X 



30 HUMAN PHYSIOLOGY. 



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

 verting the albumen 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 hydrocliloro-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 19. 



Upon meat the gastric juice has a decidedly disintegrating action. The 

 connective tissue is first dissolved, the fibres are separated, the sarcolemma 

 softened, and the whole reduced to a grumous, pultaceous mass. Milk 

 undergoes coagulation in from ten to fifteen minutes, the casein being 

 precipitated in the form of soft flocculi, which are easy of transformation 

 into peptone. Upon Vegetable tissues, the gastric juice exerts also a dis- 

 integrating action ; the cellulose and woody fibres are dissolved and the 

 nutritive principles liberated. Bread undergoes liquefaction quite readily. 



The Principal Action of the gastric juice, however, is to transform the 

 different albuminous principles of the food into peptones or albuminose, the 

 different stages of which are due to the acid and pepsin respectively. When 

 freed from its combination, the hydrochloric acid converts the albumen 

 into acid albtimen or parapeptone ; while this intermediate product is being 

 formed, the pepsin converts it at once into peptone. In order that the 

 digestion of albumen may be complete, it is necessary that both the acid 

 and pepsin be present in proper quantity. Before digestion, the albuminous 

 principles are insoluble in water and incapable of being absorbed. After 

 digestion, they become soluble and are readily absorbed. Peptones differ 

 from the albumins in being 



1 . Diffusible, passing rapidly through the mucous membrane and walls 

 of the blood vessels. 



2. Non-coagulable by heat, nitric or acetic acids; but are readily precipi- 

 tated by tannic acid. 



3. Soluble in water and saline solutions. 



