256 ABSORPTION OF THE FOOD. 



The materials to be absorbed are : (1) water, salts ; (2) carbo- 

 hydrates ; (3) fats ; (4) proteids. 



Absorption of Water. As was stated in connection with gastric 

 absorption (p. 254), water is not absorbed to any extent from that 

 organ, most of that taken in being passed on into the small intes- 

 tine. Experiments have shown that the water which enters the 

 small intestine is absorbed by the capillaries of the villi ; and yet 

 even when large quantities are absorbed, an analysis of the blood 

 shows no change, as might be expected, the excess being elimi- 

 nated by the kidneys. 



Absorption of Carbohydrates. The dextrose and levulose, formed 

 by the action of the enzymes, are absorbed by the veins and 

 carried by the portal vein to the liver, but there is evidence that 

 saccharose, and even dextrin and starch, can be taken up by the 

 cells; although, as we have seen, the action of the intestinal 

 enzymes is very powerful, and doubtless the amount of carbo- 

 hydrates remaining in any other condition than that of dextrose 

 or levulose is very small. But, even if carbohydrates should be 

 absorbed in any form but these, they would be inverted while 

 passing through the cells. It is a remarkable fact that lactose, 

 which forms so important a part of the milk, the sole food of the 

 growing child, is unaffected by the enzymes ; however, in its pass- 

 age through the epithelial cells it is inverted, the product being 

 probably dextrose and galactose. Maltose, also, may be inverted 

 by the columnar epithelium. 



It is, then, mostly in the form of dextrose and levulose that 

 the carbohydrates of the food enter the blood and are carried to 

 the liver, and from these glycogen is formed. Saccharose and 

 maltose cannot be thus changed by the liver-cells. It sometimes 

 happens that very large quantities of sugar are taken in with 

 the food ; if the amount is so great that the liver and muscles can- 

 not convert it all into glycogen, the overplus is eliminated by the 

 kidney, and appears in the urine, constituting alimentary glycosuria. 



Glycogenic Function of the I/iver. As we have seen, 

 the result of the digestion of starch is its conversion into mal- 

 tose, or maltose and dextrin, which later becomes dextrose, in 

 which form, for the most part, the carbohydrates of the food 

 reach the liver. Some levulose may accompany it to the liver, 

 where, according to some authorities, it becomes dextrose. If 

 during the time of the absorption of sugar the blood going to 

 the liver through the portal vein and that coming from it by 

 the hepatic vein are analyzed, it will be found that the former 

 contains much more sugar than the latter; from this fact the 

 inference is inevitable that some change takes place in the sugar 

 during its passage through the liver. This change consists in its 

 conversion by the hepatic cells into glycogen, which is a process 

 of dehydration, the reverse of what takes place when the starch or 

 glycogen of the food is converted into sugar. 



