70 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



little difference ; a very small amount of acid stops its activity. 

 The conversion of starch into sugar by saliva in the stomach 

 continues for a considerable time, for the swallowed masses which 

 fall into the fundus of the stomach are not subjected to peristalsis 

 and admixture with gastric juice until a later stage in digestion ; 

 the hydrochloric acid which is poured out by the gastric glands first 

 neutralises the saliva and combines with the proteins in the food ; but 

 immediately free hydrochloric acid appears the ptyalin is destroyed, 

 so that it does not resume work even when the semi-digested food 

 once more becomes alkaline in the duodenum. 



THE SECRETION OF GASTRIC JUICE 



The uice secreted by the glands in the mucous membrane of the 

 stomach varies in composition in the different regions, but the mixed 

 juice is a solution of a proteolytic ferment called pepsin in a saline 

 solution, which also contains a little free hydrochloric acid. 



The gastric juice can be obtained during the life of an animal by 

 means of a gastric fistula. Gastric fistulas have also been made in 

 human beings, either by accidental injury or by surgical operations. 

 The most celebrated case is that of Alexis St. Martin, a young 

 Canadian who received a musket wound in the abdomen in 1822. 

 Observations made on him by Dr. Beaumont formed the starting- 

 point for our correct knowledge of the physiology of the stomach 

 and its secretion. 



We now make artificial gastric juice by mixing weak hydrochloric 

 acid (O2 to 0'4 per cent.) with a glycerin or aqueous extract of the 

 stomach of a recently killed animal. This acts like the normal juice. 



Three kinds of glands are distinguished in the stomach, which 

 differ from each other in their position, in the character of their 

 epithelium, and in their secretion. The cardiac glands are simple 

 tubular glands quite close to the cardiac orifice. The fundus glands 

 are those situated in the remainder of the cardiac half of the 

 stomach : their ducts are short, their tubules long in proportion. 

 The latter are filled with polyhedral cells, only a small lumen being 

 left : they are more closely granular than the corresponding cells in 

 the pyloric glands. They are called principal or central cells. 

 Between them and the basement membrane of the tubule are other 

 cells which stain readily with aniline dyes. They are called parietal 

 or oxyntic (i.e. acid-forming) cells. The pyloric glands, in the pyloric 

 half of the stomach, have long ducts and short tubules lined with 

 cubical cells. There are no parietal cells. 



