192 THE VITAL PROCESSES 



used by the body in doing its work. The phrase " Child 

 of the Sun " has sometimes been applied to man to express 

 his dependence upon the sun for his supply of energy. 



Why Oxygen and Food are Both Necessary. The neces- 

 sity for introducing both oxygen and food into the body 

 for the purpose of supplying energy is now apparent. 

 The energy which is used in the body is not the energy 

 of food alone. Nor is it the energy of oxygen alone. 

 It belongs to both. It is due to their attraction for each 

 other and their condition of separation. It cannot, there- 

 fore, become kinetic except through their union. To intro- 

 duce one of these substances into the body without the other, 

 would neither introduce the energy nor set it free. They 

 must both be introduced into the body and there caused to 

 unite. 



Bodily Control of Energy. A fact of importance in the 

 supply of energy to the body is that the rate of transfor- 

 mation (changing of potential to kinetic) is just sufficient 

 for its needs. It is easily seen that too rapid or too slow 

 a rate would prove injurious. The oxidations at the cells 

 are, therefore, under such control that the quantity of 

 kinetic energy supplied to the body as a whole, and to 

 the different organs, is proportional to the work that is 

 done. This is attained, in part at least, through the ability 

 of the body to store up the food materials and hold them 

 in reserve until they are to be oxidized (page 180). 



Animal Heat and Motion. Most of the body's energy 

 is expended as heat in keeping warm. It is estimated that 

 as much as five sixths of the whole amount is used in this 

 way. The proportion, however, varies with different 

 persons and is not constant in the same individual during 

 different seasons of the year. This heat is used in keeping 

 the body at that temperature which is best suited to carry- 



