DIGESTION. 169 



by the cells of the pyloric glands. Pepsin is the chief proteolytic or proteo- 

 clastic agent of the gastric juice and exerts its influence 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 coagulation 

 of milk, a condition due to a transformation of soluble caseinogen into a 

 solid flaky body, casein. 



Hydrochloric acid is the agent which gives to the gastric juice its normal 

 acidity. Though the juice frequently contains lactic, acetic, and even phos- 

 phoric acids, it is 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 analysis 

 of human gastric juice made by Schmidt shows a percentage of 0.02, while 

 that of the dog is 0.34. It is probable, however, that the low percentage of 

 HC1 in human gastric juice was due to the admixture with saliva. At pres- 

 ent it is believed from analyses made for clinical purposes that the acid is 

 present to the extent of at least 0.2 per cent. This degree of acidity is not 

 constant during the entire process of digestion. In the earlier as well as in 

 the later stages it is much less. 



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 the chlorids be withdrawn from the animal tissues by the administration of 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

 of hydrochloric acid from the stomach. On the addition of sodium or potas- 

 sium chlorids to the food, there is at once a reappearance of the acid. 



As to the nature of the process by which the acid is formed, nothing 

 definite is known. Various theories of a chemic and physical character 

 have been offered, all of which are more or less unsatisfactory. As no hydro- 

 chloric acid is found ekher in the blood or lymph, the most plausible view as 

 to its origin is that which regards it as one of the products of the metabolism 

 of the gland-cells, and more particularly of the parietal or border cells, and 

 which for this reason have been termed acid-producing or oxyntic cells. 

 From the chlorids furnished by the blood the chlorin is derived, which, 

 uniting with hydrogen, forms the HC1. The base set free returns to the 

 blood, which in part accounts for its increased alkalinity during digestion as 

 well as the diminished acidity of the urine. The acid thus formed passes 

 through the canaliculi, which penetrate and surround the cells, into the 

 lumen of the gland. 



Hydrochloric acid exerts its influence in a variety of ways. It is the 

 main agent in the derivation of pepsin and rennin or pexin from their ante- 

 cedent zymogen compounds, pepsinogen and pexinogen (Warren) ; it imparts 

 activity to these ferments; it prevents and even arrests fermentative and 

 putrefactive changes in the food by destroying microorganisms; it softens 



