348 THE HUMAN BODY 



terfered with. In cold-blooded animals such as frogs or turtles 

 this activity outside the Body may continue for hours. While we 

 refer to this activity as automatic we do not mean by the word 

 anything more than the fact just stated, that the heart continues 

 to beat independently of the rest of the Body. The rhythmic na- 

 ture of the heart's activity is as characteristic as its automaticity. 

 The regular succession of contractions and relaxations is its normal 

 response to continuous or rapidly recurring stimulation. In this 

 respect it differs strikingly from skeletal muscle, which remains 

 strongly contracted throughout the period of such stimulation un- 

 less fatigue sets in to release it. 



Another peculiarity of heart-muscle, and one that probably ex- 

 plains in part its rhythmic property, is that its contractions are 

 always maximal. By this is meant that whenever heart-muscle 

 contracts it always does so to the full extent of its ability at the 

 time. In this respect we may compare its energy liberation with 

 the discharge of a gun. When the trigger is pulled all the powder 

 in the cartridge is exploded; similarly whenever the heart contracts 

 it uses up all the energy available at the time. Because of this 

 it is necessary that the contraction be followed by a relaxation 

 during which an accumulation of energy may prepare for the next 

 contraction. 



The evidence that all the available energy of the heart-muscle is 

 used up at each systole is furnished by the existence of the refrac- 

 tory period. During this period, which coincides with the systole, 

 external stimulation of the heart-muscle is altogether ineffective, 

 although during diastole the heart responds to adequate stimu- 

 lation by contraction. It is observed, also, that the irritability of 

 the heart increases steadily from the end of the refractory period 

 to the beginning of the next systole. We may assume, then, that 

 during diastole there is a gradual replacement of the energy supply 

 used up during the preceding systole, and that the more energy has 

 accumulated the more irritable is the tissue. 



The Passage of the Beat over the Heart. In the first paragraph 

 of the chapter it was stated that the beat of the heart takes a 

 certain course, beginning at the mouths of the great veins, spread- 

 ing thence over the auricles, and passing from them to the ven- 

 tricles. In all vertebrates there is a distinct pause between the 

 contraction of the auricles and of the ventricles. In animals, such 



