550 ABSORPTION OF DIFFUSIBLE SUBSTANCES. [BOOK 11. 



laws which regulate their disappearance from one side of an 

 ordinary diffusion septum. This can be ascertained by introducing 

 solutions of the substances, of various strength, into a loop of 

 intestine, isolated in the living animal by the method described in 

 250, and watching their disappearance by analysis of the con- 

 tents of the loop. Experiments thus made shew the difference 

 on which we are dwelling. For instance, sodium sulphate passes 

 through an ordinary diffusion septum with a rapidity rather 

 greater than that of dextrose, whereas dextrose disappears from 

 the intestine distinctly more rapidly than sodium sulphate ; 

 peptone which diffuses very slowly indeed through an ordinary 

 diffusion septum disappears rapidly (though not so rapidly as 

 dextrose) from the intestine ; and when the details of the disap- 

 pearance from the intestine of weak solutions of two salts which 

 diffuse through an ordinary membrane at different rates, which 

 have as it is said different osmotic equivalents, are studied, these 

 details are quite different from those of ordinary diffusion. The 

 more the matter is studied the more decidedly apparent becomes 

 the difference between ordinary diffusion and the absorption of 

 diffusible substances from the intestine. 



Moreover, in such experiments on an isolated loop of intestine, 

 the disappearance of material from the intestine is accompanied 

 by the appearance of material in the intestine, namely proteid and 

 other substances ; these are derived from the blood. And the 

 question arises, If we allow ourselves to regard the passage of 

 material from the interior of the intestine into the blood as 

 carried out by ordinary diffusion, why should we not regard the 

 passage of material from the blood into the interior of the intestine 

 as being also carried out by means of diffusion ? But such a 

 passage we speak of elsewhere as a "secretion"; and everything 

 which we have hitherto learnt has led us to the conclusion that 

 secretion is a different and much more complex thing than mere 

 diffusion. Even admitting that the succus entericus is of subor- 

 dinate importance in carrying out digestive changes, we cannot 

 doubt that the glands of Lieberkuhn secrete, and may with some 

 reason suppose that the columnar cells of the villi do so also. 

 Hence even if we assume the existence of an ordinary diffusion 

 current from the blood into the intestine, accompanying and 

 complementary to an ordinary diffusion current from the intestine 

 into the blood, we are compelled to admit that with this there 

 coexists, at times at all events, and in varying intensity, a current 

 of a different and more complex nature, a current which is the 

 result of secretory activity. And results which at first sight seem 

 explicable by the former, may, after all, be due to the latter. 

 Thus the flow of water into the intestine with the subsequent 

 production of a watery stool, which follows upon the introduction 

 into the alimentary canal of a concentrated solution of magnesium 

 or sodium sulphate, may at first sight seem to be simply the 



