182 NERVOUS MECHANISM OF THE BEAT. [BOOK i. 



heart in its normal condition within the living body. Hence we 

 may infer that the beat of the heart is an automatic action : the 

 muscular contractions which constitute the beat are caused by 

 impulses which arise spontaneously in the heart itself. 



The beat goes on even after the cavities have been cleared of 

 blood, and indeed when they are almost empty of all fluid. A beat 

 cannot therefore be, as was once thought, a reflex act excited by 

 the entrance of blood into the cavities of the heart. 



In the frog's heart, as in that of the mammal, there is a distinct 

 sequence of events. First comes the beat of the sinus venosus, 

 preceded by a more or less peristaltic contraction of the large veins 

 leading into it, next follows the sharp beat of the two auricles 

 together, then comes the longer beat of the ventricle, and lastly 

 the beat of the bulbus arteriosus completes the cycle. If the 

 incisions, by which the heart is removed, be made carefully, so as 

 not to injure at all the sinus venosus, the beats will continue after 

 a very short pause, or sometimes without any real interruption, 

 with great vigour for a very considerable time. 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 as to leave 

 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 one, two 

 or several beats, and then come to rest. But it will in the majority 

 of cases, the animal having previously been in a vigorous condi- 

 tion, recommence after a while its spontaneous beating, the systole 

 of the ventricle following that of the auricles ; but the rhythm of 

 beat will not necessarily be the same as that of the sinus venosus 

 left in the body, 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 generally re- 

 mains 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. 



