110 THE WONDERFUL HOUSE THAT JACK HAS 



and that of females eighty times, a minute. The 

 number may be greatly increased by either mental or 

 physical exercise, and is greater in youth and less in 

 old age. The beating of the heart is often spoken of 

 as a pulsation, and the arteries are so elastic that they 

 contract and relax as the blood flows through them. 

 This movement of the walls of the arteries helps to 

 hasten the flow of blood through them. By putting a 

 finger on an artery the heart pulsation, or pulse, as it is 

 usually called, can be felt and counted. The inside of 

 the wrist below the base of the thumb is the place where 

 the pulse is most readily felt. As the muscles are 

 involuntary (that is, work day and night without our 

 willing it), we can see that this wonderful little pump 

 of ours makes about four thousand five hundred con- 

 tractions an hour and over one hundred thousand a 

 day, to keep the blood traveling on its ceaseless round 

 through the body. The statement that the heart never 

 rests is sometimes made, but this is not true. There 

 is a short pause for rest between the beats. 



But why cannot the heart have long periods of 

 complete rest just like other muscles of the body? 

 This question brings us to the second chapter of how 

 the air assists in body building. We know how air 

 gets into the numerous cells of our lungs. We have 

 also seen how r the dark blood is forced from the right 

 ventricle of the heart through the pulmonary artery 

 and into the minute capillaries of the lungs. By 

 examining a drop of blood with a microscope, we can 



