DIGESTION IN THE MOUTH. 273 



contains a substance of the nature of a ferment, which is termed animal 

 diastase or ptyalin, whose presence may be demonstrated by the power 

 possessed by the saliva of converting starch mucilage into sugar. Of 

 the albuminoid bodies, serum-albumen and a globulin-like body which 

 may be precipitated by carbonic acid are the representatives. 



The most important constituent of the saliva is the ptyalin. This 

 substance belongs to the group of soluble ferments, and is a product of the 

 cells of the salivary glands. It may be obtained, according to the method 

 of Colmheim, by adding a little phosphoric acid to mixed saliva and then 

 stirring with milk of lime until the alkaline reaction is restored ; the 

 white precipitate is then filtered off, and the filtrate shows scarcely any 

 albuminoid reaction, while it still possesses in an almost undiminished 

 degree its diastatic power. A considerable quantity of the ptyalin still 

 remains clinging to the albuminoid matters deposited in the precipitate, 

 and if this is washed with water the ptyalin is extracted, while it leaves 

 the albuminous matters still on the filter. If alcohol is added to the 

 watery extract of this precipitate, a flocculent, whitish precipitate is 

 formed, which may be collected by decantation and dried over sulphuric 

 acid. A gra3 T ish-white powder is thus obtained, which consists of ptyalin 

 mixed with phosphates ; the latter may be removed by dissolving in 

 water, precipitating again by absolute alcohol, washing the precipitate 

 with dilute alcohol and then with a small quantity of water, and drying 

 at a low temperature. Ptyalin so obtained is a nitrogenous substance, 

 but not an albuminoid. It is readily soluble in water and glycerin and 

 possesses the power of converting starch and glycogen into maltose, 

 and this property is exerted whether in a neutral or very faintly acid or 

 alkaline medium. An excess of alkali or of acid, as will be again referred 

 to, prevents its activity. The ferment may also be extracted from 

 the salivary glands by mincing fresh glands and covering them with 

 glycerin. As the ferment is soluble in glycerin, it is extracted from the 

 gland-tissue, and may be precipitated again from the glycerin extract by 

 alcohol. 



The saliva contains appreciable volumes of different gases in 

 solution, as determined by Pfluger in the case of the submaxillary gland 

 of the dog. He estimates the different amounts of gases contained in 

 the saliva as follows : — 



Oxygen 0.4 to 6 volume per cent. 



Nitrogen 0.7 to 0.8 



Carbon dioxide, . . . 49.2 to 64.7 ' 



It is thus seen that saliva is the richest in CO, of any fluid in the 

 animal body. Only a small proportion of the above amount, however, 

 can be extracted with the gas-pump, showing that the remainder is held 

 in chemical combination. The amount capable of being pumped out 



18 



