122 THE ENGINES OF THE HUMAN BODY 



we may know and yet never clearly perceive that the 

 breathing system of the human machine is simply a pair 

 of bellows — but bellows so marvellously constructed that 

 we can find nothing even approaching them in perfection 

 of workmanship among human inventions. They may 

 work on continuously seventeen or twenty times a minute, 

 day after day, for a stretch of seventy years or more. 

 Unfortunately that is not always the case ; but we can 

 say that when the bellows of the human machine break 

 down the fault does not lie either in their workmanship 

 or in their construction. 



In quiet, dry, crisp, frosty mornings about Christmas 

 time we can often see our breath breaking on the air 

 as little wisps of steam, so like the puffs which issue from 

 the funnel of steam engines that we may almost fancy 

 that they too have a pulmonary system. But the ill- 

 smelling fumes which shoot out from the exhaust pipe 

 of a motor cycle never lead us to suppose that they are 

 issuing from a true respiratory chamber. Even those 

 who are most familiar with motor cycles never credit 

 them with any part corresponding to a chest and lungs 

 or suppose that they have a respiratory system. Yet I 

 think we may say with truth that the engine of a motor 

 cycle does breathe — does possess a respiratory mechanism. 

 The diagrams of the movements of the piston, reproduced 

 on p. 12, show that in the first stroke of a four-cycle 

 engine the mixture of air and petrol vapour rushes into 

 the cylinder to occupy the space left above the descending 

 piston. The air rushes in because we human beings, like 

 crawling crabs, live on the bottom of a deep and heavy 

 sea — not of water but of air. Hence as the piston 

 descends and leaves an empty space behind it the air 

 rushes into the cylinder, carrying the petrolised air with 

 it. The cylinder and piston represent a thorax of a 

 peculiar kind ; the suction stroke of the piston constitutes 

 the act of taking a breath — the act of inspiration. Then 

 when combustion has taken place an upward stroke of 

 the piston thrusts the burnt air from the cylinder out 

 through the exhaust or outlet passage. The exhaust 



