358 



DIGESTION AND ABSORPTION 



in the stomach of man, as well as in the frog, are located a number 

 of tiny pits. These form the mouths of the gastric glands, which 

 pour into the stomach a secretion known as the gastric juice. The 

 gastric glands axe httle tubes, the lining of which secretes the fluid. 

 This fluid is largely water. It is slightly 

 acid in its chemical reaction, containing about 

 .2 per cent free hydrochloric add. It also 

 contains a very important enzyme called 

 pepsin, and another less important one called 

 rennin. 



Action of Gastric Juice. — If proteid is 

 treated with artificial gastric juice at the 

 temperature of the body, it will be found to 

 become swollen and then gradually to change 

 to a substance which is soluble in water. 



Most proteid substances are insoluble. 

 They belong to the class of substances 

 known as colloids — substances that do not 

 easily pass through a membrane by osmosis. 

 After proteid is digested in the stomach, it 

 is known as a peptone. Digestion of proteid 

 results in a change of a colloid substance to 

 one which will diffuse readily through a 

 membrane, or a crystalloid. Peptones are 

 crystalloid substances. 



A peptic gland, from the 

 stomach, very much 

 magnified. A, central 

 or chief cell, which 

 make pepsin ; B, bor- 

 der cells, which make 

 acid. (From Miller's 

 Histology.) 



The other enzyme of gastric juice, called rennin, curdles or coagulates 

 a proteid found in milk ; after the milk is curdled, the pepsin is able to 

 act upon it. "Junket" tablets, which contain rennin, are used in the 

 kitchen to cause this change. 



The hydrochloric acid found in the gastric juice acts upon lime and 

 some other salts taken into the stomach with food, changing them so 

 that they may pass into the blood and eventually form the mineral part 

 of bone or other tissue. 



Movement of Walls of Stomach. — The stomach walls, provided with 

 three layers of muscle which run in an oblique, circular, and longitudinal 

 direction (taken from the inside outward), are well fitted for the constant 

 churning of the food in that organ. Here, as elsewhere in the digestive 

 tract, the muscles are involuntary, muscular action being under the con- 

 trol of the so-called sympathetic nervous system. Food material in the 

 stomach makes several complete circuits during the process of digestion 



