524 ABSORPTION OF DIFFUSIBLE SUBSTANCES. [BOOK n. 



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 subtances, 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 

 substances, part of the sugar for instance, arrived inside the 

 basement membrane should slip by the capillary blood vessel and 

 passing through the meshes of the capillary network make its 

 way into the lacteal. And indeed, as we have seen, 308, 

 under certain circumstances some amount of sugar appears to 

 take this course. But, as we have also seen, under ordinary 

 circumstances the current, whatever be its exact nature, from 

 the narrow lymph-spaces lying between the epithelium and the 

 capillary into the blood-stream is strong enough to carry all or 

 nearly all the sugar into the blood. In the establishment of this 

 current, in this second stage of absorption diffusion always plays 

 a part, and probably a still more conspicuous and decided part 

 than in the first stage, seeing that the epithelioid plate of the 

 capillary wall is a far less active structure than the columnar cell 

 of a villus. Indeed it might be open for us to contend that this 

 second stage was merely a matter of diffusion, whatever might be 

 the nature of the first stage. But remembering what was said 

 above, 302, in discussing the transudation of lymph, it seems 

 more in accordance with what we already know, to conclude that 

 in this second stage also diffusion is the servant and not the 

 master of the living capillary wall. 



A word may be added concerning the special case of the 



