FROM THE SMALL INTESTINE 625 



of molecules and ions in the solution, and this cannot be 

 determined by chemical analysis. Further, organic substances, 

 such as proteids, present in physiological fluids yield on in- 

 cineration organic acids, &c., capable of exerting a considerable 

 osmotic pressure, which would have been impossible in the 

 original solution. If, therefore, we wish to know the osmotic 

 pressure of one of these fluids we must determine its A. From 

 what has been said it would appear that we could then deter- 

 mine the concentration of the electrolytes present by estimating 

 the electrical conductivity of the fluids. This is true for such 

 a fluid as urine, but in the case of blood, lymph, and other fluids 

 containing large quantities of proteid it would not hold good. 

 For it has been found that the presence of colloids, whilst not 

 altering the diffusion velocity of an electrolyte, does diminish 

 its electrical conductivity. In order to carry out a complete 

 osmotic analysis of serum, &c., it is necessary to employ com- 

 plicated indirect methods which cannot be discussed here. 



When the osmotic pressure of a fluid has been determined, 

 we know only the pressure which the solution would exert 

 on a true semipermeable membrane ; but this tells us nothing 

 about how much of this pressure would be exerted on a cell 

 wall, unless we know the permeability of tissue cells for the 

 various substances in solution. Of the actual permeability of tissue 

 cells for various substances we know very little. De Vries esti- 

 mated the osmotic pressures of various solutions by immersing 

 in them tangential sections of Tradescantia leaves. When the 

 solution had an osmotic pressure just higher than the contents 

 of the cell, the protoplasm was . observed to undergo plasmolysis 

 and shrink up. There is no doubt that this vegetable cell 

 wall is a more or less semipermeable membrane for some sugars 

 and salts. In the case of animal cells we know little of the 

 permeability of any, excepting red blood- corpuscles. Hamburger 

 estimated osmotic pressures by adding red blood - corpuscles 

 to various strengths of different salt solutions. If the solution 

 is hypotonic to the corpuscles, they swell up, and their haemo- 

 globin is liberated. If two strengths of a solution are found, 

 one of which just does and the other does not produce haemolysis, 

 then the mean strength would be isotonic with the animal's 

 blood plasma. Another method of determining the same thing 

 is to measure directly the volume of red corpuscles in the 



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