CHAPTER XXII 

 THE PHYSIOLOGY OF THE HEARTBEAT (Cont'd) 



THE ORIGIN AND PROPAGATION OF THE BEAT (Cont'd)- 



FIBRILLATION 



Mode of Propagation in the Auricles 



From the mass of evidence we have little doubt that the heartbeat 

 originates in the sinoauricular node, and the question now presents itself 

 as to how the beat is propagated over the remainder of the auricles and 

 into the ventricles. Regarding the propagation of the beat over the 

 auricles, two possibilities exist: (1) it may spread uniformly over the 

 muscular tissue of the auricular wall until it reaches the auriculoventric- 

 ular node, or (2) there may be laid down between the sinoauricular and 

 the auriculoventricular node a special strand of highly conducting tissue. 

 It is no argument against this second possibility that we should so far- 

 have been unable by histological methods to differentiate any such struc- 

 tures. 



There is considerable practical importance attached to the solution of 

 these questions, particularly with regard to the cause of certain types 

 of cardiac arrhythmia, such, for example, as that known as nodal rhythm. 

 Thus, it is evident that if the beat is transmitted uniformly over the 

 muscular tissue of the auricle, then the whole auricle will have con- 

 tracted before the beat has reached the auriculoventricular bundle, by 

 which it is then transmitted to the ventricles. On the other hand, if the 

 beat should travel between the two nodes by special conducting tissue, 

 then the impulse will have arrived at the auriculoventricular node be- 

 fore the auricle has contracted. As a matter of fact, it is not quite settled 

 yet as to which of these two views is the correct one, although the balance 

 of evidence seems to favor the former that is, that the wave is transmitted 

 uniformly over the muscular tissue of the auricle. (Lewis.) 



The methods employed in attacking the problem have been essentially 

 the same as those described above. One of them may be called the direct, 

 the other the indirect. In the former, a series of pairs of contacts is 

 placed on the auricle, each pair being in a radial direction to the sino- 

 auricular node. The time at which the excitatory process arrives at that 

 contact of each pair which is proximal to the sinoauricular node is accu- 



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