MILK. 365 



503. What substances cause the clotting of blood, how are they sepa- 

 rated, and what are their properties? 



504. How may blood stains be recognized ? 



505. What is the active principle of saliva, and how does it act on 

 starch ? 



506. State the composition of gastric juice, and explain its physio- 

 logical action. 



507. State the general properties of bile, and mention its chief con- 

 stituents. 



508. Give Gmelin's test for biliary pigments, and Pettenkofer's test 

 for biliary acids. What precautions are necessary in using the latter 

 test? 



509. What is cholesterin ? State its properties and reactions. 



510. Mention the principal constituents of muscles, bone, teeth, and 

 hair. 



52. MILK. 



Properties and composition. Milk is the secretion of the mam- 

 mary glands, the presence of which is characteristic of the class 

 of animals known as mammalia. The milk of different animals 

 differs somewhat in composition, but it always contains all the 

 constituents necessary for a normal development of the various 

 tissues, liquids, organs, etc., of the young mammal, which gen- 

 erally feeds exclusively upon milk for a shorter or longer period 

 of its early life. 



Milk is a slightly alkaline, aqueous solution of casein, albu- 

 min, lactose, and -inorganic salts, holding in suspension small 

 globules of fat. Its specific gravity ranges from 1.029 to 1.033. 



The average composition of various kinds of milk is given 

 below, but, it must be remembered that milk not only differs 

 in certain species, but also in the same animal at different times ; 

 for instance, the quality and quantity of food taken, as also 

 various physiological changes, have a decided influence upon the 

 milk secreted. 



Human milk. Cow's milk. 



Water, 



Casein and albumin, 



Fat (butter), 



Lactose, 



Inorganic salts, 



