688 PHYSIOLOGICAL CHEMISTRY. 



gastric enzyme acting upon carbohydrates, but a small amount of the 

 simple sugars may be absorbed here. In the intestine the starches 

 are energetically attacked by the amylopsin (diastase) of the pancreas, 

 and the disaccharides are converted into the monosaccharides by the 

 inverting enzymes of the succus entericus. In this manner almost 

 all of the starch and sugar of the food is normally reduced to the 

 hexose form, the greater part being dextrose, with some laevulose, 

 galactose, and pentose (from cane-sugar, milk-sugar, and various vege- 

 tables respectively). These simple sugars are absorbed by the small 

 intestine, transferred as such to the blood-stream, carried directly to 

 the liver by the portal vein, and here stored up as glycogen. If an 

 excessive amount of sugar, particularly a simple sugar, be eaten, it is 

 absorbed more rapidly than the organism is able to care for it, and it 

 will appear unchanged in the urine (alimentary glycosuria, Icevulo- 

 suria, etc.). The exact mechanism of this fact is not known. It is 

 commonly believed that the liver is unable to convert more than a 

 certain amount of sugar into glycogen, hence there results an exces- 

 sive quantity of sugar in the blood, which excess is excreted by the 

 kidneys. The amount of sugar which can be eaten at one time with- 

 out a resulting excretion is termed the assimilation limit of that sugar. 

 The assimilation limit differs for the different sugars and in different 

 individuals. 



Fats undergo less digestive change than either carbohydrates or 

 proteins. Their digestion occurs mainly in the intestine, and consists 

 primarily of a saponification into glycerin and fatty acid by the lipase 

 of the pancreas. The biliary fatty acids combine with the alkali of 

 the intestinal contents to form soaps, which produce an emulsification 

 of the neutral fat still present. This emulsification is believed to be 

 of importance in offering a greater surface of fat for the action of the 

 lipase. The bile has two important actions in fat digestion, the bile 

 salts aid the action of the lipase and also act as solvents for both the 

 fatty acids and the soaps. It is believed now that little or no fat is 

 absorbed in the form of an emulsion, and that the greater part is 

 taken up as glycerin and fatty acid (soap). The glycerin is, of 

 course, readily soluble, while the fatty acid is very likely in solution 

 with the bile salts. Apparently the glycerin and fatty acid are at 

 once resynthesized in the mucous membrane of the intestine to neutral 

 fat and passed into the lacteals as an emulsion, which, in turn, is 

 carried to the general circulation by the thoracic duct. 



Proteins are digested by three enzymes, pepsin (stomach), trypsin 

 (pancreas), and erepsin. The three carry out what is fundamentally 



