A "HEAD OF PRESSURE" 109 



blood spurts out ; blood is being pumped into it all 

 the time. 



An engineer who has to think out a method of pro- 

 viding the inhabitants of a hill-set city with a constant 

 supply of water knows something of the difficulties which 

 Nature had to overcome before she could succeed in pro- 

 viding each living unit of her complex machines with a 

 constant supply of blood. A head of pressure has to be 

 maintained so that the flow of water or blood may be 

 constant and plentiful. The water-engineer may employ, 

 as Nature had done before him, a pump to force the water 

 up to the required level ; nor does the plan of the pump 

 he adopts differ in essentials from the one worked out by 

 Nature in her master-pump — the heart. In fig. 29 a 

 force-pump of a simple construction is compared with the 

 main pump of the body — the lefc ventricle. In both 

 there is an inflow pipe conducting to the main-pump 

 chamber ; only in the human pump the inflow passes 

 through a loading chamber — a contrivance which i as far 

 as I can learn, has never been fitted to a pump made of 

 metal or of wood. As the inflow enters the main chamber 

 it passes an inlet valve, a simple trap-door contrivance in 

 the water-pump, but an elaborate arrangement of sail- 

 like cusps in the blood-pump — forming the mitral valve 

 (fig. 29). From the main chamber opens the outflow 

 pipe — guarded by the outlet valve — again a simple trap- 

 door contrivance in the water-pump, but a triple pocket 

 valve in the blood-pump (fig. 29). 



The arrangement of valves is the same in both kinds of 

 pump, but the manner in which the contents are forced 

 out from the main chamber is quite different. In the 

 water-pump a rigid piston descends within a metal cylinder, 

 and thus obliterates the space of the main chamber and 

 forces the water through the outflow pipe, past the outlet 

 valve. The power or strength which drives the piston 

 or plunger is obtained from an engine which is placed 

 at some distance from the pump. In the heart there are 

 no separate plungers or pistons ; every part of the wall of 

 the main chamber moves inwards, as if it formed part of a 



