626 THE MECHANISM OF ABSORPTION 



capillary tube of a hiemocrit ; any solution in which the corpuscles 

 do not alter in volume will be isotonic with the plasma. Although 

 these physiological methods, as a way of estimating osmotic 

 pressures, are open to many limitations and fallacies, they do 

 show that the red corpuscle behaves to some substances in 

 solution as a partial semipermeable membrane. But the actual 

 permeability of red corpuscles can be estimated in the following 

 ways. Hedin's method consists in dissolving equal quantities 

 of the same substance in whole blood and in plasma ; the 

 corpuscles are centrifugal ised away in the one case, and the A 

 of both lots of plasma is observed. If the A of the centri- 

 fugalised plasma is less than that of the other plasma, then 

 some of the dissolved substance has been taken up by the 

 corpuscles. Oker-Blom's method depends upon the fact that 

 electrolytes contained in red blood- corpuscles or taken up by 

 them from the plasma do not influence the electrical conduc- 

 tivity of blood. If a conducting substance is dissolved in blood 

 and the observed electrical conductivity of the solution com- 

 pared with what it ought to be if none of the substance has 

 passed from the plasma into the corpuscles, it can be ascer- 

 tained whether or not the corpuscles are permeable to this 

 substance. By these means it has been shown that red 

 corpuscles are practically impermeable to NaCl, KC1, NaN0 3 , 

 KNo 3 , KBr, K 2 S0 4 and a few ammonium salts, but are readily 

 permeable to NH 4 C1, NH 4 Br, NH 4 N0 3 and most other ammo- 

 nium salts. Of the permeability of other cells we know practi- 

 cally nothing, but it is clear that in any question dealing with 

 filtration, diffusion, or osmosis, through living membranes, the 

 permeability of those cells is of fundamental importance. 



When we consider the ways in which various solutions could 

 produce an effect upon living cells, two obvious possibilities at 

 once present themselves : ( 1 ) that they act in virtue of their 

 osmotic pressure, and (2) by ionic action. That osmotic pressure 

 does affect the properties of living cells is best shown in the 

 case of non-electrolytes ; for, by using them we exclude the 

 possibility of ionic action. Mathews has shown that non-electro- 

 lytes in isotonic solution will not stimulate a sciatic nerve, Loeb 

 and others have shown the same thing for muscle. On the other 

 hand, any non-conductor in solution having an osmotic pressure 

 of about 14 atmospheres will stimulate a nerve. The same 



