DOMESTIC ANIMALS 249 



cheese; the by-products are skim milk, buttermilk, and whey. 

 Milk is a fluid formed in the glands of the cow's udder. It is an 

 ideal food, containing in readily digestible form water for thirst, 

 ash to make bones, protein to form flesh and muscle, and fat and 

 sugar to supply heat and energy. 



The globules of fat are light and rise to the surface, forming 

 cream. These globules vary in size. They are large in the 



6 ^ ~ o o 



A, PURE MILK, AND B, IMPURE MILK, AS SEEN UNDER THE MICROSCOPE 



A shows fat globules ; B shows globules and also many forms of bacteria. B stood sev- 

 eral hours in a warm room in a dirty dish. 



milk of the butter breeds and small in that of the cheese 

 breeds. 



Bacteria, always present in the air, grow in milk and change 

 its sugar to an acid which gives it a sour taste. Where strict 

 cleanliness is observed, there are fewer bacteria and milk does 

 not sour so quickly. Souring, or turning, of milk is retarded also 

 by keeping it cool, as bacteria thrive in warmth. Cooling milk 

 does not destroy the germs but only checks their action. 



Skim milk and buttermilk, from which fat has been removed, 

 and whey, left from the manufacture of cheese, are wholesome and 



