284 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



tion. In the saliva, however, the sulphocyanide is found in such minute traces 

 and its presence is so inconstant that no special functional importance can be 

 attributed to it. It is supposed to be derived from the decomposition of 

 proteids, and it represents, therefore, one of the end-products of proteid metab- 

 olism. Potassium sulphocyanide may be detected in saliva by adding to the 

 latter a dilute acidulated solution of ferric chloride, a reddish color being 

 produced. 



Ptyalin and its Action. — From a physiological standpoint the most 

 important constituent of saliva is ptyalin. It is an unorganized ferment or 

 enzyme belonging to the amylolytic or diastatic group (p. 280) and possessing 

 the general properties of enzymes already enumerated. It is found in human 

 saliva and in that of many of the lower animals — for example, the pig and 

 the herbivora — but it is said to be absent in the carnivora. Ptyalin has not 

 been isolated in a sufficiently pure condition for satisfactory analysis, so that 

 its chemical nature is undetermined ; we depend for its detection upon its 

 specific action — that is, its effect upon starch. Speaking roughly, we say that 

 ptvalin converts starch into sugar, but when we come to consider the details 

 of its action we find that it is complicated and that it consists in a series of 

 livdrolytic splittings of the starch molecule, the exact products of the reaction 

 depending upon the stage at which the action is interrupted. To demonstrate 

 the action of ptyalin on starch it is only necessary to make a suitable starch 

 paste by boiling some powdered starch in water, and then to add a little fresh 

 saliva. If the mixture is kept at a proper temperature (30° to 40° C), the 

 presence of sugar may be detected within a few minutes. The sugar that is 

 formed was for a time supposed to be ordinary grape-sugar (dextrose, C 6 H, 2 6 ), 

 but later experiments have shown conclusively that it is maltose (C 12 H 22 O n ,- 

 H 2 G), a form of sugar more closely related in formula to cane-sugar (see 

 Chemical section). In experiments of the kind just described two facts 

 may easily be noticed : first, that the conversion of starch to sugar 

 is not direct, but occurs through a number of intermediate stages ; second, 

 that the starch is not entirely converted to sugar under the conditions of 

 such experiments — namely, when the digestion is carried on in a vessel, 

 digestion in vitro. The second fact is an illustration of the incomplete- 

 ly ss of action of the enzymes, a general property that has already been 

 noticed. We may suppose, in this as in other cases, that the products of 

 digestion, as they accumulate in the vessel, tend to retard and finally to sus- 

 pend the amylolytic action of the ptyalin. In normal digestion, however, it 

 is usually the case that the products of digestion, as they are formed, are 

 removed by absorption, and if the above explanation of the cause of the 

 incompleteness of action is correct, then under normal conditions we should 

 expect a complete conversion of starch to sugar. Lea l states that if the 

 products of ptyalin action are partially removed by dialysis during digestion 

 in vitro, a much larger percentage of maltose is formed. His experiments 

 would seem to indicate that in the body the action of the amylolytic ferments 

 1 Journal of Physiology, 1890, vol. xi. j». 227. 



