172 TEXT-BOOK OF PHYSIOLOGY 



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 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, 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 

 connective tissue, it dissolves and acidifies the proteins, thus making possible 

 the subsequent action of pepsin. 



The inorganic salts of the gastric juice are probably only incidental and 

 play no part in the digestive process. 



Mode of Secretion. Based on the observations of Dr. Beaumont and 

 the experiments of many physiologists certain statements regarding the 

 mode of secretion of the gastric juice have had wide acceptance, e.g.: 

 that the secretion is intermittent and not continuous; that the juice is 

 discharged into the stomach in normal amounts only on the entrance of 

 food, that during the intervals of digestion the stomach is free from all 

 traces of the juice: that the juice will be secreted in response to mechanical 

 stimulation. 



In opposition to these statements, the results of modern methods of 

 investigation have made it apparent that the stomach at all times contains 

 a fluid composed of gastric juice, saliva and probably fluid regurgitated 

 from the duodenum. The amount of the fluid that can be thus obtained 

 from the stomach before breakfast either by means of the stomach tube or 

 through a gastric fistula varies from a few centimeters to as much as 1 50 

 cc. Average amounts of 10 to 25 cc., 24 to 60 cc., 30 to 180 cc., have been 

 reported by different observers. Inasmuch as this fluid contains hydro- 

 chloric acid and is capable of digesting proteins the assumption is that 

 the stomach is more or less continuously active. The stimulus to this 

 secretion is unknown. It may be due to a tonus of the secretor cells 

 excited by nerve impulses transmitted to the vagus center and then re- 

 flected to the gastric glands. The secretion of gastric juice that precedes 

 and accompanies gastric digestion is the result of two different stimuli, 

 a primary and a secondary. 



The primary stimulus to gastric secretion, according to Pavlov, is a 

 psychic state induced, on the one hand, by the sight or the odor of food 

 especially if the animal is hungry and the food appetizing; and on the other 



