92 HUMAN PHYSIOLOGY. 



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 acidu- 

 lates 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. It has the power, when brought into relation with acidulated proteins, 

 of transforming them into peptones. 



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 coagulat- 

 ing 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 phos- 

 phate is essential to the action of this enzyme. 



Gastric Glands. Embedded within the mucous membrane are to be 

 found enormous numbers of tubular glands, which though resembling 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 base- 

 ment membrane lined by epithelial cells. In the duct the epithelium is of the 

 columnar variety, resembling that covering the surface of the mucous mem- 

 brane. The secretory portion of the tubule is lined by a layer of short, poly- 

 hedral, granular, and nucleated cells, which, as they border the lumen of the 

 tubule, and thus occupy the central portion of the gland, are termed central 

 cells. At irregular intervals, between the central cells and the wall of the 

 tubule, are found large oval, reticulated cells, which, on account of their 

 position, are termed parietal cells. (See Fig. 13.) 



Each parietal cell is in relation with a system of fine canals, which open 

 directly into the lumen of the gland. It is estimated that the fundus contains 

 about five million glands. In the pyloric end of the stomach the glands are 

 generally branched at their lower extremities, and the common duct is long 

 and wide. The duct is lined by columnar epithelium, while the secreting 

 part is lined by short, slightly columnar, granular cells. The parietal cells 

 are entirely wanting. The epithelium covering the surface of the mucous 



