THE CAUSATION OF THE HEART-BEAT 



943 



them, either functionally by crushing the intervening tissue, or by actual 

 section. Under such conditions it is found that there is a descending 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 ven- 

 tricular rhythm on the beat of the 

 sinus may be shown by a simple 

 experiment. The ventricle is con- 

 nected with a lever suspended by 

 a spring so as to record its con- : 

 tractions on a drum. A platinum 

 loop connected with' a galvanic 

 battery is put round the heart, 



B.A. 



either round the sinus or round 

 the ventricle (Fig. 424). When 

 a current is allowed to pass 

 through the inner loop the cor- 

 responding part of the heart is 

 warmed. When the ventricle 

 alone is warmed the beats .be- 

 come larger, but the rhythm is 



unaltered. On lowering the loop FIG. 424. 



so as to warm the sinus, the 



rhythm of the whole heart is quickened, but the size of the ventricular 

 beats is unaffected. The different rhythmic power of these parts of the 

 heart is apparently connected with the histological characters of the 

 muscle fibres at each part. The lowly differentiated sinus cell has well- 

 marked rhythmic power and a quick rhythm of beat, but is not able to 

 exert such force in its contraction. The more highly differentiated 

 ventricle cell has only a slight rhythmic power, but beats forcibly and 

 is a good servant of the sinus. 



THE PROPAGATION OF THE WAVE OF CONTRACTION 



The normal contraction started in the sinus venosus is propagated to the 

 auricles, thence to the ventricle, and thence to the bulbus aortse. Between 

 the contractions of each of these cavities there is a slight pause, whereas 

 the contraction spreads so rapidly over each cavity that all parts, say of the 

 auricles or ventricle, appear to contract simultaneously. It is obvious that 

 the excitatory wave might be propagated through the heart from one 



