256 PHYSIOLOGY OF ALIMENTATION. 



parchment paper, may be taken as a type of a permeable 

 membrane. By this is meant that these allow most crystal- 

 loids which are held back by semipermeable membranes to 

 pass through them with ease. But substances having a high 

 molecular weight (as the various colloids) pass through these 

 membranes not at all or only to a slight extent. 



Do true semipermeable membranes exist in the animal 

 organism? So far as we know they do not. Many cells 

 are impermeable to a large number of substances, but all are 

 permeable to some dissolved substances, The red blood- 

 corpuscles, for example, are impermeable to many different 

 salts, but they readily allow ammonium compounds, urea, 

 etc., to pass into them. The majority of cells are even more 

 permeable than the red blood-corpuscles, and this even to 

 very large molecules. The fact that colloids can to some 

 extent diffuse into other colloids already points in this direct 

 tion, and later we shall become acquainted with experiments 

 in which colloidal sodium silicate was found to be absorbed 

 from the intestinal tract and excreted in the urine. The 

 intestinal epithelium must therefore be looked upon as made 

 up of cells which are permeable to at least certain very large 

 molecular aggregates, if only sufficient time be allowed for 

 their absorption. But the intestinal epithelium is not equally 

 permeable to all substances, even when they possess approx- 

 imately the same molecular weight, and show in pure sol- 

 vents approximately the same rates of diffusion. What is 

 true for the intestinal epithelium holds still more when we 

 deal with different tissues. Each tissue varies in the ease 

 with which it will absorb different chemical compounds. This 

 selective permeability is probably more confusing in endeav- 

 oring to unravel the problem of absorption than any one 

 other factor, though, as will be seen shortly, great strides 

 have been made in the solution of this problem within the 

 last decade. 



We have not as yet said where these different membranes 

 exist in tissues. Every cell is surrounded by a membrane, 



