430 ABSORPTION 



which undergo no chemical changes in the gut e.g., salt solutions 

 of different kinds and different concentrations chemical energy 

 must be transformed, and on no mean scale, in the intestinal 

 mucosa, for the consumption of oxygen and the production of 

 carbon dioxide by the intestine is markedly increased (Brodie and 

 Vogt). 



It may be taken, then, as quite certain that in absorption from 

 the alimentary canal an essential factor is the activity of the living 

 cells of the mucosa, which, in some way at present unknown, main- 

 tain the ' set of the tide ' from the lumen to the bloodvessels (or 

 lymph spaces), whether the slope of concentration of the dissolved 

 substances favours or opposes it, or when the concentration is the 

 same on both sides of the membrane. It is even probable that this 

 action of the cells is much the most important of all the factors 

 involved. It would be highly misleading, however, to assume thai, 

 because this is so, other factors osmosis, diffusion, possibly eveu 

 nitration due to differences of pressure caused by the intestinal 

 movements or the contractions of the muscular fibres of the villi 

 (p. 437) are of necessity negligible. On the contrary, these other 

 factors cannot be adequately taken account of, nor can there be 

 any possibility of assigning to them their proper value until it is 

 recognized that their influence in absorption from the digestive 

 tract is never under ordinary conditions expressed as a simple and 

 uncomplicated effect, such as may be observed in experiments with 

 dead membranes, but, on the contrary, is constantly overlaid, 

 thwarted, or totally reversed, by the special action of the cells. 

 For this reason the discussion of the mechanism of absorption under 

 the time-honoured captions of ' mechanical or physical theory ' 

 versus ' physiological or vital theory,' as if the process must of 

 necessity be purely ' physical ' or purely ' vital,' has lost interest. 

 It may be confidently assumed, indeed, that just because the 

 physiological factor is so dominant, the familiar physical forces 

 must often appear to exert a smaller influence than is really the 

 case, and that, could we disentangle the currents which they create 

 and sustain from that steady drift of material out of the lumen of 

 the gut maintained at the expense of its chemical energy by the 

 still unknown machinery of the cells, we should be impressed with 

 the magnitude rather than the insignificance of their total effect. 



The following attempt by Hober to analyze on these lines an old 

 experiment of Heidenhain, which the latter observer had interpreted 

 as showing that diffusion and osmosis play no essential part in absorp- 

 tion from the alimentary canal, is of interest in this connection. A 

 loop of small intestine was tied off at both ends, but its circulation was 

 not otherwise interfered with. Solutions of sodium chloride of dif- 

 ferent strengths were introduced into the loop, and in each observation 

 after fifteen minutes the contents of the loop were recovered and 

 analyzed for the chloride. 



