474 APPLIED BIOLOGY 



after allowing time for a change, try for osmosis of the food 

 through membranes, such as fish-bladder or parchment. 

 There are good reasons for thinking that any food which will 

 osmose through a dead membrane will also be absorbed under 

 conditions which exist in the linings of the stomach and 

 intestine. Hence any digestive changes which will prepare 

 foods in test-tubes for osmosis will serve to illustrate the 

 changes which take place inside the living alimentary canal. 

 But not all the digestive processes can be illustrated by test- 

 tube experiments, for the living digestive organs cause some 

 changes in foods which cannot be imitated in the lifeless 

 conditions in test-tubes. However, it is possible to perform 

 a number of experiments which will throw much light on the 

 various secretions and parts of the alimentary canal with 

 regard to their part in the digestion of the various kinds of 

 foods. We shall consider the digestive processes in the 

 order in which foods come in contact with the secretions in 

 passing from the mouth-cavity through the esophagus into 

 the stomach, and thence into the intestine ; this means con- 

 tact, in succession, with the salivary, gastric, intestinal, 

 pancreatic, and hepatic (liver) secretions. 



398. Digestion by Saliva. The fluid secreted by the 

 salivary glands consists chiefly of water and an enzyme 

 called ptyalin, which is very similar to the diastase of plants. 

 The dry ptyalin may be purchased from chemists, or saliva 

 may be collected in a test-tube (when chewing a piece of 

 rubber or gum it is secreted rapidly). The work of saliva is 

 the digestion of starch to a kind of sugar, which is absorbable. 

 This is illustrated by the following experiments: 



(D) Make some thin starch paste, by heating starch in water. 

 Notice that the resulting fluid is not clear, but opalescent. Place a 

 small quantity of the paste in a test-tube and add a few drops of 

 iodine-solution. Note the color. 



(D) Place some of the paste in a small bag made from a piece of 

 gold-beaters' membrane, or fish-bladder, and suspend the bag so 



