SALIVARY AND GASTRIC DIGESTION 191 



The saliva assists greatly in making the food manage- 

 able during mastication and swallowing. In some animals 

 it seems to have no other service. In others, including 

 man, it is a real digestive juice. The fact has already 

 been indicated that saliva has the power in such cases 

 to convert starch to a kind of sugar. 



This power is referred to the enzyme ptyalin or salivary 

 diastase. If a crumb of bread is held for some time in 

 the mouth and slowly chewed a sweetish taste gradually 

 develops. This is due to the formation of sugar or of 

 incompletely digested compounds, the dextrins, from 

 the starch of the bread. Someone has said that we can 

 thus turn bread into cake. The change made apparent 

 to the sense of taste in this simple way can be followed 

 almost as readily by chemical tests. 



When starch is formed in the leaves of plants it is in 

 grains which give evidence of being dense and brittle. 

 Boiling these disintegrates them and gives rise to a paste 

 or, if the dilution is greater, to an apparent solution. 

 It. is fair to say that boiling does not digest starch but 

 makes its subsequent digestion far easier. The surface 

 exposure which is so important is almost infinitely 

 multiplied by the destruction of the granules. Saliva 

 makes comparatively slow progress with raw starch but 

 is wonderfully efficient with that which has been cooked. 



Starch gives an intense blue color to a test solution 

 of iodin in potassium iodid. If we try successive sam- 

 ples from a mixture of warm starch and saliva we shall 

 find that the color produced with iodin soon shifts 

 through a violet to a red and then fades until the ad- 

 dition of the digesting mixture to iodin does not darken 

 the color of the reagent at all. A series of tests showing 

 such results must be taken to mean that the starch 

 has rapidly disappeared. Another type of test will 

 show that sugar has taken its place. Most sugars are 

 said to decompose the salts of copper while starch does 

 not. A solution of copper hydrate in Rochelle salts 

 (Fehling's solution) may be boiled with starch without 



