246 DIGESTION 



salt content is 0.1-0.2 per cent and the KCNS, 0.003-0.01 per cent. The 

 quantity of mixed human saliva secreted in twenty-four hours is about 

 1,500 c.c. 



The Diastatic Enzyme. In 1831 Leuchs found that saliva gradually 

 dissolves starch and converts it into the soluble carbohydrates., dextrin and 

 sugar. This action is to be ascribed to an enzyme, called ptyalin. 



Dry starch is not soluble, but it swells up in warm water, forming starch 

 paste. On heating starch with glycerin up to 190, or by acid fermentation 

 of the paste, it is rendered soluble. Soluble starch is also the first product of 

 digestion under the action of ptyalin. In the further course of this action 

 soluble starch is split by absorption of water into dextrin, isomaltose, and 

 maltose. There is present in saliva a trace of an inverting enzyme, maltase, 1 

 which converts a small quantity of maltose into dextrose (Rohmann). 



More in detail, this transformation of starch into sugar proceeds about 

 as follows: First the soluble starch is split into ery thro dextrin (red color 

 with iodine) and maltose. Then from the erythrodextrin is formed an acliro- 

 odextrin (no color with iodine) and more maltose, and achroodextrin in its 

 turn yields another achroodextrin and more sugar, etc. 



In artificial digestion starch can never be completely changed into sugar. 

 But if the experiment be so arranged that the sugar can be removed by dial- 

 ysis as it is formed, the transformation may be carried much farther than is 

 possible otherwise (Lea). Since provision for removal of the sugar is made 

 in the digestive system, it is probable that all the starch is transformed, 

 provided only that the ptyalin has the opportunity of acting long enough. 



Human saliva acts very rapidly. When equal volumes of saliva and starch 

 paste are mixed, at body temperature the starch disappears in about two and 

 a half hours. Cooked starch is digested more rapidly than raw, and pulver- 

 ized starch more rapidly than nonpulverized. 



Ptyalin appears to act more powerfully in a neutral or weakly add medium ; 

 hence best results are obtained when the alkaline saliva is carefully neutralized 

 by addition of a very small quantity of acid (not more than 0.0007-0.0012 per 

 cent HC1; Cole). 



3. GASTRIC JUICE 



Gastric juice cannot be obtained pure from an ordinary gastric fistula, for 

 even if the particles of food could be excluded, it would be mixed with some 

 saliva which had been swallowed. In the dog, however, these difficulties can be 

 overcome, by making both a stomach fistula and an cesophageal fistula (Pawlow). 

 The stomach can also be cut off from both oesophagus and duodenum, the lat- 

 ter two sutured together, and the juice collected from the isolated stomach 

 (Fremont). 



Gastric juice obtained in this way, when it is freed from the stomach 

 mucus, is perfectly clear in color, acid in reaction and is devoid of foreign 

 taste. Its specific gravity is 1.003-1.0039. In a layer 20 cm. long it rotates 

 the plane of polarized light 0.70-0.73 to the left. Its dry residue amounts 



1 In general the enzymes are named by adding the suffix ase to the name of the sub- 

 stance on which it acts. Exceptions to this rule are older names stilJ in use. 



