VEINS AND HEART. 



of the wall of the blood-vessels here receives 

 equally enormous development, so that the 

 heart may be regarded as little more or less 

 than a very considerable muscle of special form 

 and functions. 



To force the blood through the arteries, 

 arterioles, and tissues, it has to be raised to a 

 pressure equal to that produced by a column 

 of water nine feet high on the left side of the 

 heart, and three feet on the right, which sends 

 it through the lungs. And it is against this 

 pressure that the heart has to pump it. The 

 quantity expelled by each side of a man's 

 heart at each stroke is about six ounces, and 

 there are about seventy strokes a minute. 

 Thus the work done by a strong heart in twenty- 

 four hours is about equivalent to that of lift- 

 ing a weight of two tons 100 feet high. A 

 horse-power is commonly reckoned as the work 

 equivalent to lifting 400 tons the same height 

 in the same time ; so that a strong human 

 heart works up to about the two-hundredth 

 part of a horse-power. Horse-power in such 

 measurements means the total draught-power 

 of a strong horse ; and the phrase implies no 

 comparison here between the human and equine 

 heart. 



The work to be done, then, being so consider- 

 able, and no piston being available to force out 

 the blood from the heart even if it were itself of 

 the shape of a cylinder, the heart has to empty 



293 



