THE PHYSIOLOGY OF THE HEARTBEAT 175 



The experiment can be still better performed by using a wedge- 

 shaped clamp. (GaskelPs clamp.) If this is applied so that the heart 

 can be pinched either at the sinoauricular junction or at the auriculo- 

 ventricular, it will be found that, as the cardiac tissue is gradually 



Fig. 41. Heart-block produced by applying clamp at a-v junction. The clamp was tightened 



at a. (From Brubaker.) 



pinched, the portion of the heart below fails to beat as quickly as that 

 above the clamp (Fig. 41). This is known as partial heart-block, and 

 the degree of the block is indicated by the numerical expression 2 to 1, 

 3 to 1, 4 to 1, etc., meaning that the sinus is beating either twice as 

 quickly as the ventricle, or three times, or four times as the case may 



Fig. 42. Tracing of contraction of ventricle, showing the effect of the local application 

 of heat to the auricle at /, and to the apex of the ventricle at 2. Note that the rate in- 

 creased in the former case. 



be. Similar conditions of heart-block may also be produced by cutting 

 the cardiac tissue partly across at various places in the heart. 



Further evidence that the sinus dominates the beat in the heart of 



