308 ESSENTIALS OF PHYSIOLOGY. 



flow of juice is also inhibited by alkalies, and is excited by acids in 

 the stomach. 



The Origin of the Chief Constituents of the Gastric Juice. Two 

 chief types of gland are found in the mucous membrane of the stomach. 

 The tubular glands of the body of the stomach are relatively straight, 

 open into short ducts, and possess two kinds of secreting cells. The 

 gland is lined throughout by cubical, granular cells which are called 

 chief or peptic cells. Between the chief cells and the basement 

 membrane there occur at intervals somewhat larger ovoid cells, which 

 are described as oxyntic because they are believed to secrete the acid 

 of the gastric juice. The second type of gland occurs in the pyloric 

 portion of the stomach. The pyloric glands are also simple tubes, 

 which are twisted on themselves and open into relatively long and wide 

 ducts. Moreover, each is lined by one type of cell only, resembling in 

 structure the chief cells of the glands of the body of the stomach. 



The cells lining the general surface of the stomach and the ducts 

 secrete the mucin of the gastric juice. The enzymes of the juice are 

 contained in the secretion of the body of the stomach and also in that 

 of the pyloric portion, and are derived from the chief cells of the glands 

 of the body and from the cells lining the pyloric glands. Pepsin, how- 

 ever, does not exist in the secretory cells as such, because extracts of 

 the mucous membrane do not possess marked peptic activity until they 

 have been treated with acid. It is therefore a precursor of pepsin, 

 known as pepsinogen, which is found in the secretory cells, and this is 

 converted into pepsin, after its discharge from the cells, by the hydro- 

 chloric acid of the gastric juice. 



The facts from which it is concluded that the acid itself is derived 

 .from the ovoid cells are (1) that it is most abundant in the middle of 

 the stomach, where these cells are most numerous, and (2) that it is 

 absent from the secretion of the pyloric portion of the stomach, where 

 ovoid cells are also wanting. 



Various explanations have been offered as to the method of pro- 

 duction of the free acid in the gastric juice. The most probable of these 

 suggestions is that the acid is derived from the interaction of chlorides 

 with di-sodium hydrogen phosphate, according to the formula 



2Na 2 HP0 4 + 3CaCl 2 = Ca 3 (P0 4 ) 2 + 4NaCl + 2HC1. 



During the early stages of secretion the cells of the gastric glands 

 become enlarged, the chief cells, and those lining the pyloric glands, are 

 crowded with secretory granules, and the ovoid cells are distended and 

 clear. As secretion proceeds, all the cells become diminished in size, as 

 in the case of the salivary glands. It has already been pointed out 



