V. 

 BODILY STRUCTURE. 



I. Pumps without Pistons : Self-repairing 

 Valves. 



ONE of the most interesting applications in 

 Nature of mechanical contrivances for the per- 

 formance of mechanical work is seen in the 

 case of the veins and the heart. The heart 

 is a great pump for driving blood to all parts 

 of the body. The veins contain a series of 

 pumps for drawing it back to the heart. These 

 pumps have two great advantages over the 

 forms which we make of metal. They are self- 

 repairing, and, owing to their elastic and col- 

 lapsible structure, they are able, like our bellows 

 and spray and syringe-balls, to dispense with 

 pistons. It is the latter condition that makes 

 the former possible ; for, if Nature had ever 

 developed an apparatus like our piston and 

 cylinder, with the complicated arrangements 

 necessary for working the piston, it is difficult 

 to see how it could have been contrived that 

 such apparatus should have had the power of 

 growth and self-repair. 



The blood-pumps of the body, then, work 

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