XVI.] STOMACH. GASTRIC JUICE. MILK. 173 



These experiments show that acid alone (A) 

 and pepsin alone (C) will not digest fibrin, 

 and that pepsin loses its power on being 

 heated to boiling point (D). Now add acid 

 again to C, and place it in the warm chamber. 

 Digestion will take place. The neutraliza- 

 tion has only suspended, not destroyed, the 

 action of the pepsin. 



I. Take two test-tubes, with 5 c.c. of gastric 

 juice and a morsel of fibrin in each. 

 Place A in the warm. Surround B with 

 ice, or put it in a cold spot. 

 The fibrin in A will be digested rapidly; 

 that in B very little or not at all. 



3. Take 5 c.c. of artificial gastric juice which has 

 been found to digest fibrin rapidly, neutralize it, 

 filter and add an equal bulk of Na 2 C0 3 2 p. c. 

 thus obtaining pepsin in the presence of a small 

 quantity of an alkaline salt. Place at about 

 40 C. for half-an-hour to an hour. Then add 

 HC1 until the mixture is distinctly acid (or 

 neutralize and add an equal volume of HC1 '4 

 p.c.). Add a flock or two of fibrin and warm. 

 Little or no digestion will take place. The 

 pepsin has been destroyed by the alkaline salt. 



4. Place 50 c.c. of artificial gastric juice together 

 with some fibrin or other proteid in a beaker 

 and leave in the warm until a small part only 

 of the proteid remains undissolved. Filter and 

 neutralize carefully, a precipitate of acid-albumin 



