104 ADVANCED PHYSIOLOGY 



etc.) is a proteid product with very limited food value. 



We thus see that there is a genuine basis for the arguments 

 which insist on a varied diet; for not all the substances in even 

 the so-called "best foods" have real nutritional value. 



The amylopsin acts upon starches much as does the ptyalin 

 of saliva and completes their change into sugar. 



The steapsin acts upon the fats causing their digestion. 



Two very different changes take place in 

 them. First, much of the fat is broken up in- 

 to extremely minute droplets, which float in 

 the liquid of the food. In this condition it re- 

 sembles the fat of milk and, indeed, the entire 

 contents of the intestine, because of this con- 

 dition of the fat, become more or less white FIG. 56. EMULSI- 

 like milk. Such a condition of finely divided FIED FAT 

 fat droplets is called an emulsion; Fig. 56. 

 It was formerly thought that these fat drop- 

 lets were absorbed directly through the walls of the intestine 

 into the blood ; but it is now known that a second change takes 

 place in part of the fat, if not in all, before it is really absorbed. 



This is a chemical change which results in splitting up the 

 fat molecules into two different substances called fatty acids 

 and glycerine both of which are easily absorbed. Their 

 reformation into fat occurs very promptly, however, for true 

 fat droplets are found abundantly in the lining cells of the 

 intestine, immediately after fat digestion. See Fig. 64. 



No digestive gland has such varied and efficient powers as 

 the pancreas. The pancreatic fluid digests any food that has not 

 been acted on by the other digestive juices. Some foods, espe- 

 cially complex sugars, may be simplified by enzymes, e. g. 

 maltose (which acts on starch-sugars) and lactose (which 

 modifies milk-sugar) occurring in juices which are added to 

 the food mass by cells in the intestinal lining. 



After the food is completely digested, it is wholly changed 

 in nature and appearance. It was swallowed as meat, pota- 

 toes, bread and butter or milk; it has become dissolved into a 



