DIGESTION AND ABSORPTION 169 



acid, and boil for ten or fifteen minutes to hydrclyze the mucin. 

 Neutralize the acid with sodium carbonate and then make FehHng's 

 solution test (Experiment XXII, 2), by adding 5 c.c. of mixed Fehling's 

 solution to the solution in the beaker and boiling again. A positive 

 test for reducing sugar should be obtained. The protein and 

 Fehling's solution tests prove mucin to be a conjugated glyco-protein 

 (p. 89). 



(3) Action of Saliva on Starch, (a) Make up some fresh starch 

 paste (Experiment XXIV, 3). (b) To 10 c.c. of this paste add an 

 equal volume of water and warm to 35° C, then add i c.c. of saliva 

 and keep mixture at 35°-4o° C. Immediately remove a drop of the 

 mixture, and add to a drop of iodine on a porcelain drop plate. (The 

 drop plate should be prepared first by placing a drop of iodine in 

 each cavity.) If no blue color is obtained at the first test, the starch 

 paste is too dilute or too much saliva has been added. At intervals 

 of one minute repeat the test and follow the transformation of starch 

 as given on p. 166. When no starch test is obtained with the iodine, 

 test 5 c.c. of the solution with Fehling's solution. A positive test 

 proves the hydrolysis of starch to maltose or glucose, (c) Repeat 

 (b) , adding before the saHva o. i c.c. of concentrated acid. This amount 

 of acid makes the solution approximately 0.3 per cent acid. Note 

 the effect of the acid on the action of the saHva enzymes. 



The Stomach 



In the stomach the only carbohydrate digestion which takes 

 place is the continuation of the action of ptyalin which began 

 in the mouth. As we shall find when we consider protein diges- 

 tion, the stomach juice has an acidity of about 0.2 to 0.3 per 

 cent free hydrochloric acid. This is far above the limits of 

 acidity destructive to ptyalin which we have just given as 

 0.003 to 0.006 per cent. While the stomach juice has this high 

 acidity the food mass on entering the stomach does not reach 

 this acidity for a considerable length of time. It rests in the 

 front or cardiac end of the stomach for a period of time vary- 

 ing from one half to two hours before its acidity reaches the 

 limit of 0.003 P^J" cent which destroys the action of ptyalin. 

 Within this time salivary digestion of carbohydrates continues 

 in the stomach and ceases when the limit of acidity is reached. 



