382 FOOD AND DIGESTION 



the reflex secretion just described takes place as usual, but is followed after 

 an hour or an hour and a half by a second increase in the quantity of secre- 

 tion. This second increase has been ascribed to the reflexes originating in 

 the stomach, possibly from the reflex stimulating action of the digestive prod- 

 ucts themselves. 



10. Composition of Gastric Juice. Test a sample of gastric juice 

 obtained from a gastric fistula, as follows: 



Reaction. Gastric juice is strongly acid. Test for free hydrochloric acid 

 as follows: Gastric juice turns congo-red to a blue color. 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. Com- 

 bined hydrochloric acids and organic acids do not give the color. Giinz- 

 burg'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. 



Proteins. The usual protein tests (page 107) can be applied to gastric 

 juice and show that it contains small quantities. 



11. Artificial Gastric Juice. The 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 o . 2 per 

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

 solution 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 o . 2 per cent, hydrochloric acid. 



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

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

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

 the material form the best protein 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 protein goes into solution. 



13. Condition Affecting the Enzyme Action of Gastric Juice. Pre- 

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



