DIGESTION. 



103 



The water forms by far the largest part of this fluid, and serves 

 the purpose of holding the other ingredients in solution, and by its 

 saturating power brings them into relation with the constituents of 

 the food. Of the inorganic salts the sodium and potassium chlorids 

 are the most abundant and important. 



The hydrochloric acid, which exists in a free state, is present 

 in variable amounts. In the foregoing table the number of parts a 

 thousand is much smaller than is usually stated. According to most 

 observers, hydrochloric acid is present to the extent of from 0.2 to 

 0.3 part a hundred. Though secreted as soon as the food enters the 

 stomach, the acid can not be detected in the free state until after the 

 lapse of from thirty to forty minutes. It acidulates the food and 

 prevents fermentative changes. 



The pepsin, which is present in gastric juice associated with the 

 organic matter, is a hydrolytic ferment or enzyme. When freed 

 from its associations and obtained in a pure state, pepsin presents the 

 characteristics of a colloid body, and resembles in its reactions the 

 albuminoids. It has the power, when brought into relation with 

 acidulated proteids, of transforming them into new forms capable of 

 absorption into the blood. 



Rennin. In addition to pepsin a second ferment exists in the 

 gastric juice, to which the term rennin has been given. It possesses 

 the power of coagulating the caseinogen of milk. It exists in the 

 mucous membrane, from which it can be extracted by appropriate 

 means. When rennin acts on caseinogen, the latter is split into 

 insoluble casein and a soluble albumin. Calcium phosphate is essen- 

 tial to the action of this enzyme. 



Gastric Glands. Embedded within the mucous membrane are to 

 be found enormous numbers of tubular glands, which though resem- 

 bling one another in general form, differ in their histologic details 

 in various portions of the stomach. 



In the cardiac end or fundus the glands consist of several long 

 tubules, opening into a short, common duct, which opens by a wide 

 mouth on the surface of the mucous membrane. Each gland consists 

 primarily of a basement membrane lined by epithelial cells. In the 

 duct the epithelium is of the columnar variety, resembling that 

 covering the surface of the mucous membrane. The secretory por- 

 tion of the tubule is lined by a layer of short, polyhedral, granular, 

 and nucleated cells, which, as they border the lumen of the tubule, 



