174 THE CIRCULATION OF THE BLOOD 



prove))." But it is likely that 1 lie majority of the jury would vote 

 in favor of the myogenic hypothesis. Probably the safest viewpoint to 

 take at the present time is that the power of rhythmic contraction is 

 inherent in the cardiac muscle fibers, being most highly developed in 

 those of the venous end of the heart, and least developed in those of 

 the arterial end. Such a conclusion does not deny. to the nervous struc- 

 tures of the heart the power under certain conditions of also assuming 

 rhythmic activity. In one case at least — namely, the heart of Limulus — 

 we know that this is so. For some reason in this animal the cardiac 

 muscle fiber has lost its inherent rhythmic power, and is now dependent 

 for its activities upon rhythmic nervous discharges transmitted to it 

 from the neighboring nerve cords, a condition which is paralleled in 

 the higher animals in the innervation of the respiratory muscles. The 

 respiratory center rhythmically discharges impulses to the muscles, which 

 are quiescent in the absence of these impulses. 



The Pacemaker of the Heart and Heart-block 



In a volume of this nature, devoted primarily to the practical appli- 

 cation of physiology, the discussion of these problems may seem a little 

 out of place, but that this is not the case is seen when we consider that 

 the experiments upon which the various points of evidence depend 

 bring to light facts of the very greatest importance in the study of the 

 physiology of the heartbeat. One fact which stands out prominently 

 is that the greatest rhythmic power resides in the basal portion of the 

 heart — that is, in what corresponds, in the more primitive hearts, to the 

 sinus venosus. 



Although the muscle of the entire heart possesses rhythmic power, it 

 does not do so to an equal degree; in the sinus the rhythmic power is 

 extraordinarily developed, while in the bulbus arteriosus it is scarcely 

 recognizable. This observation suggests the possibility that the sinus 

 may dominate the heartbeat — that it may be the "pacemaker" for the 

 heart as a whole. The most natural method for demonstrating such a 

 possibility would be to observe the effect on the heartbeat of some block 

 between the sinus and the rest of the heart. Such a block can be intro- 

 duced in the heart of cold-blooded animals by local compression around 

 the various junctions. If a thread is tied around the sinoauricular 

 junction, the sinus will go on beating uninterruptedly, but the auricles 

 and ventricles — that is, the greater part of the heart below the ligatures 

 —will cease beating, sometimes entirely (Stannius' ligature). After a 

 while, however, the heart below the ligature will usually begin to beat, 

 but at a rhythm which is slower than, and independent of, that of the 

 sinus. 



