THE ENGINES OF 

 THE HUMAN BODY 



CHAPTER I 



HOW THIS BOOK CAME TO BE WRITTEN 



There is a machine which every one of us has to drive 

 morning, noon, and night, day by day, year in and year 

 out, until the end of life's journey. So quietly does this 

 machine run, so easy is it to guide, that we never imagine 

 it as contrived in the same way as the machines which 

 clever men have invented and made — machines which can 

 run, fly, weave, sew, count, and even speak. These 

 machines made of metal or of wood seem so different from 

 our bodies ; they are fed with coal or oil, or electricity ; 

 they become very hot when they work and very cold when 

 they stand idle ; they never feel pain, laugh, cry, play, or 

 know what it is to have a good time. Some of these 

 machines are wonderfully made, complex, and really 

 difficult to understand ; but not one of them, not even 

 the most intricate, is so hard to understand as the human 

 machine — your body and mine. You will see how difficult 

 it is to find out how its parts work when 1 tell you that 

 for two thousand years and more a countless succession of 

 clever men have studied it, both when it was living and 

 when it was dead, have taken it to pieces — or, as medical 

 students say, have dissected it — have examined its flesh 

 and textures with the most powerful of microscopes, have 

 applied to it all the arts and crafts known to chemists, and 



i i 



