DIGESTION AND ABSORPTION 337 



Test the contents of the different tubes for peptone with the biuret test. 

 (See below.)^ 



Action of Gastric Juice. — Most proteid substances are insoluble 

 in water. They belong to the class of substances known as '' col- 

 loids" — substances that do not easily pass through a membrane 

 by osmosis. Protein is changed by the pepsin of the gastric fluid 

 to a substance readily soluble. After protein is digested 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. 



Gastric juice acts most perfectly at the temperature of the normal body. 

 The enzyme pepsin will not act in an alkaline medium; boiling or freezing 

 prevents its action as well, A slightly acid medium is necessary for proteid 

 digestion in the stomach. 



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

 the proteid found in milk; after the milk is curdled, then the enzyme, 

 pepsin, is able to act upon it and change it to a soluble crystalloid substance. 



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

 some other salts taken into the stomach with food. 



Experiment. — Add, drop by drop, very dilute hydrochloric acid to a 

 test tube containing limewater. Notice the change that takes place. By 

 this means lime is changed from an insoluble to a soluble form and can be 

 absorbed into the blood, to be later used in the building of bone. 



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 control 

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

 makes several complete circuits during the process of digestion in that 

 organ. While this is taking place, the gastric juice acts upon proteins, 

 softening them, while the constant churning movement tends to separate 

 the bits of food into finer particles. Finally, some of the partly digested 

 food is allowed to pass in small amounts through the pyloric valve, into 

 the small intestines. This is done by the expansion of the ringlike muscles 

 of the pylorus. 



^ The food test for proteid already given (nitric acid followed by ammonium 

 hydrate) is known as the xanthoproteic. 



Another test is made in the following manner : Place in a test tube containing 

 proteid some concentrated caustic soda solution. To this add, drop by drop, a little 

 weak copper sulphate solution. Note the resulting color (violet). Heat the material 

 and the color deepens. 



If the proteid has been digested to peptone the above test will show a rose pink. 

 This test, known as the biuret test, is used for the detection of peptones. 



hunter's BIOL. — 22 



