INTRINSIC ACTIVITY OF SECRETING CELLS 149 



served by the balance of opposing factors, reactions still occur 

 between the molecules, but these are equal and opposite. 



So also in the case of the living cell in equilibrium, the case 

 is not that of an impenetrable membrane through which an ion 

 of potassium or sodium never passes, but a labile equilibrium 

 with both potassium and sodium ions passing in and out all the 

 time, but the numbers passing in and out are equal, so that the 

 concentrations are preserved unaltered. 



That this is the true state of affairs there is abundant experi- 

 mental proof. For let the resting cell divide, and the two 

 daughter cells commence to grow, then the supposed impermeable 

 membrane for potassium ions quits the scene of action, and the 

 growing cell readily takes up potassium ions. 



Further proof of the existence of a labile balance of equilibrium 

 is seen in the physiological behaviour of the cell when the appro- 

 priate ions are absent from its circulating fluids. 



As we have seen in the preceding chapter in describing the 

 effects of inorganic salts upon living cells, in order that the 

 physiological properties of tissues may be maintained in a normal 

 condition, it is necessary that normal amounts of different ions 

 shall be present in the circulating medium. Thus the normal 

 heart-beat cannot be retained unless a certain definite low con- 

 centration of potassium ion is maintained in the perfusing fluid. 

 What explanation of this can be given on the basis that the active 

 cells are impermeable to potassium ions ? If the cells of the heart 

 muscle are impermeable to potassium ions, how can the presence, 

 or absence, or variation in concentration, of such ions in the 

 circulating fluid affect the physiological activity of the cells ? 

 Obviously the cells are permeable to the potassium ions and in 

 both directions ; for when there is no potassium ion in the circu- 

 lating fluid, the balance, for potassium ion between circulating 

 fluid and cell contents, becomes upset and corresponding to the 

 low pressure in potassium ion in the circulating fluid, potassium 

 ion must be given out by the cell until a new equilibrium is reached. 

 On the other hand, if potassium ion is present in the circulating 

 fluid at the proper concentration to correspond to and balance 

 the concentration in the cell, then exchange will be equal, the 

 concentration of potassium ion in the cell will not change, and 

 the cell will preserve its normal activities. Finally, if the con- 

 centration of the potassium ion in the circulating fluid be greater 



