522 ABSORPTION OF DIFFUSIBLE SUBSTANCES. [BOOK n. 



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 we should 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 from 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 Lieberkiihn 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 

 osmotic current passing from the weaker solution of the salt, 

 namely the blood, to the stronger solution of the salt, namely the 

 intestinal contents. But the difference between these effects of a 

 dose of magnesium sulphate and those of a corresponding dose of 

 sodium chloride are much greater than can be accounted for by 

 the diffusion phenomena, by the differing osmotic equivalents of 

 the two substances ; and the more the matter is studied the more 

 reason have we to believe that the flow of water produced by 

 the former is to a large extent the result of suddenly increased 

 secretory activity. So also the fact that the contents of the small 

 intestine throughout its length retain the same amount of water 

 relatively to the solids, that is to say maintain the same or nearly 

 the same fluidity, whereas in the large intestine the water relatively 

 diminishes until at last the faeces become firm and even dry, cannot 

 be wholly explained without calling into our aid variations in active 

 secretion as distinguished from mere physical diffusion. And in 

 the case of a purgative such as croton oil producing a watery 

 stool, when only a minimal, we might almost say an infinitesimal 

 amount of its own substance can at any one time be present in 

 the intestinal walls, the result is obviously due to active secretion. 



