CHAPTER IV 

 THE SOURCES OF ENERGY 



IT is clear that the animal has a double relationship with its 

 environment. On the one hand it becomes aware, by means of 

 its sense organs, of the changes that occur in the outer world. 

 Events may happen there that menace it, and so there are places 

 and things that are to be avoided; while other things present 

 advantages food and shelter, for instance and these things it 

 must seek and utilise. As the result of sensation and perception 

 it acts upon the external world : its second relation to the latter. 



This acting involves mobility of body and limbs that is, the 

 performance of mechanical work and we have next to enquire 

 into the source of this kinetic energy, or vis viva, of the animal 

 body. Common experience shows that it is obtained from the 

 food that the animal eats, and so we regard this food as a store of 

 potential energy which becomes transformed into the movements 

 that make up the locomotory, defensive, and aggressive actions. 

 The whole series of energy transformations falls into three stages : 



(1) The digestion, assimilation, and distribution of food material; 



(2) the oxidation of the assimilated food substances, with the 

 liberation of the bound energy contained in it ; and (3) the excre- 

 tion of the products of the process of oxidation. All this train of 

 changes undergone by the substances that are taken into the 

 animal body, built up to form its tissues, oxidised and excreted, 

 is called metabolism. 



The Inanimate Engine. Now it will help us greatly in our 

 study of the animal mechanism if we discuss first the way in 

 which energy is transformed in the inorganic engine. In the 

 most common form of the latter, the steam engine, there is a 

 working substance, the water, which is heated in the boiler 

 until it forms steam under pressure. This steam is admitted 

 into a cylinder, where it is allowed to expand, thus forcing out 

 the piston, and so doing mechanical work. It is again expanded 

 in intermediate and low-pressure cylinders, thus doing more 

 work, and by the time it has entered the COD denser, and attained 



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