50 THE HEART 



of the sinus. Stannius had observed this, but had paid no 

 attention to it. Gaskell further found that if the second 

 hgature were replaced by slow compression of the auriculo- 

 ventricular junction the ventricle first stopped beating, but 

 afterwards began to beat with a rhythm slower again than 

 that of the auricle. Gaskell was therefore led to the view 

 that the origin of the beat was to be found in the inherent 

 property of rhythmicity possessed by the heart-muscle. 

 In other words, he founded the Myogenic Theory of the 

 Heart-beat. 



In confirmation of this view came the later observations 

 that the heart of the developing chick begins to beat before 

 any nerves have migrated into it, and that the separated 

 apex of the frog's ventricle, demonstrably free from nerves, 

 continues to beat if properly nourished. 



Rhythmic contraction then, being a property of heart- 

 muscle, what is the cause of the conduction of the beat 

 from the sinus to the ventricle ? Gaskell proved that the 

 conduction of the beat was muscular by two experiments 

 performed on the heart of the tortoise. In this animal sinus 

 and ventricle are connected together by a band of auricular 

 tissue. When a series of interdigitating cuts is made into 

 the band the conduction of the beat is unaffected. This 

 would not be the case if the conduction were due to nerves. 

 Again, if this band is little by little cut almost completely 

 across (Fig. 7), a stage is reached when the part of the 

 auricle distal to the cut responds only to every alternate 

 beat of the proximal part. On cutting further, it responds 

 only to every third beat, and so on until eventually the 

 bridge of tissue becomes so much narrowed that no wave 

 can pass along it. The distal part then develops a rhythm 

 of its own. Clearly, therefore, conduction is dependent 

 upon and due to the integrity of the muscle itself. 



Both the origin and the conduction of the beat being 

 myogenic, why does the beat travel from sinus to ventricle 

 and not in any other direction ? Gaskell showed that this 

 was due to a greater rhythmicity possessed by the sinus. 



