176 ELEMENTARY PHYSIOLOGY. [XVI. 



5. Place a small quantity in a warm place for one 

 or two days; then test the reaction, it will be 

 found to be acid ; this is due to fermentation, 

 in the process of which the milk-sugar is con- 

 verted into lactic acid. 



0. Action of Gastric Juice on Milk. 



Neutralize with dilute Na 2 C0 3 a little artificial 

 gastric juice prepared by method (a) 1, filter 

 and add 5 c.c. of the filtrate to 5 c.c. of fresh milk, 

 place in the warm. 



Observe at short intervals the condition of the 

 milk, it will soon form a firm clot so that the 

 test-tube can with safety be held upside down, 

 later the clot shrinks and presses out a nearly 

 clear fluid; the clot continues to shrink for 

 some time. 



The rennet-ferment in the extract has coagu- 

 lated the casein, and this has carried with it 

 the greater number of the fat globules. 

 If the amount of rennet-ferment contained in 

 the extract is large the clotting may be almost 

 instantaneous ; in this case the experiment should 

 be repeated taking a smaller quantity of the 

 extract and without warming. The extract is 

 neutralized since (cp. 4) excess of acid of itself 

 precipitates casein. 



7. To the milk clotted by rennet-ferment add 

 5 c.c. HC1 '4 p.c. and warm for an hour or so, the 

 casein will be converted into peptone by the 

 pepsin of the extract in the presence of acid. 



