FOOD AND AIR TURNED INTO FLESH AND ENERGY. 79 



The animal body, however, not only needs a constant 

 supply of the substances from which this energy may be 

 produced, but also a constant supply of those substances 

 which compose its body. In every young animal there 

 is a growth, an increase of size and weight, and in the 

 adult a constant replacing of body material. And by 

 far the greater part of an animal body is made up of just 

 those things, namely, oxygen, carbon, hydrogen, and 

 nitrogen, that are used in the supply of energy. 



How animals breathe. — The animal gets its oxygen 

 from the free air, or from the air mixed with water, by a 

 process called respiration. It obtains from its food all 

 the other substances used. Food is prepared for use by 

 the process of digestion. Oxygen obtained by respiration 

 and the substances obtained from food are distributed 

 throughout the body by the process of circulation. We 

 may now consider the ways in which respiration, diges- 

 tion, and circulation are carried on among animals. 



As to respiration, mention has been made only of its 

 service in providing the animal with oxygen. It has, 

 however, one other object. When oxygen combines with 

 carbon, carbon dioxide is formed; if this remains in the 

 muscle or other tissue cells it interferes with the activity 

 of those cells. It is therefore just as necessary for the 

 carbon dioxide to be removed as for the oxygen to be 

 supplied. Carbon dioxide, like oxygen, is soluble in water. 

 Blood, which is composed largely of water, and which can 

 carry the one serves also to carry the other. Further- 

 more, since carbon dioxide is made by a combination with 

 oxygen, it arises just where it can be carried away by the 

 very apparatus that has brought the necessary oxygen. 

 Thus the respiratory apparatus manages both the supply 

 of oxygen and the disposal of carbon dioxide. 



The fundamental fact in the process of respiration is 

 that gases, whether free or dissolved in water, will readily 



