356 FOOD AND DIGESTION 



as follows: Gastric juice turns congo-red to a blue color. Organic acids pro- 

 duce violet. Gastric juice plus 0.5 per cent alcoholic solution of dimethyl- 

 amido-azobenzol develops a cherry-red color, a reaction that is given by free 

 hydrochloric acid. Combined hydrochloric acids and organic acids do not give 

 the color. Giinzburg's reagent, consisting of 2 per cent phloroglucin and i 

 per cent vanillin in 80 per cent alcohol, produces a rose-colored mirror 

 on porcelain in the presence of free hydrochloric acid. The test is very 

 delicate. 



Proteids. The usual proteid tests can be applied to gastric juice and show 

 that it contains small quantities. 



11. Artificial Gastric Juice. An active principle, pepsin, of gastric 

 juice can be obtained by extracting the gastric mucous membrane of the 

 dog, pig, etc. Scrape off the mucous membrane, grind it to a fine pulp by 

 repeatedly running it through a sausage machine, or by pounding in a mortar 

 with clean sand. The mucous membrane should be allowed to stand for 

 three or four hours before extraction, otherwise the zymogen, and not the 

 enzyme, will be obtained. Extract a portion of this gastric pulp in water, and 

 filter. Or extract with glycerin for several weeks and filter. Either of these 

 extracts contains the enzyme. A solution of the glycerin extract in 0.2 per 

 cent hydrochloric acid contains all the properties of gastric juice. This is 

 known as artificial gastric juice. 



Commercial pepsin already prepared can be obtained on the market. 

 Artificial gastric juice is made from commercial pepsin by adding 3 to 5 grams 

 to a liter of 0.2 per cent hydrochloric acid. 



12. Digestive Action of Gastric Juice, or Artificial Gastric Juice. 

 The digestive action of gastric juice is limited to proteids. Shreds of fibrin 

 which permit the gastric juice to come in intimate contact with all parts of 

 the material, form the best proteid for testing the action of this enzyme. 

 Prepare a series of test tubes, a, b, c, d, each containing 5 c.c. of artificial 

 gastric juice. Add to a some shreds of fibrin; to b some boiled white of an egg; 

 to c some fibers of boiled meat; to d some fibers of raw meat; place in a warm 

 bath at 40 C. and examine at intervals of 20 minutes. Tabulate the results 

 by the plan indicated in experiment 13, noting particularly the rapidity with 

 which the proteid goes into solution. 



13. Conditions Affecting the Enzyme Action of Gastric Juice. 

 Prepare a series of test tubes containing 5 c.c. each of gastric juice, according 

 to the table on the following page. Add a definite quantity of fibrin to each 

 and note the changes at intervals of 20 minutes. 



14. Cleavage Products of Gastric Digestion. Add 5 to 10 grams 

 of fibrin to 100 c.c. of artificial gastric juice in a flask and place in a 

 water bath at 40 C. After one hour filter off 40 c.c. Exactly neutralise this 

 filtrate with i per cent potassium hydrate. A precipitate makes its appearance, 

 and can be collected on the filter paper, washed with distilled water, and dis- 



