i 4 8 TEXT-BOOK OF PHYSIOLOGY. 



on the addition of carmine. When treated with water or dilute acids, the 

 globules swell up, coalesce, and form a uniform mass. The chemic relations 

 of this substance indicate that it is the precursor of mucin namely, mucigen. 

 During secretor activity the cells discharge these mucigen granules into the 

 lumen of the acinus where they are transformed into mucin. Though the 

 appearance of the gland-cell indicates it, there is no evidence for the view 

 that the cell itself undergoes disintegration in the process. 



The Physiologic Actions of Saliva. The constant presence of salivary 

 glands in the different classes of animals and the large amount of secretion 

 they pour daily into the alimentary canal point to the conclusion that this 

 mixed fluid plays an important r61e in the general digestive process. Experi- 

 ment has demonstrated that it has a two-fold action; viz., physical and 

 chemical. 



Physically, saliva softens and moistens the food, unites its particles into 

 a consistent mass by means of its contained mucin, and thus facilitates 

 swallowing. 



Chemically it converts 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 condition each starch grain con- 

 sists 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 con- 

 verted 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 transparent. On testing the solution from time to time 

 with iodin the characteristic blue reaction will be found to disappear, gradu- 

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

 solution 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 dextro- 

 rotatory. 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 investi- 

 gators 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: 



