CHAP, iv.] THE VASCULAR MECHANISM 283 



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 

 one, two or several beats, and then comes 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 

 retaining 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 vigor- 

 ously, with their proper sequence, but the ventricle, after a few 

 rapid contractions due to the incision acting as a stimulus, generally 

 remains for a long time quiescent. When stimulated however 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 consider- 

 able 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. 



If a transverse incision be carried through the ventricle at 

 about its upper third, leaving the base of the ventricle still 

 attached to the auricles, the portion of the heart left in the body 

 will go on pulsating regularly, with the ordinary sequence of 

 sinus, auricles, ventricle, but the isolated lower two -thirds of the 

 ventricle will not beat spontaneously at all however long it be 

 left. Moreover in response to a single stimulus such as an in- 



