428 ABSORPTION 



that after all physical explanations have been exhausted, the real secret of 

 the cell will still lurk in some ultimate ' vital ' property of structure or of 

 function, and still elude our search. The only sense in which this attitude 

 can be said to be a useful one is that it presents a standing protest against 

 the acceptance of superficial ' physical ' explanations merely because they 

 are physical. Both the optimists and the pessimists, the adherents 

 of the physical and the adherents of the vitalistic hypothesis, have, 

 unfortunately, as a rule taken up an extreme position, as if their theories 

 were mutually exclusive. They agree, however, in admitting that the 

 phenomena of absorption are essentially connected with the cells that 

 line the alimentary canal, and not with any more or less inert ' cement ' 

 substance between them. But the one school must confess what the other 

 proclaims, that while the processes carried on in these cells are definite, 

 well ordered, and evidently guided by laws, these laws have as yet 

 for the most part denied themselves to the modern physiologist, with 

 chemistry in one hand and physics in the other, as they denied them- 

 selves to his predecessor, equipped only with his scalpel, his sharp eyes, 

 and his mother- wit. So that in the present state of our knowledge all 

 we can really say is that, while absorption is certainly aided by physical 

 processes, like osmosis and diffusion, possibly by physical processes 

 like imbibition, and is very likely not unrelated to the molecular proper- 

 ties of surfaces (surface tension, adsorption), it is at bottom the work 

 of cells with a selective permeability which we do not fully understand, 

 or at least which we cannot as yet explain in terms of known physical 

 processes acting through a membrane of known physico-chemical 

 structure. 



Thus, dissolved substances pass with equal ease in either direc- 

 tion through an ordinary diffusion membrane, but in general they 

 pass, with the water in which they are dissolved, more readily out 

 of the intestine than into it. This normal direction of the stream 

 is still maintained for a considerable time after stoppage of the 

 circulation, provided that the intestine is kept in good condition 

 for example, by being suspended in well-oxygenated blood. Water 

 or solutions of sodium chloride or sugar disappear from the lumen. 

 And this is not due to mere imbibition by the intestinal wall, but 

 the liquid is actually transported across it. The theory that liquids 

 might be taken up from the gut by imbibition, and the water then 

 mechanically removed by the blood flowing on the other side of the 

 imbibing cells, is incompatible with this experiment (Cohnheim). 

 Nor is it necessary that differences of concentration of the dissolved 

 substances on the two sides of the absorbing intestinal membrane, 

 which would permit osmosis and diffusion to go on, should exist. 

 When the excised intestine of a holothurian was filled with sea- 

 water and suspended in the same sea-water, its contents continued 

 to diminish in bulk for hours or entirely disappeared. Here a liquid 

 identical in composition and concentration with the external liquid 

 was moved in a definite direction across the wall of the intestine 

 from the lumen to the exterior surface. In like manner, when a 

 piece of intestine from a newly-killed rabbit is stretched across a 

 vessel of salt solution so as to divide it into two separate compart- 

 ments, the solution continues for a while to leave the compartment 



