260 INTRINSIC ACTIVITY OF SECRETING CELLS 



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. Let us 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, since it 

 cannot increase, as we have seen, the concentration above the value 

 in the lymph, and if it had the value of the lymph (or water) in 

 permeability, the concentrations would be practically the same. 

 It is when the permeability changes in the opposite direction, and 

 the degree of permeability of the secreting or absorbing cell becomes 

 progressively less and less than that of a layer of lymph or water 

 of equal thickness, that the only and indeed an important effect 

 of cell permeability becomes obvious, in slowing, never in hastening, 

 the rate of secretion of constituents. For as the permeability of 

 our imaginary secreting or absorbing lamina to any constituent 

 becomes less and less, its resistance to the passage of that con- 

 stituent becomes greater and greater, and the concentration of the 

 constituent in the secretion or absorbed fluid less and less, until 

 in the limit none may pass through at all. 



It is in such a resisting action that the value of differences in 

 permeability comes in, by causing the retention of substances 

 in the lymph, and not in a high degree of permeability causing 

 increased rate of passage, and increased concentration of sub- 

 stances in secretion. Examples are the retention of the plasma 

 proteins in the glomerular secretion or filtration, and the pre- 

 vention of ingress of poisonous substances in many cases to the 



