DIGESTION 395 



systhesis and must be oxidized. 1 It is also possible that the pro- 

 ducts of digestion of certain proteids may be injurious. 



Fats are absorbed as fatty acids, as glycerol, as soaps, 

 and in a finely divided form suspended in the digested 

 solution, as an emulsion. There is a union of fatty acids and 

 glycerol in the absorbing membrane, so that only fats enter the 

 chyle or blood. 



Carbohydrates are converted into simple sugars (grape sugar, 

 fructose, etc.,) or by fermentation, into acids, such as lactic acid, 

 butyric acid, etc. These appear to some extent in the chyle, but 

 more largely in the blood. 



Various methods are used in studying the processes of 

 digestion. The digestive juices have been secured from animals, 

 and their action tested upon various constituents of the food. 

 The process of digestion has also been to some extent observed 

 through openings made into the digestive organs by accident or 

 intention. The contents of digestive organs have been removed 

 and examined. 



Rennet, prepared from the stomachs of calves ; and pepsin, pre- 

 pared from animals killed in the slaughter house, are commercial 

 products, the former being used in coagulating milk in the manu- 

 facture of cheese, and the latter for medicinal purposes. None 

 of the digestive ferments so far isolated have the power of 

 causing crude fiber to go into solution. The intestinal bacteria, 

 however, when inoculated into a medium containing crude fiber, 

 cause it to be partly dissolved, producing marsh gas, carbon 

 dioxide, organic acids, and soluble products which can be absorbed 

 and utilized. 



Excretion. The undigested residues, mixed with gallic acid, 

 mucus, with other animal products (metabolic products), and with 

 digested but unabsorbed material, are finally ejected. 



The excrement is by no means free of digestible materials. The 

 quantity of digestible matter is small, however, unless the food 

 is imperfectly masticated, or unless its premature evacuation is 

 caused by digestive disturbances. 



1 See Wisconsin Research Bulletin No. 21. 



