CHAPTER XIII 



MILK, BLOOD AND URINE 



Before dismissing the general subject of animal food and 

 nutrition, and taking up the study of plant physiology, it is 

 best to consider the three animal fluids mentioned at the head 

 of this chapter. Milk is a normal secretion of all mammalian 

 animals, serving as food for the offspring during the first period 

 of its life. Blood is the main circulatory liquid of the animal 

 body by means of which the absorbed food nutrients are carried 

 to the different parts of the body and in the muscle cells brought 

 in contact with oxygen, also carried by the blood, and finally 

 oxidized. Urine is an excretion liquid which serves as the 

 medium by which the nitrogen end products of protein katab- 

 olism are removed from the body. 



MILK 



Milk is the normal secretion of the mammary glands of 

 mammalian animals. It is provided by the animal as the sole 

 food of the young for the first period of its life. In harmony 

 with this use it is natural to find that it contains all of the 

 substances essential as animal food. These substances, as we 

 have previously stated, embrace carbohydrates, fats, proteins 

 and certain inorganic salts. As an agricultural product milk 

 means almost exclusively cows' milk and, unless otherwise 

 stated, what we shall say refers to this particular milk, though 

 in general the statements will apply also to all milk. 



Constituents 



Carbohydrates. — There is only one carbohydrate found in 

 milk, viz. milk sugar, or lactose. This sugar is a disaccharose 

 of the same composition as cane sugar, i.e. C12H22O11. It is 

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