A "HEAD OF PRESSURE" 119 



cylinders are supplied from the capillary field with the 

 materials of a combustion mixture — oxygen and blood- 

 sugar. The supply is carried into the cylinders by 

 arteries, and the effete products are carried away from 

 them by veins. It is the heart which circulates the blood 

 in the capillary field and maintains the supply of com- 

 bustion material to the muscular engines. When we 

 examine the mechanism of a motor-cycle engine we find 

 the circulation of a combustion mixture through its 

 cylinder ; the petrolised air enters by the inlet pipe or 

 artery and leaves by the outlet pipe or vein. That 

 circulation is maintained by the movements of the 

 piston ; it is drawn in by the suction stroke, and expelled 

 by the exhaust stroke ; for two strokes in each cycle the 

 engine acts as a pump or heart. Thus in the engine of 

 a motor cycle we have a primitive condition — a machine 

 which serves the purposes of muscle and heart combined. 

 In the human machine we reach a much higher point of 

 evolution — one where the driving power and circulating 

 force are carried on by separate machines. 



