HUMAN STRUCTURE AND LIFE-ACTIVITIES 477 



been demonstrated within recent years that saliva continues 

 its action on starch for some minutes after the food reaches 

 the stomach, or until the acid gastric juice stops the action. 



399. Digestion by Gastric Juice. Physiologists have 

 found that the human stomach secretes from five to ten 

 quarts of gastric juice in twenty-four hours. This secretion 

 consists chiefly of water containing a small amount of hydro- 

 chloric acid and three enzymes pepsin, rennin, and lipase. 

 The last named seems to be of most importance after food 

 reaches the intestines. The action of pepsin and rennin are 

 most easily demonstrated. Pepsin may be extracted by 

 soaking stomach-membranes in glycerine, but the commercial 

 extract sold at drug-stores is most convenient for experi- 

 ments. Rennin is especially abundant in the stomachs of 

 calves, and is sold in grocery and drug stores under the 

 names of " liquid rennet " and " junket tablets." 



Numerous experiments by physiologists have proved that 

 gastric juice digests proteins, curdles (coagulates) milk, dis- 

 solves some minerals in foods, digests a small amount of fat, 

 but it has no effect upon starch. In this course, we have 

 time for only a few experiments which illustrate some of 

 these points. 



(D) The action of pepsin on proteins. (1) Make some albumen- 

 solution by mixing white-of-egg in cold water, fill a test-tube half 

 full, add some dry pepsin or glycerine extract of pepsin, add enough 

 hydrochloric acid to make the mixture slightly acid to litmus paper, 

 place the tube in a warm place near a stove or radiator, or in a "fire- 

 less cooker," or bucket of water heated to 37 C. (98 F.) and pro- 

 tected from cooling rapidly. After from three to ten hours, pour the 

 contents of the test-tube into a membrane bag or osmose-apparatus 

 and suspend in pure water. (Start the next experiment at this time.) 

 Allow an hour for osmosis. Test the water for proteins ( 377), 

 using Millon's reagent. Have proteins gone through the membrane 

 into the water? 



(2) Pour some fresh undigested albumen-solution into a membrane 

 bag, and after an hour test for osmosis as in above experiment. 

 Does this albumen osmose ? What conclusion is to be drawn con- 



