INTRINSIC ACTIVITY OF SECRETING CELLS 153 



and pressure than in the plasma. Thus if a cell is immersed in 

 a fluid containing any given constituent in solution, it will, if it 

 possesses any degree of permeability whatever for that constituent, 

 become ultimately saturated to the equilibrium point with the 

 constituent, and the point of equilibrium will not vary with the 

 permeability, the only thing which will vary with the permeability 

 will be the time in which equilibrium is attained. In considering 

 the effects of change in permeability upon the time relationships 

 of absorption and secretion, the factors to be borne in mind are 

 the thickness of the layer through which diffusion has to occur, 

 the difference in concentration of the diffusing dissolved substance 

 or ion at the two surfaces bounding the layer through which 

 diffusion is taking place, and the coefficient of diffusion for the 

 particular substance through the layer. The rate of diffusion, 

 regarded as a purely physical process unaided by the cellular 

 activity (and dependent only upon the difference in osmotic 

 pressure at the two sides of the layer or membrane, the thickness 

 and the coefficient of permeability or diffusion), may be said to 

 be directly proportional to the difference in osmotic pressure and 

 to the coefficient of permeability, and inversely proportional to 

 the thickness of the layer or membrane, that is, the length of the 

 absorbing or secreting cell. Hence diffusion can only occur so 

 long as there is a fall in osmotic pressure in the direction in which 

 diffusion is taking place ; when the two pressures become equal 

 diffusion must stop, and if by any chance the pressure became 

 greater in the direction in which diffusion had been taking place, 

 then the purely physical process of diffusion would carry out or 

 tend to carry out the process in the opposite direction. Accordingly 

 any separation of a constituent at a higher osmotic pressure must 

 be carried out against diffusion, with increase in osmotic energy, 

 and heaping up of difference in osmotic pressure or increase in 

 the potential factor of osmotic energy. 



It is, then, only when the concentration of a substance, either 

 secreted or passing through as an absorption product to the other 

 side of the active cell, is diminished that diffusion due to osmotic 

 pressure can be regarded as a factor in the process, and it is here only 

 that we have to consider the possible effects of changes in the per- 

 meability of the cell. If the secreted or absorbed product is carried 

 rapidly away from the other side of the cell after having passed 

 through, so that it does not tend to approach in concentration, as a 



