380 PRACTICAL PHYSIOLOGY 



mann's reagent (30 c.c. 1 % phenol solution and a few drops of ferric 

 chloride solution). The blue violet colour of the reagent changes to a 

 characteristic yellow when lactic acid is present. 1 



A much more sensitive and characteristic test for lactic acid is 

 Hopkins', which is applied as follows : 



EXPERIMENT VI. Mix some of the dry ethereal extract of stomach 

 contents with 5 c.c. concentrated sulphuric acid, and transfer to a dry 

 test tube. Add 3 drops of a saturated solution of copper sulphate, 

 mix, heat the mixture in a boiling water bath for 2 minutes. Cool 

 under the tap, and add 2 drops of a 0-2 per cent, alcoholic solution of thio- 

 phene and shake. Eeplace the tube in the boiling water bath, and 

 examine it frequently, when a cherry red colour will be found to 

 develop if lactic acid is present. Prolonged heating causes the solution 

 to become very dark. 



The lactic acid is produced by the action of the bacillus acidi lactici 

 and other organisms on sugars (see Milk, p. 329). 



C 12 H 22 11 + H 2 = 4C 3 H 6 3 

 Lactose. Lactic acid. 



The fermentative process seldom stops at the production of lactic 

 acid. Other bacteria act on the lactic acid and produce butyric acid, 

 carbon dioxide gas, and hydrogen. 



4C 3 H 6 O 3 = 2C 4 H 8 2 + 4Co 2 + 4H 2 



Lactic acid. Butyric acid. 



These gases accumulate in the stomach, causing flatulence. The 

 presence of butyric acid usually reveals itself by the odour of the 

 gastric contents. When its presence is doubtful, boil a portion of 

 the fluid and hold a strip of blue litmus paper in the steam. If this 

 turns red, it indicates a volatile acid (butyric or acetic). Butyric acid 

 has a characteristic odour. 



Acid phosphates (NaH 2 Po 4 ) when present in gastric contents are 

 demonstrated by mixing calcium carbonate with a portion of the fluid. 

 If an acid reaction still remains towards litmus paper, acid phosphates 

 must be present, since the calcium carbonate will have combined with 

 the free acids. 



In the clinical examination of the stomach contents numerous methods have been 

 introduced for the purpose of estimating the total acidity, the total amount of 

 hydrochloric acid, and the amount of free (uncombined) hydrochloric acid contained 

 therein. It would be out of place to go exhaustively into these methods here, 

 but a brief outline of the most trustworthy may be of value. 



1 It is indispensable to make an ethereal extract for this test, because alcohol, 

 sugar and phosphates give the reaction, and one or other of these is always 

 present in the gastric contents. 



