u6 /'A7.\r//'/-/:.S Ol' GENERAL PHYSIOLOGY 



vesicle, by the process known as osmosis. This must for the present be taken as 

 an experimental fact. If the water outside be replaced by a solution of sugar, 

 but of a lower concentration than that within the membrane, water will enter 

 until the concentration is equal on both sides ; if the solution outside is stronger 

 than that inside, water will escape, until again the concentration is the same on 

 both sides. It is not a necessity, moreover, that the two solutions, inside and 

 outside, be of the same substance, so long as the membrane is impermeable to it. 

 The amount of distension or collapse is clearly in exact proportion to the molecular 

 concentration of the solutions, since on this depends the degree of dilution or 

 concentration necessary to bring the inner and outer solutions into osmotic 

 equilibrium. Now, careful investigations of the behaviour of the cells of the 

 kidney by Siebeck (1912) and of the muscle cells by Beutner (1913, 1) have shown 

 that living cells react in the same way as the semi-permeable membrane described 

 above. The changes in volume are simply proportional to the molar concentration 

 of the solutions used. 



All the various members of the " Hofmeister series," in equal concentration, have the same 

 effect. The process of imbibition, as we have seen (page 100 above), follows a different law. The 

 series of electrolytes just referred to, in equal concentration, have different effects on imbibition 

 according to their action on the properties of water, so causing it to be distributed between 

 the two phases of the colloidal system in a different proportion. Moreover, sugar behaves, 

 as regards its effect on the volume of cells, just as a salt of the same osmotic pressure, 

 provided that the salt is one to which the membrane is impermeable, whereas, according 

 to certain investigations, it is devoid of action on imbibition processes. Martin Fischer 

 and G. Moore (1907, p. 339) find that non-electrolytes in general have no effect on the 

 swelling of fibrin. 



Further facts are, I think, unnecessary to show that the imbibition theory is insufficient to 

 account for more than a small part of the behaviour of cells towards solutions of varying 

 concentration. At the same time, there is no doubt that the power of changing the water 

 content of cell constituents must play an important part in cell mechanics. 



We may now pass on to consider the nature and properties of the cell 

 membrane. It will clear the way somewhat if I state the general conclusion 

 which is forced upon us by consideration of the whole of the evidence on this 

 disputed question, although, at first sight, it may seem rather a lame one. It is, 

 in fact, that the cell membrane is sometimes permeable to crystalloids, sometimes 

 not. This will seem more satisfactory when we find that the apparently capricious 

 behaviour is in relation to functional changes in the cell, or dependent on the 

 action of definite substances. As regards colloids, the membrane itself is probably 

 always impermeable ; although in special cases, as the cells of secreting glands, 

 there appear to be arrangements by which colloids can get in or out, probably by 

 rupture of the membrane. 



Impermeability to Crystalloids. If a slice of living red beetroot be allowed to 

 soak in tap water, it will be found that neither the red pigment nor the cane-sugar 

 escapes from the cells. This fact can only be explained on two hypotheses : 

 either the cell membrane is impermeable to these substances, or they are combined 

 in an irreversible manner with the insoluble matter of the cells. Now, Moore and 

 Roaf (1908, p. 80) appear to regard the existence of some kind of chemical com- 

 bination between the proteins of cell protoplasm and electrolytes as sufficient to 

 account for the difference of composition between cell and surrounding liquid, without 

 the necessity of assuming the existence of a semi-permeable membrane. But, if 

 this compound is reversible, as an adsorption process would be, there can be 

 merely a quantitative difference between the cell contents and the outer solution, 

 because an adsorption process is only in equilibrium with a finite concentration of 

 adsorbed substance in the solution with which the surface is in contact. This is 

 contradictory to experience in the case of the beetroot, and we shall find other 

 instances as we proceed. If the hypothetical compound is a more strictly chemical 

 one, it must be neither hydrolytically nor electrolytically dissociated, and, in fact, 

 completely insoluble and inert. It is difficult to see of what value such a substance 

 can be in the dynamics of the cell. Moreover, direct measurements by Hober 

 (1912, 2) of the electrical conductivity of the interior of cells show that a part, at 

 least, of the inorganic constituents &refree. 



We are compelled, therefore, to assume the existence of a membrane of some 



