THE WORK IT DOES 159 



and their results are almost identical. The 

 following is an outline that may bring the 

 matter clearly before us, while it ought to 

 make us value the mechanical achievements 

 of our hearts and the magnificence of the 

 structure capable of such heavy work : 



1 In estimating the amount of work done 

 by any machine it is usual to express it in 

 terms of " unit of work.' 7 The "unit of 

 work" is defined to be the energy expended 

 in raising a unit of weight (1 Ib.) through 

 a unit of height (one foot). In England the 

 "unit of work" is the "foot-pound." The 

 work done by the heart at each contraction 

 can be readily found by multiplying the weight 

 of the blood expelled by the ventricles by the 

 height to which the blood rises in a tube 

 tied to an artery. This height has been 

 found to be about 9 feet in the horse, and it 

 has been shown that this estimate is nearly 

 correct for a large artery in man. 



c Taking the weight of blood expelled from 

 the left ventricle at each systole as 4 ozs., 

 that is, J Ib., we have 9 x J = 2J foot-pounds 



