion whether a current passes or not. Suppose we have a sodium salt of a 

 colloidal acid, such as caseinogen or Congo-red, and that the electrode in this 

 solution is the positive one or anode. The current must pass through the 

 membrane from inside to outside ; that is, positively charged ions must pass through 

 to the negative electrode and negative ions from outside to inside and be 

 discharged there ; unless this can happen, no current will pass. Now, sodium ions 

 can freely pass through the membrane and the opposite negative ions are already 

 inside, so that current will flow when the internal electrode is the anode. On the 

 contrary, if the outer electrode is the anode, in order that a current shall pass, the 

 negative ions must reach it. This cannot happen, since there is an impassable 

 barrier between them and the electrode. 



Such conditions would clearly account for the resistance of cells to the passage 

 of currents. The boundary surface on the one side of the cell would oppose 

 currents in one direction, and that on the other side, those in the opposite 

 direction. They would appear to be non-conductors. But it is to be remembered 

 that this state of affairs holds only as long as the colloidal ion is the only one 

 available of the right sign. Jf any diffusible ion is present, the current will pass 

 by means of it, and we know that there are in the cells inorganic ions of both 

 signs. A high resistance might be accounted for by the existence of most of the 

 inorganic constituents of the cell in the form of salts with colloids, while the non- 

 colloidal salts of the cells and the plasma of the blood were freely diffusible. But, 

 as we have shown (page 1 20), if this were the case, the ratio of the different cations, 

 say of potassium and sodium, must be the same inside and outside the blood 

 corpuscles, and this is not what is actually found. 



FUNCTIONAL CHANGES IN PERMEABILITY 



It appears from the preceding section that we must regard the surface mem- 

 brane of cellSj.at all eveiftsTin the condition in which they are usually investigated, 

 as Detligmipermeable both to colloids and to the majority of crystalloids. 



There are, however, certain substances ammonium salts, urea, glycerol, alcohol, 

 etc. to which the membrane is more or less permeable at all times. When placed 

 in hypertonic solutions of these, there is a preliminary plasmolysis or shrinking of 

 the cell, greater or less according to the diffusibility of the solute, but this 

 disappears as the concentration becomes equal on the two sides of the membrane. 



On the other hand, we know that it is necessary for cell processes that such 

 things as glucose and amino-acids, which are usually unable to pass the membrane, 

 should get into the cell. For this reason certain recent work, showing that it is 

 possible to produce reversible changes of permeability without killing the cell, are 

 of great importance. 



Osterhout (1912) showed, as already stated, that the cells of Laminaria are 

 impermeable to the ions of sea water, when immersed therein. But, if immersed 

 in pure sodium chloride of the same conductivity (and temperature) as sea water, 

 their conductivity rapidly rises, until they oppose very little more resistance to the 

 passage of the current than the salt solution itself does. If the exposure to the 

 sodium chloride has not been too prolonged, the normal state of the cells is 

 recovered on return to sea water. 



It may be remarked, in passing, that this fact seems impossible to account for on the view 

 of the membrane being only semi -permeable as regards colloids ; for it would be necessary t<> 

 assume that it becomes permeable to colloids under the action of sodium chloride ; in \vhirh 

 case the protoplasmic substance of the cells would diffuse away and no recovery be possible 

 on replacing in pure sea water. 



Lillie (1909) found that the larva of Arenicola, if placed in pure sodium 

 chloride, isotonic with sea water, constricts up and the pigment contained in its 

 cells diffuses out freely. This pigment is soluble in water, and does not appear to 

 be in colloidal solution. The addition of one volume of 0*5 molar calcium chloride 



