ABSORPTION 405 



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). 

 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. And in their normal state they do not take 

 up indiscriminately all kinds of diffusible substances, or absorb 

 those which they do take up in the direct ratio of their diffusibility. 

 Nor do they 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 different 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. 416), 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 re- 

 concile this experiment with any purely physical theory of absorp- 

 tion. The same investigator, summing up the result of careful 

 experiments on the absorption of weak solutions of glucose, con- 

 cludes that ' with the intestinal membrane as normal as the experi- 

 mental 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.' 



But if it be true that the action of the columnar epithelium of the 

 intestinal mucous membrane is secretory and selective, making use 

 of purely physical processes, but not mastered by them, the possi- 

 bility must be admitted that in the cells of endothelial type which 

 line the serous cavities, the lymphatics, the bloodvessels, the alveoli 

 of the lungs, and the Bowman's capsules of the kidney (p. 452), the 

 element of secretion may be less marked, and more overshadowed 

 by the physical factors. And it may very plausibly be urged that 

 changes of considerable physiological complexity can only be 

 wrought on substances that have to pass through a cell of con- 

 siderable depth, while a mere film of protoplasm suffices for, and 

 indeed favours, mechanical filtration and diffusion. We have 

 already seen (p. 258), in the case ">f the lungs, that whatever the 



