THE HEART-BEAT IN ITS PHYSIOLOGICAL RELATIONS 145 



heart have been thrown by the first section, or more probably to 

 the loss of the accustomed impulses from the sinus which normally 

 give the signal for the auricular contraction. A stronger argument 

 in favour of the myogenic theory is the fact that the embryonic 

 heart beats with a regular rhythm at a time when as yet no ganglion- 

 cells have settled in its walls. But it may well be that this primitive 

 automatic power of the cardiac muscle, absolutely necessary at first, f 

 since the early establishment of the circulation is essential for the 

 development of the tissues in general and of the nervous system in 

 particular, falls into abeyance when the intrinsic cardiac nervous 

 mechanism is completed, or at least becomes subordinated to the 

 latter. The advocates of the myogenic theory further state that 

 the isolated bulbus aortae of the frog, and even tiny fragments of it, 

 will pulsate spontaneously, and that the same is true of small 

 portions of the great veins which open into the sinus. The rhyth- 

 mical contraction of the veins of the bat's wing has also been con- 

 sidered an argument in favour of myogenic automatism. In none 

 of these cases, however, can the complete absence of ganglion-cells 

 be considered satisfactorily demonstrated. The statement that a 

 portion of the apex of the dog's ventricle continues for a considerable 

 time to beat with a rhythm of its own when connected with the rest 

 of the heart by nothing but its bloodvessels and the narrow isthmus 

 of visceral pericardium and connective tissue in which they lie has 

 not been confirmed by all observers. But even if it be accepted, it 

 can hardly be used as a decisive argument against the neurogenic 

 theory so long as the absence of ganglion-cells from such a ventricular 

 strip has not been demonstrated. 



The fact that under the influence of a constant stimulus portions 

 of the heart can be made to beat rhythmically has been sometimes, 

 though erroneously, brought forward as evidence of myogenic 

 automatism. Thus the supposedly ganglion-free apex of the frog's 

 heart, lifeless as it seems when left to itself, can be caused to execute 

 a long and faultless series of pulsations when its cavity is distended 

 with defibrinated blood or serum, or certain artificial nutritive 

 fluids, or even physiological salt solution. The passage of a constant 

 current through the preparation may also start a regular rhythm. 

 But apart from the question whether nervous elements would not 

 be subjected to the constant stimulus impartially with the muscular 

 elements (and nerve-fibres, at any rate, are acknowledged to be 

 present), the beat here produced ought not to be considered as an 

 automatic beat, but as a contraction evoked by an external stimulus. 

 Such experiments, in fact, throw no light upon the automatism of 

 the heart, but prove clearly its rhythmicity i.e., its power of 

 responding to a continuous stimulus by regularly recurring con- 

 tractions. While we are hardly at present in a position to dis- 

 criminate sharply between the influence of constant stimulation 



