334 The Outline of Science 



not yet know what regulates its "beat." There seems to be some 

 mechanism in the heart itself for regulating it. Seventy-two 

 times in every minute, in a healthy and resting man, the chambers 

 draw their walls together and pump out the blood. There are 

 tens of thousands of muscular fibres built so wonderfully into 

 the walls that the chambers can close in from every side, like a 

 man closing his fist, and give the blood a start that will carry it 

 all round the body and back to the starting-point. 



It is, of course, a mistake to say that the heart never rests. 

 It rests, and recovers, between each beat. But its function is 

 remarkable. As we said, it beats seventy-two to the minute 

 when a man is resting. But let there be some sudden call for 

 action, and almost before you get from your chair, the great 

 pump beats faster, as if it knew that the distant muscles and 

 brain had now work to do and must have more blood. When 

 we are sitting still, it throws five pints of blood (a little more 

 than a third of all the blood in the body) into the arteries every 

 minute. During a quick walk the heart pumps seventeen pints 

 a minute; and the man who runs upstairs is asking his heart to 

 pump thirty-seven pints a minute! During even less violent 

 exercise than this, all the blood in the body (about fourteen pints) 

 passes twice through the left ventricle of the heart and all round 

 the body in a single minute. 



From the left ventricle, the chief pump, the blood passes 

 into a thick broad tube called the aorta. The elastic walls of 

 this tube expand as the blood rushes in, and then slowly close 

 again, driving the blood onward. In this way, and by the general 

 resistance of the tubes (the arteries), the jerky discharge from 

 the heart is converted into a steady flow after a time. The 

 arteries branch out in every direction, and as they approach the 

 tissues they have to feed they break into myriads of very fine 

 tubelets, often not more than 1-3000 of an inch in thickness. The 

 wall of the blood-vessel has now to be so thin that the nourishing 

 matter in the blood can flow through it to the tissues, and the 



