228 PRACTICAL PHYSIOLOGY 



polysaccharide starch (blue with iodine and no reducing power), 

 first into a simpler polysaccharide dextrin (red with iodine, no 

 reducing power), and then into the disaccharide maltose (no 

 colour with iodine, reduces cupric salts). 



The first effect of ptyalin on starch is to convert it into so-called 

 soluble starch (sometimes called amylodextrine). This gives a clear 

 solution with water and a blue colour with iodine. Then erythro- 

 dextrine giving a red brown colour with iodine and finally achroo- 

 dextrine which gives no colour with iodine is formed. During each 

 step in the breakdown a certain amount of maltose is formed. 

 This is small in amount at first, but becomes progressively more 

 and more with each successive dextrine formed. 



EXPERIMENT VII. Place some 0-5 per cent, solution of starch in 

 the mouth. After about two minutes transfer it to a test tube 

 and test its reducing power. Repeat this experiment with a 

 suspension of unboiled starch. 



EXPERIMENT VIII. Repeat experiment VI (1) with saliva 

 which has been boiled ; (2) with saliva and starch kept in a test 

 tube surrounded with ice ; (3) with saliva and starch in a test tube 

 immersed in boiling water ; and (4) with saliva and starch to 

 which a drop of mineral acid or strong caustic soda has been added. 

 (These same experiments may be repeated with pepsin and trypsin.) 



Gastric Juice 



Acidity of Gastric Juice. In marked contrast to most of the 

 other fluids of the animal body, gastric juice has a strong acid 

 reaction towards all indicators. In disease alterations may take 

 place in the degree and nature of the acidity of gastric juice and 

 these variations may have a diagnostic value. 



Acidity from a chemical standpoint is invariably due to the 

 presence of excess of hydrogen ions in the solution. For the pre- 

 sence of these hydrogen ions, one or other of three general causes 

 may be responsible, viz. the presence of free mineral acid, free 

 organic acid, and acid salt. The acidity in each case is in direct 

 proportion to the dissociation of the acid in watery solution, being 

 greatest for mineral acid. One of the first questions, therefore, 

 to be answered is : to which of the above causes is the presence 

 of hydrogen ions in gastric juice due ? The question is most simply 

 answered by the use of indicators, for it has been found that the 

 behaviour of these varies with the nature and cause of the 

 acidity. (See also p. 338.) 



Is the acidity due to a free acid or to an acid salt ? Congo red is 

 the most useful indicator for this purpose. 



EXPERIMENT I. To a 0-2 per cent. HC1 solution add a few drops 

 of congo red solution : l the red turns to blue. Repeat with a 

 dilute solution of acid sodium (NaH 2 PO 4 ) phosphate no blue 



1 Congo red solution dissolve 0-5 gm. of congo red in 100 c.c. of 10 per cent. 

 alcohol. 



