i68 TEXT-BOOK OF PHYSIOLOGY. 



as an indication as to the phenomena which are taking place in the natural 

 stomach. 



By the first procedure it is possible to feed an animal with different kinds 

 of food and to observe the effects of psychic states on the secretion of gastric 

 juice. As the swallowed food is discharged from the lower end of the 

 divided esophagus the appetite continues, and hence the animal will eat for 

 several hours. By the second procedure it is possible to collect gastric juice 

 from the miniature stomach and to study the effects on its quantity and 

 quality produced by psychic states, mastication, different articles of food, 

 and by the process of digestion itself as it goes on in the large stomach. In 

 both instances the juice is obtained free from admixture with saliva or food. 



Gastric Juice. The gastric juice obtained from the human stomach 

 free from mucus and other impurities is a clear, colorless fluid with a con- 

 stant acid reaction, a slightly saline and acid taste, and a specific gravity 

 varying from 1.002 to 1.005. The juice obtained from the dog's stomach 

 possesses essentially the same characteristics, though its acidity as well as its 

 specific gravity are slightly greater. When kept from atmospheric influences, 

 it resists putrefactive change for a long period of time, undergoes no apparent 

 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 com- 

 position 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. 

 (Schmidt.) 



Water 994-04 973 .06 



Organic matter 3.19 17 . 13 



Hydrochloric acid o . 20 3 .34 



Calcium chlorid o .06 0.26 



Sodium chlorid i . 46 2 . 50 



Potassium chlorid 55 i .12 



Calcium phosphate ] 1.73 



Magnesium phosphate V 0.12 o .23 



Ferric phosphate J o .08 



Ammonium chlorid o .47 



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

 protein, products of the metabolic activity of the epithelial cells on the sur- 

 face of the mucous membrane and of the chief or central cells of the gastric 

 glands respectively. Associated with the protein 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, propepsin or pepsinogen, 

 when the latter is treated with hydrochloric acid. This antecedent com- 

 pound is related to the granules observed in and produced by the cell proto- 

 plasm during the period of rest. Though pepsin is largely produced by the 

 central cells of the preantral glands, it is also produced, though in less amount, 



