112 THE DIGESTIVE FLUIDS. 



ma tic processes, and is pmhahly referable to the establishment of a 

 certain equilibrium of reaction. A complete transformation of the 

 >taivh could occur only if the resulting sugar were removed as 

 rapidly as it is formed. So long as it is present, the reversible 

 action of the enzyme becomes manifest, and, analogous to the rever- 

 sion of glucose to maltose, a similar ^transformation of maltose to 

 dextrin no doubt occurs. 



The amount of ptyalin which is secreted in the twenty-four 

 hours has not been determined. Its activity, as would be ex- 

 pected, is subject to considerable variations. It is greatest in the 

 morning on rising, and then steadily diminishes during the day. 

 Immediately before meals, however, a temporary increase is ob- 

 served, which is then followed by a marked decrease. 



Of the chemical nature of ptyalin but little is known. Like all 

 other ferments, it is generally regarded as an albuminous substance, 

 and on the application of dry heat it develops the characteristic odor 

 of burning albumins. It is nitrogenous, but does not give the 

 xanthoproteic reaction. From its solutions it can be precipitated 

 with acetate and subacetate of lead, while bichloride of mercury and 

 the salts of platinum, as also tannic acid, are without effect. 



In the human being ptyalin is formed in the parotid and the 

 submaxillary glands and, as will be seen later, also in the pancreas, 

 while the sublingual glands apparently yield no ptyalin. In other 

 animals its presence in the saliva is variable. In the typical 

 carnivora it is said to be absent, while in the saliva of all herbivor- 

 ous animals it is uniformly found. 



Of other ferments, human saliva apparently also contains traces 

 of maltase, and of an oxydase of unknown character; invertin, 

 however, has not been found. Consequently an inversion of maltose 

 to glucose may also take place in the early stages of carbohydrate 

 digestion, but is certainly insignificant in extent. 



The digestive importance of the saliva is, in man, at least, but 

 slight, as ptyalin is rapidly destroyed by contact with the acid 

 gastric juice. During the process of mastication and deglutition, 

 moreover, it has scarcely time to effect much change, and in experi- 

 ments in vitro we find that the amount of maltose which is formed 

 by the saliva from starch is small. The importance of the salivary 

 glands as digestive glands has thus been much overrated, and it has 

 been conclusively demonstrated that their function has mostly to do 

 with the preparation of the food for the act of deglutition. This is, 

 of course, greatly facilitated by its thorough lubrication with the 

 mucus that is furnished by the salivary glands, and which in reality 

 represents the most important constituent of the saliva. 



Mucin. The mucin of the saliva is derived from the submaxil- 

 lary and sublingual glands, as also from the small mucous glands 

 which are found imbedded in the mucous membrane of the mouth. 

 Its formation in the salivary glands is apparently under the control 

 of the sympathetic nervous system, as it is secreted in much larger 



