DIGESTION. 



and to the other a few drops of a solution of potassium car- 

 bonate, mix a little starch mucilage with both, and let them 

 >,id in a water-bath at 40 C. for half an hour. Test a small 

 portion of the liquid from both tubes, and having ascertained 

 that neither contains sugar, put a drop of litmus solution in 

 eaeh, and neutralize with dilute hydrochloric acid. After both 

 have stood for another half hour, sugar will be found in the 

 one to which the carbonate was added, but not in the other. 



* 80. Action of Saliva on Raw Starch. As has been 

 seen, the saliva rapidly converts starch mucilage into sugar, 

 it it does not act so quickly on raw starch. The starch 

 mules consist of a number of layers arranged in an eccentric 

 manner round a point called the hilum. These layers consist 

 alternately of two substances which have been termed respec- 

 tive! v, starch-cellulose and starch-gran ulose. The latter is 

 colored blue by iodine alone; th.- former is not colored unless 

 the granules have been previously acted on by sulphuric acid 

 or zinc chloride. When starch is digested with saliva, the 

 Lrrannlo.se only is dissolved, and although the starch granules 

 Mill retain their form, they are no longer colored blue by 

 iodine. 



To show this, potato starch must be mixed with saliva, and 

 subjected for two or three days to a temperature of 35 C. 

 The saliva used must be decanted off, and a fresh quantity 

 added every two or three hours. The starch is prepared for 

 the purpose by placing a quantity of the pulp obtained by 

 .aping tin- cut surface of a raw potato on a bit of calico 

 strctchi-d over tlie mouth of a beaker, and then washing it 

 with a gentle stream of water. The starch granules pass 

 through into the beaker, leaving a fibrous residue on the 

 calico. 



81. Artificial Saliva. As ptyalin is present, ready 

 formed, in the salivary glands, a lluid which, like saliva, will 

 convert starch into sugar, can be obtained by making an infu- 

 siou of the glands. 



Take the salivary glands of an ox, sheep, rabbit, or guinea- 

 pig. Remove the cellular tissue from them, chop them up fine, 

 and let tin-in stand with a little water upon them for several 

 hours. Strain through muslin and filter. The filtrate may 

 lie iiM-d instead of saliva for the experiments already described. 

 * 82. Preparation of Ptyalin from the Salivary 

 Glands. Ptyalin may In; separated from the infusion of the 

 glands in the ame manner as from saliva, but as it dissolves 

 readily in glycerin, it is much more advantageous to ex- 

 tract it by that agent. For this purpose prepare the salivary 

 glands oi' an ox or sheep, as above directed. Place the welf- 

 miiuvd gland in a llask. and cover it with absolute alcohol. 

 Cork the mouth of the llask, and let it stand for twenty-four 



