90 HUMAN BIOLOGY 



occupied by their whole lives. (By this rule how long 

 ought man to live?) Animals, moreover, move from place 

 to place, and work with their muscles. The energy for this 

 comes from the food they eat. Plants do not use food for 

 this purpose. Another need for food comes from the 

 necessity for heat in all living things. The activities of 

 animals cause the tissues to wear out, or break down, and 

 food furnishes material with which new living matter is 

 built up by the cells and the tissues repaired. We have 

 already stated the role of oxygen in setting free energy in 

 the living substance of the cell by oxidizing it. There is 

 no furnace in the body as in an engine, but the oxidation 

 occurs in the cells themselves and the fuel is built up into 

 living matter by the cells before it is oxidized. Plants 

 must lift mineral from the inorganic to the organic world 

 before it can be food for animals. Plants can assimilate 

 minerals ; animals cannot. The body cannot make bone 

 out of limewater. The iron in iron tonics cannot be used. 

 Iron makes the grain brown, and the peach red. There 

 is ten times as much iron in our food as the body needs. 



State four reasons why animals need food. Which of 

 these reasons is very powerful with plants ? Least powerful? 

 Absent altogether ? Why is constant breathing necessary 

 for life ? When is breathing more rapid ? Why? People 

 who lead what kind of lives usually have poor appetites ? 

 Good appetites ? Why ? What was the first distinct or- 

 gan evolved by animals ? (Animal Biology, Chap. IV.) 



The Body is a Machine for transferring Energy. — Energy 

 cannot be destroyed, but it can be transferred and changed 

 in form. When a coin is rubbed on the table, muscular 

 energy, supplied by oxidation in the muscle, produces the 

 motion. Friction may change motion into heat, and the 

 coin will become very hot. The uniting of food and 



