80 THE MILK QUESTION 



epithelial cells lining the alveoli. Normal milk has rela- 

 tively few or no leucocytes: pathological milk may con- 

 tain many. 



Composition of milk 



Milk is an enormously complex substance; being a mix- 

 ture, a suspension, and a solution of the representatives of 

 each one of the great classes into which foodstuffs are usu- 

 ally divided, namely, proteins, fats, carbohydrates, salts, 

 and water. In addition to this, milk contains various fer- 

 ments or enzymes ; gases ; and also antibodies, including pro- 

 tective antitoxins which doubtless serve a useful purpose. 

 In short, milk contains all the elements necessary for the 

 growth and development of the young animal. It is the 

 only single article of food which is complete enough in 

 itself to sustain life for a very long time. 



The milk of different animals shows a general agreement 

 in physical properties and composition. It contains essen- 

 tially the same ingredients, but differs in the amount of 

 these several constituents. 



In its perfectly fresh state milk is a yellowish-white 

 opaque liquid. The opacity of milk covers a multitude of 

 dirt and bacteria, and other sins. 



When allowed to stand for some time it separates into 

 two distinct layers. The upper, lighter layer is called 

 " cream " and consists of all the constituents of the milk, 

 but containing an increased number of fat globules. The 

 lower, heavier layer is white or bluish-white in color and 

 is known as " skim milk." 



Milk^ is heavier than water, having a specific gravity 

 ranging from 1.027 to 1.040; that is, if a quart of water 

 weighs 2 pounds and ij ounces, a quart of milk weighs 

 from 1.027 to 1.040 times as much, or about 2 pounds and 

 2}/2 ounces. Such wide differences as from 1.027 to 1.040, 



' Unless otherwise specified milk is understood to refer to cow's 

 milk. 



