SALIVARY DIGESTION. 45 



test-tube a few c.c. of the solution and add a drop of iodine 

 solution. What color is obtained? Replace the mixture 

 in the bath and as the amylolysis proceeds remove successive 

 portions (2 c.c.) and test with the iodine solution. What 

 successive variations in color do you obtain? Finally the 

 iodine fails to give a color to the solution. This is called the 

 achromic point; why? Test the remainder of the solution 

 with Fehling's solution. What is the character of the sugar 

 present? What is the cause of these changes indicated by 

 the iodine reaction? Make a scheme of the successive 

 hydrolytic products formed in the digestion. 



Influence of Conditions upon the Activity of Ptyalin. 



(6) Make up the following mixtures in separate test-tubes 

 and keep them in the water-bath at 40 C., using in each case 

 5 c.c. of the diluted saliva and 10 c.c. of the starch paste. 



(1) Starch paste + saliva. 



(2) Starch paste + saliva first boiled and then cooled. 



(3) Starch paste + saliva exactly neutralized with 0.2% 



HC1. 



(4) Starch paste + saliva made acid with 0.2% HC1. 



(5) Starch paste + saliva made alkaline with 0.5% 



Na,C0 3 . 



In all these test-tubes note carefully the changes which 

 are taking place from time to time, testing the rapidity of 

 digestion by means of the iodine reaction. Determine the 

 varying lengths of time necessary for the achromic point to 

 be reached. 



After some time it will be noticed that in experiments (4) 

 and (5) the starch paste has not changed; change the reac-' 

 tion in each to that corresponding to the normal saliva and 

 again allow them to digest. What result? 



Make deductions from these test-tube experiments. 



