I 3 2 A MANUAL OF PHYSIOLOGY 



happens to the auricles and ventricle of the frog's heart when 

 the sinus is cut off. The answer is that, as a rule, while the sinus 

 goes on beating, the rest of the heart comes to a standstill, in 

 spite of the numerous ganglion -cells in the auricular septum and 

 the auriculo-ventricular groove. Not only so, but if the ventricle 

 be now severed from the auricles by a section carried through 

 the groove, it is the former, poor in nerve-cells though it be, 

 which will usually first begin to beat. We shall again have to 

 discuss this experiment (p. 151). It, at any rate, cannot be 

 interpreted as proving that the automaticity of the heart does not 

 depend upon the presence of ganglion -cells. For although a 

 portion of the heart rich in ganglion-cells may, under the cir- 

 cumstances mentioned, refuse for a time to beat, there is good 

 evidence that this is due either to a peculiar condition called inhibi- 

 tion into which the muscular tissue or the nerve-cells of the lower 

 portions of the 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, 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 

 rhythmical contraction of the veins of the bat's wing has also 

 been considered 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 



