CHAPTER VIII 



ABSORPTION 



BY absorption we understand all those processes by which the digested 

 foodstuffs are taken up from the cavity of the alimentary canal into its mucous 

 membrane and are forwarded thence to the general circulating fluids. 



1. ABSORPTION IN GENERAL 



After Dutrochet had discovered the osmotic phenomena, it was thought 

 that absorption in the intestine could be easily explained by osmosis. Diges- 

 tion was for the purpose of changing the foodstuffs contained in the food 

 into easily diffusible substances, if they were not already diffusible. Hence 

 absorption took place according to the well-known physicochemical laws of 

 osmosis. 



More searching investigation, however, of matters as they are, have made 

 us acquainted with facts which preclude so simple a process, and have led 

 us for the present to the view that the activity of the living mucous membrane 

 plays an essential part in absorption. It is perfectly evident that purely 

 physicochemical processes, like filtration, osmosis, imbibition, etc., are in- 

 volved, and this requires no further argument. 



Among the more important observations for the theoretical explanation of 

 absorption, those upon the behavior of weak salt solutions and of blood serum 

 should be mentioned first (Voit, Heidenhain and others). If normal or slightly 

 diluted blood serum be placed in an intestinal loop of a dog, notwithstanding 

 that the conditions of the experiment exclude the cooperation of osmotic pres- 

 sure, water and salts are absorbed in almost the same proportion as that in which 

 they exist in the serum introduced, whereas the organic substances take part in 

 absorption in far less proportion. If a solution of common salt whose osmotic 

 tension is higher than that of the blood be introduced, according to the laws of 

 osmosis no water should be absorbed ; but it is absorbed. And, vice versa, common 

 salt is absorbed from a solution in which the osmotic tension is less than that 

 of the blood. The absorption of water from a weak dextrose solution is not 

 changed, if the osmotic pressure of the blood be raised by intravenous injection 

 of common salt (Reid). From equimolecular and therefore isosmotic solutions 

 of different kinds of sugar which are stereoisomeric, the quantities of sugar 

 absorbed in unit time are not equal (Rohmann and Negano). 



Moreover it has been shown that the movement of salt in the normal intes- 

 tine takes place in the direction from lumen to tissues, much more easily than 

 in the opposite direction (O. Cohnheim) ; that an intestine cut out. of an animal 

 in full digestion, if bathed both within and without with a salt solution of the 



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