THE CAUSATION OF THE HEART-BEAT 1061 



inhibitory fibres of the vagus. Since tetanisation of the heart with weak 

 currents also causes local inhibition, it would seem that the finer nerve 

 fibres ramifying throughout the muscular substance are, to a large 

 extent at all events, inhibitory in their function. This is confirmed by 

 the fact that atropine, which paralyses the inhibitory fibres of the vagus, 

 also abolishes the direct inhibitory effect of tetanisation on the heait 

 muscle. Gaskell and Engelmann therefore came to the conclusion that 

 the source of the cardiac rhythm was to be found, not in the ganglia 

 scattered about its cavities, but in the muscular cells themselves. 



The normal sequence of events i.e. the subordination of the 

 ventricle to auricles and auri- 

 cles to sinus, so that the beat 

 always follows in the order, 

 sinus, auricles, ventricle, bul- 

 bus can be ascribed to the 

 variation in the natural 

 rhythm of these different 

 cavities. It is possible to 

 record the contractionsof each 

 of these parts of the heart 

 separately, after having 

 divided them, either function- 

 ally by crushing the interven- 

 ing tissue, or by actual section. 

 Under such conditions it is 

 found that there is a descend- 

 ing scale of rhythm from sinus 

 to bulbus, the contractions of 



the sinus being most frequent, those of the ventricle and bulbus the 

 least frequent. Thus it is impossible for the ventricle to beat at its 

 own rhythm, since before it is ready to beat again after performing 

 one beat it receives an impulse from the auricles causing an excited 

 beat. That the normal sequence of contractions is dependent simply 

 on the natural rhythm of the sinus is shown by the fact that by 

 exciting the ventricle by means of induction shocks repeated at a 

 rhythm slightly greater than that of the sinus it is possible to excite 

 a reversed rhythm, the order of the beat being now ventricle, auricles, 

 sinus venosus. 



The dependence of the ventricular rhythm on the beat of the sinus 

 may be shown by a simple experiment. The ventricle is connected 

 with a lever suspended by a spring so as to record its contractions on 

 a drum. A platinum loop connected with a galvanic battery is put 

 round the heart, either round the sinus or round the ventricle (Fig. 426). 

 When a current is allowed to pass through the inner loop the 



Sm. 



FIG. 426. 



