A STUDY OF BLOOD MANUFACTURE 91 



Certain mineral food substances, however, like phosphate 

 of lime found in milk, are not soluble in water, and these 

 insoluble salts reach the stomach unchanged. The hydro- 

 chloric acid of the gastric juice soon converts them into 

 soluble salts, which are then dissolved. Thq, following ex- 

 periment will illustrate this process. Put a little of the 

 phosphate of lime into a test tube, add water, and shake. 

 The mixture assumes a milky appearance, and after a time 

 the phosphate of lime settles to the bottom, showing that 

 this kind of mineral matter will not dissolve in water. The 

 addition of a small amount of hydrochloric acid immediately 

 clears the mixture, for the phosphate of lime is converted 

 into chloride of lime, which is a soluble salt. 



Digestion of Proteids. 1 The most important function of 

 the stomach is its digestive action on proteids. The white 

 of egg, lean meat, or the gluten of wheat reaches the stomach 

 in a pulpy mass, and this is insoluble in water. The casein 

 of milk, which is a liquid when swallowed, becomes curdled 

 by the action of the hydrochloric acid. Before these most 

 important proteids can be absorbed to become a part of the 

 blood, they must be chemically changed into a kind of 

 soluble proteid called pep'tone. The principles of proteid 

 digestion can be illustrated in the following way : Cut into 

 fine bits a piece of hard-boiled egg, and divide the minced egg 

 into four portions. Put one fourth into a test tube (No. 1) 

 and add water. Into another test tube (No. 2) put a second 

 portion of minced egg, water, and a little hydrochloric acid. 

 To a third portion (test tube No. 3) add water and a little 

 pepsin. (Pepsin readily dissolves in water.) The rest of 

 the egg in test tube No. 4 should be treated with all the in- 

 gredients of gastric juice (water, pepsin, and hydrochloric 

 acid). Since the food in the stomach is in constant motion, 

 and since the temperature within the body (98 i F.) is 

 considerably higher than that of the surrounding air, these 

 conditions should be imitated so far as possible by frequently 

 1 See "Laboratory Exercises," No. 21. 



