THE DIGESTIVE SYSTEM. 293 



tube and the mixture kept at 40 C., the changes which occur may be 

 conveniently observed. The original starch solution is slightly opalescent. 

 Within a few seconds it becomes clear, but if a drop of the clear 

 solution be added to a drop of dilute iodine a blue colour results, as in 

 the case of starch which has undergone no digestive change. This, the 

 first stage in the process, is that of soluble starch. A little later, a 

 drop of the fluid gives a purple colour with iodine, later still a reddish 

 brown, and finally an " achromic point " is reached, that is, a stage 

 when the digest does not give a colour reaction with iodine. The 

 digestive power of any particular saliva can be estimated by the time 

 taken to reach the achromic point. 



The purple and reddish brown reactions with iodine indicate the 

 presence of erythrodextrin, at first mixed with a certain amount of 

 unaltered starch so that the dextrin reaction is complicated by the blue 

 starch reaction, but later without any such admixture. If, at any 

 time after the indications of the presence of dextrin appear, a little of 

 the solution be boiled with an alkaline -solution of cupric sulphate, 

 reduction of the latter will take place, a precipitate of yellow cuprous 

 oxide being formed. This reaction indicates the presence of a reducing 

 sugar, maltose. In the achromic stage the solution is found to contain 

 maltose and a form of dextrin, which gives no colour reaction with 

 iodine and is therefore called achroo-dextrin. The ultimate result of 

 the digestion usually consists of about 80 per cent, of maltose and 

 20 per cent, of achroo-dextrin, and although the proportion of the 

 latter may be reduced in favourable circumstances to 5 per cent., the 

 conversion of starch into maltose by ptyalin is never complete. 



The salivary digestion of starch consists in the taking up of water 

 by the starch molecule, and for each molecule of water taken up a 

 molecule of maltose is split off. In this way the original mqlecule 

 becomes progressively smaller, and passes through a series of dextrins 

 which are grouped as erythro- and achroo-dextrins according to their 

 reaction with iodine, finally reaching the stage of maltose and achroo- 

 dextrin, on which ptyalin has no further action. The process may be 

 diagrammatically shown thus : 



Starch. 



Soluble starch. 



Erythro-dextrins Maltose. 



Achroo-dextrins Maltose. 



