DIGESTION. 673 



quantity in the saliva of different individuals. Pathologically, saliva 

 may contain sugar in diabetes, melanin in Addison's disease, bile- 

 pigment in icterus. Leucine and urea have been found in saliva 

 during uremia. The iodides and some other drugs are habitually 

 secreted by the salivary glands. This function is used in measuring 

 the rapidity of absorption. 



Ptyalin, the diastatic enzyme, occurs in the saliva of all animals 

 except the pure carnivora. It is characterized by its action in con- 

 verting starch into sugar. It acts best at 40 C. (104 F.) in a 

 neutral solution, although it is active in a weak alkaline solution, 

 and also in acid solution up to 0.2 per cent, of mineral acid. 



The conversion of starch into sugar by ptyalin is a progressive hydrolysis. 

 The first change is the formation of soluble starch, which gives a blue color 

 with iodine. Soluble starch is split into maltodextrin and erythrodextrin ; the 

 latter gives a red color with iodine. These dextrins are next split and yield 

 maltose, maltodextrin, and achroodextrin, which are not colored by iodine. 

 From achroodextrin more maltodextrin and maltose are derived, and finally 

 the hydrolysis results in the formation of maltose with some maltodextrin. 

 The maltose is converted into glucose by the action of the enzyme maltase. 



Experiment 79. Mix intimately 1 gramme of starch with 10 c.c. of water, 

 pour this mixture into 90 c.c. of boiling water and stir until a smooth paste is 

 formed. Place 10 c.c. of paste in a test-tube, heat to 40 C. (104 F.), add 1 

 c.c. of saliva and 1 c.c. of 1 per cent, solution of sodium carbonate, mix well, 

 and keep at the stated temperature. At the expiration of one minute take out 

 a drop of the mixture, place it on a white plate, and add a drop of dilute 

 iodine solution. The mixture will turn blue. Eepeat testing the mixture, 

 which is to be kept at the same temperature, every minute until iodine has no 

 longer any effect on the solution, indicating the conversion of all starch into 

 simple sugars. With normal saliva the color reaction will cease within six 

 minutes ; a longer time would indicate an insufficient quantity of ptyalin in 

 the saliva used for the experiment. 



When the solution no longer is affected by iodine add to 1 c.c. of solution 6 

 c.c. of alcohol : a precipitate of dextrin is formed. Allow the digestion to 

 proceed for half an hour, then heat some of the digested mixture with Feh- 

 ling's solution: the formation of red cuprous oxide shows the conversion of 

 starch into glucose. 



Action of adds and alkalies on salivary digestion. In each of three test-tubes, 

 A, B, and C, place 5 c.c. of starch paste, prepared as above, and 1 c.c. of saliva. 

 To A add 3 c.c. of 0.25 per cent, hydrochloric acid ; to B add 3 c.c. of 1 per 

 cent, sodium carbonate. Keep the tubes at 40 C., and note the time required 

 for the disappearance of the iodine color reaction with the contents of each 

 tube. The reaction will disappear first in B, then in C, and finally in A. 



In order to obtain saliva for experimental purposes a small glass 

 rod or a piece of rubber should be placed in the mouth. This will 

 stimulate the flow of saliva, which is to be collected and filtered. 



43 



