CHEMISTRY OP SALIVA. 47 



ing sugar when it is hydrolyzed, as by boiling with acid. It was 

 at one time suggested that sugar might sometimes appear in the 

 saliva, as a result of bacterial action in the mouth, and be respon- 

 sible for caries of the teeth. The amount is, however, so very 

 small in comparison with the ingested carbohydrates that it can 

 be entirely disregarded. 



Ptyalin. This belongs to the class of diastatic or amylolytic 

 enzymes, converting starch into sugar. It is not so powerful as 

 the similar enzyme in pancreatic juice (see p. 74), for it has 

 no action on uncooked starch, which the latter has. It acts best 

 in neutral reaction and in the presence of sodium chloride, but 

 is little affected by a small degree of akalinity. On the other 

 hand, it is readily destroyed by acids and by higher degrees of 

 alkalinity. These facts are of importance in connection with the 

 continuance of action of saliva after it has been swallowed, for 

 although the food remains in the mouth for much too brief a 

 period to permit of more than a trace of sugar being formed 

 here, yet, after the stomach is reached, ptyalin may continue to 

 act for about half an hour. The ptyalin content, however, varies 

 very considerably in different individuals. 



Ptyalin converts starch into the sugar maltose, so called be- 

 cause it is also formed by the action of the diastase of malt. As 

 intermediate substances are formed the dextrins, two of which 

 are distinguishable on account of their behavior towards iodine ; 

 the first dextrin, called erythrodextrin, gives a brown color, while 

 the next gives no color and is called achroodextrin. 



It has been suggested that a deficiency of ptyalin may pre- 

 dispose to caries of the teeth because, under such circumstances, 

 a large amount of dextrin is formed, which being very sticky in 

 character adheres to the teeth and becomes a suitable nidus for 

 bacterial growth. 



Potassium Sulphocyanide (sulphocyanate) . This salt has 

 the formula KCNS, and is usually present in human saliva 

 to the extent of about 0.01 per cent. It is produced in the blood 

 whenever cyanides or organic nitrites make their appearance in 

 the organism, one source for these being possibly protein meta- 

 bolism (p. 108). It is excreted from the blood into the urine as 



