THE BIOLOGY OF HEALTH 127 



to do justice to the deep differences between an animal 

 and a whirlpool in the river, or between an animal and a 

 machine (which is the embodiment of a human idea), 

 and we hold firmly to the belief that a philosophical 

 justification can be given for the common-sense belief 

 that one enters a new world when one passes from the 

 domain of things — the inorganic — to the realm of organ- 

 isms, the obviously animate. But for purposes of 

 investigation it is often very useful to treat the body as 

 if it were an engine, and for the purposes of study it is 

 also very useful to think of it in this way. 



Many books have been written about the structure 

 of our body, but perhaps the most illuminating plain 

 account is that given by Professor Arthur Keith under the 

 title The Engines of the Human Body (1920). His idea 

 was to compare the structure of the living body to that 

 of an engine, and to follow the comparison till it broke 

 down, as break down it did. An engine is a mechanical 

 contrivance for changing matter and energy from one 

 form to another in order to do work ; and our body is 

 a living contrivance which does this and more besides. 

 For we cannot forget that we think and feel. 



If we ascend a hill road on a motor-cycle, we get to 

 the top by mechanical work, the power or energy being 

 supplied by the internal-combustion engine which makes 

 the wheels go round. If we ascend a hill road on foot, 

 we get to the top by mechanical work, the power or 

 energy being supplied by our muscles which move those 

 levers we call our limbs. But the metal engine works by 

 pushing the crank-pin ; it has a rigid cylinder and a 



