PHYSIOLOGICAL PARADOXES. 



in the same way as the ball of a sprayer, by 

 alternate compression and relaxation, or systole 

 and diastole, as the physiologists call these 

 movements. The compression squeezes the 

 blood away, forcing it along the pipes, and 

 then the relaxation allows a fresh quantity to 

 accumulate ready for the next compression. 



Though the method of action is different, 

 the result is the same as in our metallic pumps ; 

 and in each case there is the same necessity 

 for valves to prevent the fluid flowing back 

 while the fresh charge is accumulating. In 

 the physiological as well as in the metallic 

 pump the valves are fittings of great import- 

 ance. If they do not close tightly, some of 

 the fluid already once driven out leaks back 

 into the space which ought to be accumulating 

 an entirely fresh supply, much of the energy of 

 the pump is wasted, and it has a difficulty in 

 maintaining the proper pressure. 



Now the inside of the veins and heart can- 

 not be got at for constantly recurring opera- 

 tions, such as the paring of the nails, the rub- 

 bing of the eyes, or the blowing of the nose ; 

 and so it is essential that the valves should be 

 of a nature which admits of their coming into 

 existence by ordinary processes of bodily growth 

 and keeping themselves repaired and in good 

 working order by similar means. Instead of 

 being separate pieces, like the valve of a metallic 

 pump, they must be an outgrowth or modifi- 



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