CHAP. L] DIGESTION. 235 



Action of Saliva on Starch. If to a quantity of boiled starch, 

 which is always more or less viscid and somewhat opaque or turbid, 

 a small quantity of saliva be added, it will be found after a short 

 time that an important change has taken place, inasmuch as the 

 mixture has lost its previous viscidity and become thinner and 

 more transparent. In order to understand this change, the reader 

 must bear in mind the existence of the following bodies (described 

 more fully in the Appendix) all belonging to the class of carbo- 

 hydrates : 1. Starch, which forms with water not a true solution 

 but a more or less viscid mixture, and gives a characteristic blue 

 colour with iodine. 2. Dextrin, differing from starch in forming a 

 clear solution and in giving a red colour with iodine. 3. Dextrose, 

 also called glucose or grape-sugar, giving no colouration with iodine, 

 but characterised by the power of reducing cupric and other 

 metallic salts; thus, when dextrose is boiled with a fluid often 

 known as Fehling's fluid, which is a solution of cupric sulphate with 

 an excess of sodium hydrate, the cupric salt is reduced and a red 

 or yellow deposit of cuprous oxide is thrown down. This reaction 

 serves with others as a convenient test for dextrose. Neither starch 

 nor dextrin, nor that commonest form of sugar known as cane-sugar, 

 give this reaction. 4. Maltose, very similar to dextrose, and like it 

 capable of reducing cupric salts. Besides having a slightly different 

 formula, it differs from dextrose, chiefly in its smaller reducing 

 power, i.e. a given quantity will not convert so much cupric oxide 

 into cuprous oxide as will the same weight of dextrose, and in having 

 a stronger rotatory action on rays of light (see Appendix). Besides 

 the above we may mention the peculiar body, achroodextrin, which 

 differs from dextrin in giving no colouration at all with iodine ; and 

 the so-called soluble starch, which like dextrin forms a clear solution 

 with water, but unlike dextrin gives a blue colour with iodine. 



Hence when a quantity of starch is boiled with water we may 

 recognize in the viscid imperfect solution, on the one hand the 

 presence of starch, by the blue colour which the addition of iodine 

 gives rise to, and on the other hand the absence of sugar (dextrose, 

 maltose), by the fact that when boiled with Fehling's fluid no 

 reduction takes place and no cuprous oxide is precipitated. , 



If however the boiled starch be submitted for a while to the 

 action of saliva, especially at a somewhat high temperature such as 

 35 or 40 C., it is found that the subsequent addition of iodine 

 gives no blue colour at all, or very much less colour, shewing that 

 the starch has disappeared or diminished ; on the other hand the 

 mixture readily gives a precipitate of cuprous oxide when boiled 

 with Fehling's fluid, shewing that dextrose or maltose is present. 

 That is to say the saliva has converted the starch into dextrose 

 or maltose; and there are reasons, which we need not enter into 

 here, for thinking that while some dextrose is formed the greater 

 part of the sugar which appears is in the form of maltose. As 

 the conversion of the starch by the saliva is going on the addition 



