DIGESTION OP FOOD. 297 



THE DIGESTIVE JUICES. 



Saliva. — The saliva as found ia the mouth is a mixture of N 

 the secretion of three pairs of glands, alkaline in reaction, of a 

 low specific gravity (variable ia different groups of animals), 

 with a^small percentage of solids consisting of salts and organic 

 bodies (mucin, proteids). 



Saliva serves mechanical functions ia articulation, in moist- 

 ening the food, and dissolviag out some of its salts. But its 

 principal use in digestion is ia reducing starchy matters to a 

 soluble form, as sugar. So far as known, the other constituents 

 of the food are not changed chemically in the mouth. 



The Amylolytic Action of the Saliva.— Starch exists ia grains 

 surrounded by a cellulose covering, which saliva does not digest: 

 hence its action on raw starch is slow. 



It is found that if a specimen of boUed starch not too thick 

 be exposed to a small quantity of saliva at the temperature of 

 the body or thereabout (37° to 40° 0.), it will speedily undergo 

 certaia changes : 



1. After a very short time sugar may be detected by Feh- 

 ling's solution (copper sulphate in an excess of sodium hydrate, 

 the sugar reducing the cupric hydrate to cuprous oxide on 

 boiling). ' •" 



2. At this early stage starch may still be detected by the 

 blue color it gives with iodine ; but later, instead of a blue, a 

 purple or red may appear, indicating the presence of dextrin, I 

 which may be regarded as a product intermediate between/ 

 starch and sugar. ' 



3. The longer the process contiuues, the more sugar and the 

 less starch or dextrin to be detected ; but, inasmuch as the 

 quantity of sugar at the end of the process does not exactly 

 correspond with the origiaal quantity of starch, even when no 

 starch or dextrin is to be found, it is believed that other bodies 

 are formed. One of these is achroodextrin, which does not 

 give a color reaction with iodiae. 



The sugars formed are : (a) Dextrose. (6) Maltose, which 

 has less reducing power over solutions of copper salts, a more 

 pronounced rotatory action on light, etc. 



It is found that the digestive action of saliva, as in the 

 above-described experiment, will be retarded or arrested if the 

 sugar is allowed to accumulate in large quantity. That diges- 

 tion in the mouth is substantially the same as that just de- 



