CHAP, iv.] THE VASCULAR MECHANISM. 299 



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 movements 

 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 stimula- 

 tion 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 frequently 

 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 endur- 

 ance 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 result is different. The 

 sinus venosus beats forcibly and regularly, having suffered hardly 

 any interruption from the operation. The excised heart, however, 

 remains, in the majority of cases, for some time motionless. 

 Stimulated by a prick or an induction shock, it will give perhaps 



