THE ACTION OF THE HEART 313 



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 

 as the frog and turtle that have a pulsating sinus, there is likewise 

 a pause between the contraction of the sinus and of the auricles. 



If in a beating heart a cut be made between the sinus and the 

 auricles so that they are completely separated, the sinus con- 

 tinues to beat exactly as before; the other chambers of the heart 

 may not beat for a moment, but after a short interval usually 

 resume activity. The rate of beat of these chambers under such 

 circumstances is slower than that of the sinus. Similarly the ven- 

 tricles may be separated from the auricles without affecting the 

 auricular beat, but with the result that the ventricles either fail to 

 beat at all, or beat at a much slower rate than the auricles. Such 

 experiments as these show that the rhythmic power increases the 

 nearer we go toward the venous end of the heart, and also that in 

 the normal heart the most rhythmic portion imposes its rate on 

 the rest of the organ. In order for the heart-rate to be determined 

 as a whole by the beat of the venous end it is evident that there 

 must be a conduction of the impulse to activity from one chamber 

 to the next throughout the heart. This conduction moves over 

 the heart in the form of a wave. 



There are in the frog's heart two places and in that of the mam- 

 mal one place where there is a delay in the passage of the con- 

 traction wave. These are, as already noted, at the junction of the 



