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. The observations of Dr. Beaumont and the experi- 

 ments of many physiologists have made it certain that the secretion of the 

 gastric juice is intermittent and not continuous, that it is only on the intro- 

 duction and digestion of the food that the normal amount is poured out. 

 During the intervals of digestive activity the stomach is practically free from 

 all traces of the juice. The mucous membrane is pale and covered with a 

 layer of mucus having an alkaline or neutral reaction. The introduction, 

 however, of small portions of food or irritation with a glass rod causes a 

 change in the appearance of the mucous membrane. At the points of 

 irritation the membrane becomes red and vascular and in a few minutes 

 small drops of a secretion make their appearance; these coalesce and run 

 down the sides of the stomach. 



The statements of Beaumont and many subsequent investigators that 

 the secretion thus obtained is gastric juice have been apparently disproved 

 by ^ Pavlov, who asserts that it is only an alkaline mucous the function of 

 which is protective in character. According to this investigator, mechanic 

 stimulation ^is incapable of exciting the secretion. The results of modern 

 methods of investigation make it apparent that the production and discharge 

 of the secretion is the result of the action 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 

 hand by the mastication of food which is agreeable to the animal. Thus 

 when a dog was tempted by the sight of food, the secretion made its appear- 

 ance at the end of six minutes and during the time of the experiment, which 



