534 THE OUTGO OF ENERGY 



into the ventricle during the period in which it 

 is refractory towards stimuli ; (3) the extra-con- 

 traction is followed by a pause, called the com- 

 pensatory pause because it usually restores the 

 rate of beat to that existing before the extra- 

 contraction took place. 



Using induction currents of equal intensity, 

 find the limits of the refractory period and note 

 them on the drum. Note also the point in the 

 cardiac cycle at which the maximum extra- 

 contraction can be obtained. 



The Transmission of the Contraction Wave in the 

 Ventricle ; Engelmann's Incisions. The action 

 current of the heart is taken to be an expression 

 of the ; excitation process, although the nature of 

 the latter is not yet understood. It has already 

 ben shown (page 310) that the action current 

 sweeps rapidly over the ventricle preceding the 

 contraction. The excitation might be propagated 

 by nerves or by muscle fibres. The following 

 experiment affords some evidence that the 

 transmission is by means of muscular tissue. 



'Leaving the heart in situ, cut the ventricle 

 into a zigzag strip by obliquely transverse in- 

 cisions beginning near the apex. The nerve 

 fibres in the ventricle will thereby be severed 

 at some part or other of their course, but muscular 

 continuity will be preserved. 



