INTRINSIC ACTIVITY OF SECRETING CELLS 259 



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 diffu- 

 sion 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 

 result of stagnation, that concentration it possesses in the fluid from 

 which secretion or absorption is occurring, then the rapidity of secre- 

 tion or absorption of the substance will be greater the thinner the 

 secreting or absorbing cell and the higher its coefficient of per- 

 meability. In other words, accordingly as the cell grows thinner 

 and more permeable, the more nearly will the secretion approach 

 in concentration of its constituents to the fluid from which the 

 secretion has been formed. 



In so far as the cell has a lower permeability than the plasma 

 or lymph, it will form a resistance of varying amount upon the 

 rate of secretion, and in so far as the cell has a greater permeability 

 than these fluids it will form a less resistance than a layer of equal 

 thickness of these fluids, and to this extent the increased per- 

 meability will aid the rate of secretion. But it must clearly be 

 pointed out that change in permeability can only act as a variation 

 in resistance, and hence the concentration can never be increased, 

 nor the dissolved substance be expedited through het cell at a 

 greater rate than if the cell did not exist on the path that is, than 

 if the resistance for the length of the cell were zero in other words, 

 as far as diffusion is concerned the cell can have no positive effect, 

 such as is actually seen for some constituent or other in every 

 secretion. 



