124 THE DIGESTIVE FLUIDS. 



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

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

 expected, 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 

 observed, which is then followed by a marked decrease. 



Of the chemical nature of ptyalin nothing definite is known. As 

 it is obtained in the laboratory it gives some of the reactions of 

 albumins (not the xanthoproteic reaction), but this may be due to 

 contamination. 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 is variable. In the typical carnivora (dog) it is 

 said to be absent, while in the saliva of all herbivorous animals it is 

 uniformly found. 



Of other ferments, human saliva contains traces of maltase, and 

 of an oxydase of unknown character ; invertin has not been found. 



The digestive importance of the saliva in man is now recognized to 

 be greater than was formerly supposed ; for it has been shown that 

 food may remain in the fundus portion of the stomach for an hour 

 or longer without coming into contact with the acid gastric juice. 

 Salivary digestion may thus proceed uninterruptedly for a much 

 longer time than was formerly thought to occur. The resulting 

 maltose is probably not absorbed to any great degree until it 

 reaches the small intestine and is then inverted by maltase. 



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 

 amounts on stimulation of these fibres than of the corresponding 

 cerebral fibres. According to Levene, the submaxillary mucin con- 

 tains the chondroitin-sulphuric acid complex, or a closely allied 

 group. 



To the presence of the mucin the viscid, stringy character of the 

 saliva is due. The substance can be obtained by precipitation with 

 acetic acid, and it is to be noted that, in contradistinction to the 

 mucin-like substances which belong to the class of the nucleo-albu- 

 mins, the precipitated mucin is insoluble in an excess of the acid. 

 In dilute solutions of the alkalies it is soluble, and it is thus 

 possible by repeated precipitation and solution to obtain the mucin 

 in fairly pure form. In the dry state it occurs as a white or yellow- 

 ish-gray powder. In alcohol and water it is insoluble, although in 

 the latter it swells to form a jelly-like material. Unlike the albu- 

 mins, it is not coagulated by heat ; but, like these, it gives the 



