THE CIRCULATION. 173 



pounds. For convenience sake let us say two foot-pounds. 

 In seventy-two beats (one minute) the amount of work 

 would be 144 foot-pounds. In an hour, 8,640 foot-pounds. 



As the right ventricle lifts the blood only one-third as 

 far, the amount of work it does is just one-third that of the 

 left ventricle. Adding this one- third to the amount of work 

 the left ventricle does in an hour makes 11,520 foot-pounds. 

 Re-stating that again, it means that in one hour the heart 

 has expended an amount of energy that would have lifted 

 11,520 pounds one foot high. 



In the case of a man weighing 160 pounds, it would lift 

 him in one hour to a distance of 72 feet, or in one day 1,728 

 feet. Even if the strong muscles of the skeleton should be 

 asked daily to carry the body up a mountain side to a dis- 

 tance of 1,728 feet, it would by no means prove an easy 

 task. Yet such is the amount of work the heart does day 

 in, day out, without showing signs of fatigue. To state 

 this in still another way if the heart weighing 300 grams 

 should expend all its energy in lifting itself only, it would 

 raise itself in one hour to a distance of 13,000 feet. In 

 ability to do work, the heart is equal to -g-J-^- of a horse- 

 power. 



THE INNERVATION OF THE HEART. 



1. The Source of the Stimuli. So far the question 

 has been ignored, what causes the heart to contract? Does 

 it beat in obedience to stimuli which reach it from the out- 

 side, like the right arm in obedience to impulses from the 

 brain, or does it beat by stimuli that originate within itself ? 

 This general question can be easily answered definitely by 

 observing that a frog's heart cut out of the body will con- 

 tinue under proper circumstances to beat for days, and the 

 heart of a warm-blooded animal will beat as long as it is 

 properly supplied with nourishment and oxygen. This 

 settles conclusively that the impulses to beat do not reach 

 the heart from the outside, but are intrinsic. The question 

 then arises whether these intrinsic stimuli are produced by 

 nerve centers within the heart or by the heart muscle itself. 



