674 PHYSIOLOGICAL CHEMISTRY. 



General tests for mixed saliva. 



1. Allow a few c.c. of saliva to stand a day or two : a cloudiness 

 will be observed, due to the precipitation of calcium carbonate, which 

 has been held in solution by carbon dioxide. 



2. Acidify saliva with acetic acid : a precipitate of mucin is formed 

 insoluble in an excess of the acid. 



3. Apply the xanthoproteic, Millon's, and biuret reactions for 

 proteins (see page 626). 



4. Acidify with acetic acid and add ferric chloride : a red color, 

 due to the formation of ferric sulphocyanide, is produced. 



Gastric digestion. The food, after mastication, passes through 

 the oesophagus into the stomach. Here the mass is kneaded by the 

 contractions of the muscular wall of the stomach and is acted on by 

 the gastric juice. By this treatment with the aid of the fluids 

 ingested the food is converted into a turbid liquid, known as chyme. 

 A small portion of the digested mass is absorbed through the stomach 

 wall, but most of the food, after being completely acted upon, passes 

 through the pylorus into the duodenum. 



Gastric juice is a liquid secreted by the follicles of the stomach. 

 It can be obtained, in a fairly normal condition, either from animals 

 (dogs) or from man, by the aid either of gastric fistulse or of the 

 stomach-pump. It is a thin, nearly colorless liquid, having a some- 

 what sour taste, an acid reaction, and a specific gravity varying from 

 1.002 to 1.003. The total solids are about 0.5 per cent., nearly one- 

 half being inorganic salts, chiefly the chlorides and phosphates 

 of alkali and alkaline earth metals. The organic matter present, 

 and amounting to about 0.3 per cent., is chiefly pepsin and a little 

 mucin. 



The secretion of gastric juice is not continuous, and is brought 

 about by chemical irritation of the gastric mucuous membrane or by 

 psychic influence. A strong desire for food will cause a flow of the 

 juice ; chemical irritation, as by the alkaline mass of food and saliva, 

 causes a slower but more continuous flow. The quantity of juice 

 secreted during digestion varies with the quantity and quality of the 

 food. 



The average composition of pure gastric juice may be approxi- 

 mately stated thus : 



Water 99. 26 per cent. 



Pepsin and other organic matter . . . .0.30 

 Rennin . y 



Free hydrochloric acid .... 0.22 



Alkali chlorides 20 



Phosphates of calcium, magnesium, and iron . 0.02 



