130 THE CONTEOL OF LIFE 



the energy is supplied by tlie explosive combustion or 

 oxidation of tbe petrol. In the muscle, tbe affair is more 

 complicated, for the using up of the oxygen is not in the 

 contraction itself, it is in the process which instanta- 

 neously restores the muscle-fibre to a state that makes 

 another contraction possible. In fact, no one yet knows 

 the secret of the muscle-engine. 



But if we say cautiously that the muscles are peculiar 

 engines, living engines, we may usefully continue to 

 compare them to the internal-combustion engines of 

 the motor-cycle. They are engines of great merit ; 

 they work so smoothly, with so little noise ; they always 

 have their ' steam up,' ready to start working at a 

 moment's notice ; some of them never stop working 

 all through life, while others are able to take a rest. 

 They work in pairs, opposed to one another, and when 

 one muscle goes into action a message travels into the 

 nervous system and orders come back so that the 

 opponent muscle yields just to the right extent. They 

 say that the best engine man has yet made does not turn 

 more than 20 per cent, of the energy of the fuel into 

 effective work, whereas our muscles can turn about 

 25 per cent, of their fuel or food into effective work. 

 Then the muscles are self-stoking, self-repairing engines, 

 requiring no looking after from us, making no demands 

 save one — that they he exercised. 



The motor-cycle works obviously by wheels, which 

 are very compHcated levers ; our movements are brought 

 about by more ordinary levers — the bones. Students 

 of mechanics know well that there are three kinds or 



