476 



auricles of a frog's or tortoise's heart, the rhythmic action imme- 

 diately, or after a few beats, ceases, and is suspended for three or 

 many more minutes, and then returns in the ventricle alone (I be- 

 lieve) at a much slower rate than it had before. 



This result is not due to the stoppage of the flow of blood into 

 the heart, for if the veins be tied separately at a distance from the 

 auricles, the rhythm continues. Neither is it due to the loss or 

 great diminution of mere motility ; for if the heart thus brought to 

 rest be stimulated, it acts once, or rarely more than once, its action 

 travelling from the part stimulated over all its substance. The result 

 seems due to an injury done to something beneath or behind the 

 ligature, from or through which some influence would naturally be 

 conducted from the conflux of the veins to the auricles, and thence 

 to the ventricle ; for in the other direction, i. e. in the trunks of the 

 veins behind the ligature, even to a great distance from the auricle, 

 rhythmic motion still continues, and is scarcely, if at all, changed. 

 And by variously placing the ligature, it would be found that, where- 

 ever tied between the venous sinus and the auriculo-ventricular 

 ring, all the parts behind it would retain, and all those before it 

 would lose (at least, for a time), their rhythm. 



This indication of the experiment (that the loss of rhythm, namely, 

 depends on injury of something beneath or behind the ligature) is 

 confirmed by another, whose whole import, however, I am quite 

 unable to explain. 



If, when a heart is thus brought to rest by a ligature tied round 

 the conflux of the veins, another ligature be tied round the boundary 

 ring of the auricles and ventricle, and including the bulbus arteriosus 

 of the frog, or the two aortse of the tortoise, the rhythmic action 

 remains suspended in the auricles, but at once begins again in the 

 ventricle, or, if it have not wholly ceased there, is much accelerated. 

 We thus obtain a condition of the heart in which the great veins and 

 the ventricle act rhythmically, though not with the same rhythm, 

 but the auricles between them are at rest. 



Now, in all these facts there appears sufficient evidence that the 

 source of the rhythmic action of the heart is not in the muscular 

 structure alone, or in its relation with the blood in the cavities, or in 

 the vessels of the heart ; for these things are not in any of the expe- 

 riments more disturbed in one part of the heart than in another. 



