CHAPTER VIII. 

 WHY WE EAT AND BREATHE. 



How is it that the body can do muscular work ? In the 



muscles we possess a set of organs capable of moving the 

 body from place to place, of changing the relative positions 

 of its parts, and of lifting external objects : as long as we 

 are alive, more or fewer of our muscles are every moment 

 doing some mechanical work. This fact suggests the 

 question, where does this power of working come from ? 

 In a few words, the answer is, it comes from the burning 

 of parts of the body itself : in the burning, work-power or 

 energy is set free and some of this is used by the muscles. 

 The conservation of energy. The different natural 

 forces known to us are not nearly so numerous as the 

 kinds of matter : we all, however, know several of them, 

 as light, heat, electricity, and mechanical work. One of 

 the greatest discoveries of the nineteenth century is that 

 these different natural forces, or forms of energy, can be 

 turned one into another, directly or indirectly : kinds of 

 energy are transmutable, while, so far as we know at pres- 



What are the functions of the muscles? Are all of our muscles ever 

 at rest at the same time ? 



What question does the constant activity of our muscles suggest? 

 How may this question be briefly answered ? 



Name some forms of energy. Can they be turned from one 

 form to another ? 



