242 GRAPHIC RECORD OF HEART BEAT. [BOOK i. 



beat of the bulbus arteriosus, which does not, like the mammalian 

 aorta, simply recoil by elastic reaction after distension by the 

 ventricular stroke but carries out a distinct muscular contraction 

 passing in a wave from the ventricle outwards. 



When the heart in dying ceases to beat, the several move- 

 ments cease, as a rule, in an order the inverse of the above. 

 Omitting the bulbus arteriosus, which sometimes exhibits great 

 rhythmical power, we may say that first the ventricle fails, then 

 the auricles fail, and lastly the sinus venosus fails. 



The heart after it has ceased to beat spontaneously remains 

 for some time irritable, that is capable of executing a beat, or 

 a short series of beats, when stimulated either mechanically as 

 by touching it with a blunt needle or electrically by an induction 

 shock or in other ways. The artificial beat so called forth may 

 be in its main features identical with the natural beat, all the 

 divisions of the heart taking part in the beat, and the sequence 

 of events being the same as in the natural beat. Thus when the 

 sinus is pricked the beat of the sinus may be followed by a beat 

 of the auricles and of the ventricle ; and even when the ventricle 

 is stimulated, the directly following beat of the ventricle may be 

 succeeded by a complete beat of the whole heart. Under certain 

 circumstances however the division directly stimulated is the 

 only one to beat ; when the ventricle is pricked for instance it 

 alone may beat, or when the sinus is pricked it alone may beat. 

 The results of stimulation moreover may differ according to the 

 condition of the heart and according to the particular spot to 

 which the stimulus is applied. 



With an increasing loss of irritability, the response to stimu- 

 lation ceases in the several divisions in the same order as that of 

 the failure of the natural beat ; the ventricle ceases to respond 

 first, then the auricles, and lastly the sinus venosus, which fre- 

 quently responds to stimulation long after the other divisions 

 have ceased to make any sign. 



It would appear as if the sinus venosus, auricles, and ventricle 

 formed a descending series in respect to their irritability and to 

 the power they possess of carrying on spontaneous rhythmic 

 beats, the sinus being the most potent. This is also seen in the 

 following experiments. 



In order that the frog's heart may beat after removal from the 

 body with the nearest approach in rapidity, regularity, and en- 

 durance to the normal condition, the removal must be carried out 

 so that the excised heart still retains the sinus venosus intact. 



When the incision is carried through the auricles so as to 

 leave the sinus venosus behind in the body, the sinus venosus 

 beats forcibly and regularly, having suffered hardly any inter- 

 ruption from the operation ; but the auricles and ventricle remain 

 motionless, often for a considerable time, and when they do re- 

 sume spontaneous beats these have a rhythm different from that 

 of the sinus, and are less vigorous and lasting than those of the 



