l82 TEXT-BOOK OF PHYSIOLOGY. 



change in composition, and loses none of its digestive power. It 



/ will also prevent and even arrest putrefactive change in organic 



< matter. The chemic composition of the gastric juice has never 



been satisfactorily determined, owing to the fact that the secretion 



as obtained from fistulous openings has not been absolutely normal. 



The following analyses represent the composition of a sample 



obtained by Schmidt from the stomach of a woman who had a fistula, 



but who was nevertheless in good health; also the composition of the 



juice from a dog: 



COMPOSITION OF GASTRIC JUICE. 



Human. Dog. 



Water, 994-40 973- 6 



Organic matter, 3- I 9 / - I 7- I 3 



Hydrochloric acid, o. 2 ?j 3.34 



Calcium chlorid, 0.06^ 0.26 



Sodium chlorid, 1.46 2.50 



Potassium chlorid, 0.55 1.12 



Calcium phosphate } i-73 



Magnesium " > 0.12 0.23 



Ferric J 0.08 



Ammonium chlorid, 0.47 



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

 and a proteid, products of the metabolic activity of the epithelial 

 cells on the surface of the mucous membrane and of the chief or 

 central cells of the gastric glands respectively. Associated with the 

 proteid material are two ferment or enzyme bodies, termed pepsin 

 and rennin. As is the case with other enzymes, their true chemic 

 nature is practically unknown. 



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

 the chief cells of the glands, but is derived from a zymogen, pro- 

 pepsin or pepsinogen, when the latter is treated with hydrochloric 

 acid.v This antecedent compound is related to the granules ob- 

 served in and produced by the cell protoplasm during the period 

 of rest. Though pepsin is largely produced by the central cells_of 

 the fundic glands, it is also produced, though in less amount, by 

 the cells of the pyloric glands. Pepsin is the chief proteolytic agent 

 of the gastric juice and exerts its iniluence most energetically in the 

 presence of hydrochloric acid and at a temperature of about 40 C. 

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

 of exciting it to activity, though with less intensity. 



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

 all 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 

 temperature, it bears a close resemblance to pepsin. Its specific 

 action is the curdling of milk, a condition due to the coagulation of 

 caseinogen. 



