GASTRIC DIGESTION 59 



tose molecule, is converted into a series of achroodcxtrins, 

 which give no color reaction with iodine. These are, by the 

 continued splitting off of maltose molecules, finally all con- 

 verted into maltose. Ptyalin acts in a neutral or slightly 

 alkaline medium. Free hydrochloric acid to the extent of 

 0.003 per cent, stops its action, which, taken in connection with 

 the fact that the food remains in the mouth but a very short 

 time, was thought to indicate that the ptyalin digestion is 

 not very important and is limited to the initial stages; but 

 recent progress in knowledge of the conditions in the stomach 

 shows that salivary digestion may continue for an hour or more 

 in the fundic end of the stomach. Cooked starch is more 

 readily acted upon than raw, owing to the fact that the cook- 

 ing destroys the cellulose envelope that surrounds the starch 

 grains. 



Gastric Digestion. The gastric juice is a thin, nearly 

 colorless liquid, of strong acid reaction and of a peculiar odor. 

 Its specific gravity varies from 1.002 to 1.003. Its constituents 

 are some protein, some mucin, inorganic salts, hydrochloric 

 acid, and the enzymes, pepsin, rennin, and lipase. The acid 

 of the gastric juice is proved to be free hydrochloric acid in 

 the following ways : 



(a) When all the chlorides are precipitated by silver nitrate 

 and the total chlorine is determined, more is found than can 

 be held by the bases present. 



(6) The secretion gives the color tests for free mineral acids 

 methyl violet solutions are turned blue, etc. The amount 

 of free acid varies, but may be put at 0.3 to 0.5 per cent. It 

 has been attempted to determine the source of the hydro- 

 chloric acid by injecting into the circulation of an animal 

 substances like ferric lactate, followed by potassium ferro- 

 cyanide, which react only in the presence of a free mineral 

 acid with the production of Prussian blue. But this only in 

 a general way proved its formation in the gastric mucous 

 membrane, leaving the method of its formation unrevealed. 

 During the active secretion of the gastric juice the alkalinity 

 of the blood is increased and the acidity of the urine is decreased, 

 corroborating the view that neutral chlorides are decomposed, 

 the chlorine going to form the hydrochloric acid, while the 

 bases pass back into the blood. 



