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THE VASCULAR MECHANISM. 



riosus, 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 

 beats, or when the sinus is pricked it alone beats. 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 stimulation 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 endurance 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 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 perhaps give one, two, or 

 several beats, and then come to rest. In the majority of cases, however, the 

 animal having previously been in a vigorous condition, it will after a while 

 recommence its spontaneous beating, the systole of the ventricle following 

 that of the auricles ; but the rhythm of beat will not be the same as 

 that of the sinus venosus left in the body, but will be slower, and the beats 

 will not continue to go on for so long a time as will those of a heart still re- 

 taining the sinus venosus. 



When the incision is carried through the auriculo-ventricular groove, so 

 as to leave the auricles and sinus venosus within the body, and to isolate the 

 ventricle only, the results are similar but more marked. The sinus and 

 auricles beat regularly and vigorously, with their proper sequence, but the 

 ventricle, after a few rapid contractions due to the incision acting as a stimu- 

 lus, generally remains for a long time quiescent. When stimulated, how- 

 ever, the ventricle will give one, two, or several beats, and after a while, in 

 many cases at least, will eventually set up a spontaneous pulsation with an 

 independent rhythm ; and this may last for some considerable time, but the 

 beats are not so regular and will not go on for so long a time as will those 

 of a ventricle to which the auricles are still attached. 



