1 64 TEXT-BOOK OF PHYSIOLOGY. 



of secretion they pour daily into the alimentary canal point to the 

 conclusion that this mixed fluid plays an important role in the general 

 digestive process. Experiment has demonstrated that it has a two- 

 fold action, physical and chemical. 



Physically j saliva softens and moistens the food, unites its par- 

 ticles into a consistenTTnass- by means oi its contained mucin, and 

 thus facilitates swallowing. 



Chemicallvitcon verts starch into sugar.'* This action is more 

 marked with boiled than with raw starch, a fact which depends 

 on the physical structure of the starch grain. In the natural con- 

 dition each starch grain consists of a cellulose envelope or stroma 

 in the meshes of which is contained the true starch material, the 

 granulose. When boiled for some minutes, the starch grain absorbs 

 water, the granulose swells and ruptures the cellulose envelope, after 

 which it passes into an imperfect opalescent solution more or less 

 viscid, depending on the relative amounts of water and starch. This 

 is the change largely brought about by the process of cooking. If 

 a portion of this hydrated starch be kept in the mouth for a few 

 minutes it will be converted into sugar, a fact made evident by the 

 sense of taste. 



The chemic action of saliva in converting starch into sugar, as 

 well as the intermediate stages, can be experimentally shown in the 

 following manner: To 5 volumes of a thin starch solution in a test- 

 tube add two volumes of filtered saliva. Place the mixture in a 

 water-bath at a temperature of 35 C. In a few minutes the starch 

 passes into a soluble condition and the fluid becomes clear and trans- 

 parent. On testing the solution from time to time with iodin the 

 characteristic blue reaction will be found to gradually disappear, the 

 color passing from blue to violet, to red, to yellow. If now the solu- 

 tion be boiled with a solution of cupric hydroxid (Fehling's solution) 

 a copious red or yellow precipitate of cuprous oxid is formed, which 

 indicates the presence of sugar. The polariscope shows that this 

 sugar is maltose, C 12 H 22 O n . During the conversion of the starch 

 intermediate substances are formed to which the term dextrin is 

 applied. After the starch has been rendered soluble it undergoes 

 a cleavage into maltose and a dextrin, which, as it gives rise to a 

 red color with iodin, is termed erythrodextrin. At a later stage this 

 erythrodextrin also undergoes a cleavage into maltose and a second 

 variety of dextrin, which, as it does not give rise to any color with 

 iodin, is termed achroodextrin. It is claimed by some investigators 

 that this form can also in time be transformed into sugar. It is 

 possible that a small quantity of dextrose is also formed. 



The successive stages of the conversion of starch into sugar may 

 be represented by the following schema : 



