DIGESTION. 183 



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 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 

 present 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 ex- 

 planation. That it is derived, however, primarily from the chlorids 

 of the food and secondarily from the blood- plasma has been estab- 

 lished by direct experiment. Mlf all the chlorids be removed from 

 the food and all chlorids be withdrawn from the animal tissue by the 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 potassium 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, noth- 

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

 character have been offered, all of which are more or less unsatis- 

 factory. As no hydrochloric acid is found either 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, und 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 

 antecedent zymogen compounds, pepsinogen and pexinogen (Warren) ; 

 it imparts activity to these ferments ; it prevents and even arrests fer- 

 mentative and putrefactive changes in the food by destroying micro- 

 organisms; it softens connective tissue, it dissolves proteids and acid- 

 ifies the proteids, thus making possible the subsequent action of pepsin. 



The inorganic salts of the gastric juice are probably only inci- 

 dental and play no part in the digestive process. 



