154 INTRINSIC ACTIVITY OF SECRETING CELLS 



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 the 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 seen for some constituent or other in every secretion. 



Further, it may be pointed out that the extent of the secreting 

 or absorbing surface is in all cases so large, and the thickness of 

 the layer so small, amounting to the length of a single cell, that 

 increase in permeability above the value for a layer of lymph or 

 plasma of equal thickness can possess but a very secondary value 

 in determining rate of secretion or absorption. If we imagine 

 the layer of secreting or absorbing cells spread out so as to form 

 a huge plane lamina, the thickness of which is that of a single 

 secreting cell, and the area of the side that of the total secreting 

 area of cells, and that this lamina forms a membrane between 

 lymph upon the one side and secretion upon the other. Then 

 if this lamina were supposed to have the same resistance to diffusion 

 through it as a lamina of lymph of equal thickness, such resistance 

 would be excessively low, and with a rapid removal of fluid 

 from the secretion side the concentration of each constituent 

 upon the secretion side would be practically the same as upon 

 the lymph side of the lamina. Hence the supposition of a higher 

 permeability or selective permeability of the secreting cell above 

 that of the lymph (or water) can have but an infinitesimal effect, 



