MECHANISM OF ABSORPTION 



435 



to which the mucosa is turned, and to accumulate in the other. In 

 the cases mentioned the transportation of the liquid depends upon 

 the survival of those properties of the mucous membrane which 

 characterize its elements as living cells. For when the cells that 

 line the intestine are injured or destroyed, or subjected to the 

 action of certain poisons, absorption from it is diminished or 

 abolished. In the dead intestine the characteristic set of the tide 

 out of the lumen across the mucosa can no longer be observed. It 

 must be remarked further, in this connection, that in its normal state 

 the mucous membrane does not take up indiscriminately all kinds 

 of diffusible substances, or absorb those which it does take up in 

 the direct ratio of their diffusibility. Nor does it reject everything 

 which does not diffuse. Albumin, for example, which does not 

 pass through dead animal membranes, is to a certain extent taken 

 up from a loop of intestine without change. Cane-sugar (after 

 inversion) and dextrose are absorbed more rapidly than their 

 velocity of diffusion would indicate, when compared with inorganic 

 salts. Glauber's salt diffuses in water fifteen times as fast as cane- 

 sugar, but cane-sugar is absorbed from the intestines ten times faster 

 than Glauber's salt. The velocity of absorption is different even 

 for simple stereoisomeric sugars i.e., sugars whose molecule, with 

 the same number of atoms combined in the same way, has a dif- 

 ferent form (Nagano). Nor is there any clear relation between the 

 rate of absorption of the various sugars and their osmotic pressure. 

 Dextrose and cane-sugar are always absorbed in greater amount 

 than lactose from solutions of the same osmotic pressure. Indeed, 

 as we shall see, lactose is practically not taken up at all as such 

 (p. 445), and in concentrated solutions may even cause a reversal 

 of the normal movement of water, and act as a purgative. Even the 

 water, organic and inorganic solids of the serum of an animal, are 

 absorbed from a loop of its intestine when the blood-pressure in the 

 capillaries of the intestinal wall is considerably greater than the 

 pressure in the cavity of the gut. Since the serum in the intestine 

 and the plasma in the capillaries must be isotonic, and practically 

 identical in chemical composition, the absorption cannot be due 

 to ordinary osmosis or diffusion. Nor can it be due to filtration, 

 since the slope of pressure is from the capillaries to the lumen of 

 the gut (Reid). It is therefore extremely difficult to reconcile this 

 experiment with any purely physical theory of absorption. The 

 same investigator, summing up the result of careful experiments on 

 the absorption of weak solutions of glucose, concludes that, ' with 

 the intestinal membrane as normal as the experimental procedure 

 will permit, phenomena present themselves which are as distinctly 

 opposed to a simple physical explanation as those previously 

 studied in the absorption of serum.' 

 There is also evidence that even during the absorption of liquids 



