276 PHYSIOLOGY CHAP. 



and impedes their passage in the reverse direction. This capacity 

 of the epithelium for letting certain solutions through m one 

 direction only is met with in many other cells, which, so long as they 

 are alive, possess a wall that is impenetrable to certain substances, 

 and lose this property after death, on which an exchange takes 

 place between the materials they contain and those of the environ- 

 ment. Cohnheim reminds us in this connection of the erythrocytes 

 and the muscles, which in many animals are devoid of sodium and 

 rich in potassium, although they may live in a medium rich in 

 soda and poor in potash. Plant cells, again, are colourless so long 

 as they are alive, although surrounded by a coloured juice, which 

 only penetrates their protoplasm when life is extinct. To the 

 same order belongs the fact discussed at length at the end of 

 the last chapter, viz. that the cells which form the intestinal wall 

 never, while living, allow themselves to be penetrated by the 

 digestive enzymes, so that auto-digestion takes place only after 

 death. 



In affirming, on the basis of well-established experimental 

 data, that intestinal absorption is not regulated by the physical 

 laws of diffusion, we do not mean that this process is of no 

 importance, and plays no part in absorption. The living cell (as 

 we saw in the first three chapters of Vol. I.) is the seat of a highly 

 complex metabolism, which consists of various chemical and 

 physical processes adapted to specific functional tasks. It would 

 be absurd to suppose that diffusion and osmosis do not figure 

 among the physical processes employed by the cell to accomplish 

 absorption. It would then be impossible to understand why non- 

 diffusible become converted into diffusible substances during 

 digestion. But the process of diffusion utilised 'in absorption is 

 adapted and modified to the service of a special function, so 

 that it differs essentially in its results from the diffusion which 

 takes place through a non-living porous septum. Foster gives a 

 suggestive interpretation of this difference : " The canals or spaces 

 are constant in a non-living septum ; but a film of a living cell 

 may be conceived of as a diffusion septum the pores of which are 

 continually varying, and, moreover, as closing up or opening out at 

 the touch of this or that substance ; hence the passage of material 

 through the pores of a living cell takes place according to laws 

 quite different from those of ordinary diffusion." 



The capacity for physiological selection which Spiro (1897) 

 attributes to living cells in general must be understood in the 

 same sense, and is entirely different from the physical selection 

 accomplished by dead tissues. 



This physiological theory of intestinal absorption is sub- 

 stantially confirmed by the later work of Hober (1898-1903) and 

 Cohnheim (1898-99). ' 



The former made a comparison in dogs between the absorption 



