RESPIRATION AND ENERGY IN MAN 123 



4. State now two evidences that oxidation is carried on 



in the human body. 



5. What element must always be present in order that 



oxidation may be carried on? 



173. Examples of energy in the human body. While 

 studying plants, we enumerated various ways hi which these 

 living organisms exhibit the energy which is developed 

 within them (P. B., 74), and we have likewise called 

 attention to evidences of energy in animals. In human 

 beings the forms of energy are much more varied and strik- 

 ing. For example, the movements of each of the five hun- 

 dred separate muscles found in the body are all due to the 

 muscular energy developed in their protoplasm; the control 

 of all these muscles is due to energy liberated in the nervous 

 system (nervous energy) ; all the glands that produce the 

 varied ferments owe their ability to do their work to the 

 release of chemical energy; and when we come to deal with 

 the highest functions, namely, feeling, thinking, and willing, 

 it seems probable that all of them are made possible by 

 the setting free of some form of energy. In connection 

 with the development of all these forms of energy, heat 

 energy, as we proved in 172, is liberated. 



174. Transformations of energy. While considering the 

 functions of green plants we found that the energy of the sun 

 is utilized and stored in the manufacture of food materials, 

 and thus is made available for the use of the plant. Con- 

 sequently, when we take into our bodies and digest the various 

 nutrients produced by green plants, these food substances 

 become available as our sources of energy. But to release 

 this stored-up energy, whether in muscle, gland, or nerve 

 cells, oxygen is always essential. Hence, a constant supply 

 of oxygen for the body is necessary. When this oxygen 



