CHAPTER X 



DYNAMICS 



Energy. Work has been defined as overcoming resistance, 

 and energy as that which, when properly applied, can do work, 

 i.e. can overcome resistance. All work, therefore, involves 

 expenditure of energy. In an ordinary engine, the energy 

 (heat) is derived from the combustion of fuel. The engine 

 itself is merely a mechanical device for applying this energy so 

 as to overcome some kind of resistance. 



Animals also can do work, i.e. they can apply energy so as 

 to overcome resistance. In this case, the energy is derived 

 from the oxidation of food. Physiologically, the process may 

 be an indirect one. The energy may possibly be derived, 

 proximately, from the oxidation of the tissues of the animal's 

 body ; these would be again restored from the materials of the 

 food, so that, in any case, the energy is derived ultimately from 

 the food. 



It was shown in the previous chapter that the energy 

 derived from the food required for the maintenance of the 

 animal is dissipated in several ways. It cannot, therefore, be 

 used to do work. If any part of it is used to do work 1 

 there will be a corresponding deficiency of energy required 

 to maintain the temperature of the animal's body and for 

 other purposes previously mentioned ; in that case, some 

 of the tissues will be oxidised (p. no), and the animal will 

 lose weight. If, therefore, an animal is required to do work, 

 it must receive additional food over and above what is 



1 This refers only to external work. The distinction between internal 

 and external work is fully explained later on in this chapter (p. 154). 



H3 



