20 The Feeding of Animals 



as a source of this element to the animal whether it 

 is supplied directly as such or is obtained from the 

 food. Sodium plays an important part in the diges- 

 tion of food, because it is the basis of certain bile 

 salts and is concerned in other ways in the digestive 

 processes. 



Calcium, when united with oxygen, forms lime, 

 which is one of our commonest commercial articles. 

 Large masses of lime rock, or calcium carbonate, exist 

 in many parts of the earth's surface, and every soil 

 contains more or less of lime compounds. As com- 

 pounds of this element are usually found in plants and 

 in the milk of all animals, normal food nearly always 

 furnishes a supply sufficient to meet the demands of 

 animal life. The growing animal makes a generous 

 use of lime, because in union with phosphoric acid it is 

 the chief building material of the bony framework. 

 A deficiency of food lime is sure to cause abnormal 

 development of the bony structures. With birds, it 

 is especially in demand during egg formation, egg 

 shells being mostly a lime compound. 



Iron, one of the elements of living organisms, needs 

 no description, because its common properties are fa- 

 miliar to every one. Iron rust and iron ore are oxides 

 of this element, and when the oxygen is removed from 

 these, we have the bright gray metal of commerce. 

 Though taken up by plants and animals in small quan- 

 tities only, iron is absolutely essential to their growth 

 and welfare, but because of its abundance the impera- 

 tive character of the demand is never realized in ordi- 

 nary experience. 



