DIGESTION 171 



organic matter, hydrochloric acid and various inorganic salts. The quan- 

 titative composition of the juice varies somewhat in different animals. 



The organic matter present in gastric juice is a mixture of mucin and a 

 protein, products of the metabolic activity of the epithelial cells on the sur- 

 face of the mucous membrane and of the chief or central cells of the gastric 

 glands respectively. Associated with the protein material are two possibly 

 three ferment or enzyme bodies, termed pepsin, rennin and lipase. As is 

 the case with other enzymes, their true chemic nature is practically un- 

 known. 



Pepsin. Pepsin, though present in gastric juice, is not present as such in 

 the chief cells of the glands, but is derived from a zymogen, propepsin or 

 pepsinogen, when the latter is treated with hydrochloric acid. This ante- 

 cedent compound is related to the granules observed in and produced by the 

 cell protoplasm during the period of rest. Though pepsin is largely produced 

 by the central cells of the cardiac glands, it is also produced, though in less 

 amount, by the cells of the pyloric glands. Pepsin is the chief proteolytic 

 or proteoclastic agent of the gastric juice and exerts its influence most ener- 

 getically in the presence of hydrochloric acid and at a temperature of about 

 4OC. Other acids e.g., phosphoric, nitric, lactic, etc. are also capable 

 of exciting it to activity, though with less intensity. 



Rennin. Rennin or pexin is present in the gastric juice not only of man 

 and all of the mammalia, but also of birds and even fish. In its origin from 

 a zymogen substance; in its relation to an acid medium and an optimum 

 iemperature it bears a close resemblance to pepsin. Its specific action is 

 the coagulation of milk, a condition due to a transformation of soluble case- 

 tnogen into a solid flaky body, casein. 



Lipase. Lipase, an enzyme found in pancreatic juice, has also been 

 shown to be present in gastric juice, the specific function of which appears 

 to be the digestion or hydrolysis of finely emulsified fat such as is found in 

 milk. 



Hydrochloric Acid. Hydrochloric acid is the agent which gives to the 

 gastric juice its normal acidity. Though the juice frequently contains lactic, 

 acetic, and even phosphoric acids, it its generally believed that they are the 

 result of fermentation changes occurring in the food, the result of bacterial 

 action. The percentage of hydrochloric acid has been the subject of much 

 discussion. The most recent investigations show that the initial acidity of 

 the freshly secreted human gastric juice is between 0.32 and 0.48 per cent. 

 HCl. This initial acidity is reduced by combination with food, admixture 

 with saliva and gastric mucus, and by regurgitation of alkaline duodenal 

 contents, to 0.15 or 0.2 per cent. HCl, the optimum acidity for the proteolytic 

 activity of pepsin. As observed clinically, following various test meals, 

 the acidity of the gastric contents is seen to rise to a maximum as diges- 

 tion progresses, after which it falls to the optimum point of about 0.2 per 

 cent. HCl. 



The immediate origin of the hydrochloric acid is difficult of explanation. 

 That it is derived, however, primarily from the chlorids of the food and 

 secondarily from the chlorids of the blood-plasma has been established by 

 direct experiment. If all the chlorids be removed from the food and all 

 the chlorids be withdrawn from the animal tissues by the administration of 

 various diuretics e.g., potassium nitrate there will be a total disappearance 



