ABSORPTION, ASSIMILATION, AND STORAGE OF FOOD 361 



We have already had illustrations of the action of these 

 ferments. Thus, the amylopsin of the pancreatic juice 

 changes the complex starch molecules to the simpler 

 molecules of malt sugar. In this form it is soluble. But 

 even the malt sugar molecule must later be broken up 

 into dextrose molecules for the use of the cells. Out of 

 the malt sugar molecule the ferment maltase produces 

 two dextrose molecules. The ptyalin of the saliva begins 

 the change of starch into sugar, but does not finish it. 

 The pepsin of the stomach, acting under acid conditions, 

 breaks up some of the complex proteid molecules into 

 peptones, which are soluble in the gastric juice. Proteid 

 digestion is not, however, completed in the stomach, so 

 the trypsin of the pancreatic juice, acting under alkaline 

 conditions, completes the process. In some similar way 

 the steapsin of the pancreatic juice changes the fats into 

 soluble form. It probably does this by decomposing the 

 fats into glycerine and acids. The acids formed are the 

 "fatty acids" (cf. 224), such as stearic acid, oleic acid, 

 etc. Butter would give chiefly butyric acid (from butyrum, 

 "butter"). Of these fatty acids butyric acid is the only 

 one that is very soluble, in the strict sense of the word. 

 But the other acids are broken up, in the small intestine, 

 into very minute particles, so that they form a part of an 

 emulsion, like milk (cf. 91) . This emulsion can be taken 

 through the cell walls of the villi, and^into the lacteals 

 (cf. 365). 



369. Absorption, Assimilation, and Storage of Food. 



Absorption (cf. 365) is not only the transferring of 

 digested food to the blood ; strangely enough, the peptones, 



