78 SCIENCE PRIMERS, [§ V. 



I lis a thin bag with a network of muscles wrapped 

 i I round it, and each time the muscles contract they 

 i ) squeeze the bag and try to drive out whatever is in it. 

 j jThere are more muscles in the ventricles than in the 

 liauricles, and more in the left ventricle than In the 



I bright, for we have already seen how much thicker the 

 Iventricles are than the auricles, and the left ventricle 

 than the right ; and the thickness is all muscle. 



] And now comes the wonderful fact. These muscles 



lof the auricles and ventricles are always at work con- 



ttracting and relaxing, shortening and lengthening, of 



! i ttheir own accord, as long as the heart is alive. The 



I I Ibiceps in your ajm contracts only when you make it 

 ^contract. If you keep quiet, your arm keeps quiet 

 jand your biceps keeps quiet. But your heart never 

 3keeps quiet. Whether you are awake or whether you 

 <are asleep, whether you are running about or lying 

 <down quite still, whatever you are doing or not doing, 



; las long as you are alive your heart keeps on steadily 

 lat work. Every second, or rather oftener, there comes 



' la short sharp squeeze from the auricles, from both 

 ^exactly at the same time, and just as the auricles 



jlhave finished their squeeze, there comes a great hug 



I / Ifrom the ventricles, from both at the same time, but a 



i ■ 



I <much stronger hug from the left than from the right ; 



I ; and then for a brief space there is perfect quiet. But 



^before the second has quite passed away, the auricles 



I ! ■ thave begun again, and after them the ventricles once 



(more, and thus the contracting and relaxing of the 



Iwalls of the heart's cavities, this beat of the heart as 



lit is called, this short snap of the two auricles, this 



longer, steadier pull of the two ventricles, have gone on 



nn your own body since before you were born, and will 



X 



