552 ABSORPTION OF DIFFUSIBLE SUBSTANCES. [BOOK n. 



may be asked why are the digestive changes directed towards 

 increased diffusibility, why are proteids converted into diffusible 

 peptones, and why is starch converted into sugar ? Because though 

 the cell is not an apparatus for diffusion, diffusion is an instrument 

 of which the cell makes use. When we say that peptone does not 

 enter the blood by ordinary diffusion we do not mean that diffusion 

 has nothing to do with the matter. The activity of a living cell is 

 an activity, built up upon and making use of various chemical and 

 physical processes ; in it the processes of ordinary diffusion play 

 their part as do the processes of ordinary chemical decomposition ; 

 but the cell uses and modifies them for its own ends. If as we 

 have every reason to believe the cell of a villus passes the sugar 

 from the intestine into the blood capillary only so far changed 

 that one diffusible kind of sugar maltose is converted into another 

 kind of sugar, dextrose, also diffusible it makes use of diffusion to 

 effect that passage ; and if it does change the diffusible peptone 

 into some proteid not diffusible before it passes it on, it receives it 

 into itself in the first instance by help of diffusion. When we say 

 that substances do not enter the blood by ordinary diffusion we 

 mean that the diffusion which takes place in a living cell is some- 

 thing so different in the results from ordinary diffusion through a 

 dead membrane that it is undesirable to speak of it by the .same 

 name. In ordinary diffusion the results depend on the relation of 

 the molecules of the diffusing substance to the minute pores or 

 canals or spaces in the diffusion septum. These canals or spaces 

 are constant in an ordinary 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. 



313. The whole act of the absorption of substances with 

 which we are dealing consists, as we have said, of two parts: 

 the passage from the interior of the intestine through the 

 epithelium cell into the lymph-spaces or reticulum of the villus, 

 and the passage thence through the capillary wall into the blood- 

 stream. In the experiments referred to above it has not been 

 possible to distinguish between these two stages of the whole 

 process ; in each case we have had to make use of the terms ' from 

 the interior of the intestine into the blood' and 'from the blood into 

 the interior of the intestine.' Nevertheless the remarks which 

 have just been made may be taken as referring more especially to 

 the first stage. They lead us to the conclusion that both fats 

 and diffusible substances, though in different ways, are carried 

 into the interior of the villus by the activity of the epithelium 

 cells. 



In respect to the second stage of the absorption of diffusible 

 substances, it might be expected that part of one or other of these 



