DIGESTION. 31 



digestion afferent impulses are passing up the pneumogastrics to the medulla ; 

 efferent impulses, in all probability, pass through the fibres of the sym- 

 pathetic nervous system to the blood vessels and glands concerned in the 

 elaboration of the gastric juice. After all the nervous connections of the 

 stomach are divided, a small quantity of juice continues to be secreted for 

 several days. This has been attributed to the action of a local nervous 

 mechanism and to the direct action of the food upon the protoplasm of the 

 secreting cells. 



The Gastric Juice is a secretion of the true peptic glands, and when 

 obtained from the stomach through a fistulous opening, is a clear, straw- 

 colored fluid, decidedly acid, with a specific gravity of 1.005 to i.oio. 



COMPOSITION OF GASTRIC JUICE. 



Water, 975- 



Pepsin, 15.00 



Hydrochloric acid, 4.78 



Inorganic salts, , 5-22 



1000.00 



The water forms the largest part of the fluid, and holds in solution the 

 other ingredients. It results from a transudation from the blood vessels 

 under the increased blood supply. Of the inorganic salts the chlorides of 

 sodium -and potassium are the most abundant. 



Pepsin is the organic nitrogenized ferment of the gastric juice, and is 

 formed, during the intervals of digestion, by the peptic cells. In the 

 presence of a small per cent, of an acid, it acquires the property of con- 

 verting the albumin of the food into albuminose or peptones. 



Hydrochloric acid is present in small quantity, and gives the juice its 

 acidity. In all probability, its production is due to the activity of the 

 parietal cells. These two characteristic ingredients of the gastric juice 

 exist in a state of combination as hydrochloro-peptic acid, and the presence 

 of both is absolutely essential for the complete digestion of the food. 



When the food enters the stomach, it is subjected to the peristaltic action 

 of the muscular coat, and thoroughly incorporated with the gastric juice. 

 This fluid has a twofold action upon the food: 1st. A physical action, by 

 which the fibrous tissues of meats, the cellulose and hard parts of grains 

 and vegetables, are dissolved away until the food is disintegrated and 

 reduced to the liquid condition. 2d. A chemical action, by which the 

 albuminous principles are transformed into peptones. The more important 

 foods with their contained albuminous principles are shown on page 21. 



