94 THE FOOD-PREPARING SYSTEM 



Saliva. — The first of the digestive enzyms is that in 

 saliva. It is called ptyalin, a sort of amylase, and its 

 action is to decompose starch into a form of sugar which 

 is familiar to the chemist but not especially familiar in 

 ordinary life. It is the so-called malt-sugar, being ex- 

 actly the same sort of sugar that is produced in germinat- 

 ing barley in connection with the manufacture of malt. 

 This malt-sugar has to undergo further digestion in the 

 small intestine before it is ready to serve as fuel for 

 living cells. 



Gastric Juice. — After the food is swallowed it enters 

 the stomach where it comes in contact with the second 

 enzym of the series, pepsin, a part of the gastric juice. 

 This proteolytic (i.e., protein-splitting) enzym begins 

 the decomposition of the proteins, reducing them from 

 the highly complex form in which they are eaten to 

 simpler forms known as proteoses and peptones. The 

 action of pepsin is like the action of ptyalin in that it 

 is not final but preparatory. Further action must take 

 place to complete the digestion that is started by pepsin, 

 as well as by ptyalin. 



Pancreatic Juice. — After the partly digested food 

 passes from the stomach into the small intestine it comes 

 in contact with the secretion of the pancreas to which 

 we give the name of pancreatic juice. It contains three 

 enzyms. The first is an enzym which has exactly the 

 same action as the ptyalin of saliva, i.e., can cause starch 

 to take up water and split into maltose or malt sugar, 

 and as far as we know is the same enzym. The second 

 is lipase, which digests fats. The third is trypsin, which 

 digests proteins. Trypsin is a more effective enzym 

 than pepsin, in that it carries the digestion, i.e., hydroly- 

 tic splitting, of proteins much further. In fact, the re- 

 sult of digestion by trypsin is to break the proteins up 

 into the amino-acids of which their molecules are 

 composed. 



The Composition of Proteins. — As has already been 



