Fibrillation in the Mammalian Heart. 15 



vigour as in the former experiment, after a time giving place to normal 

 action. What has happened in this case is that the electrical stimulation, 

 falling within the period of vagus influence, is effective in setting up the 

 mechanism of circular excitation, while the latter finds no expression in con- 

 tractile movement on account of the mechanical response to excitation being 

 kept in abeyance by the vagus inhibitory power. "When the latter wanes 

 and the mechanical response again becomes manifest, the circulating excita- 

 tationS are attended by the circulating contractions of visible fibrillation. 



When the electrical stimulation is applied in the foregoing way without 

 apparent effect on the inhibited auricles, the subsequent appearance and 

 development of fibrillation as described above is not affected by the stimulated 

 area (e.g., auricular appendix) being isolated from the rest of the auricle 

 shortly after the brief application of the stimulating current and while the 

 auricles are still kept in complete quiescence by the vagus ; the subsequent 

 fibrillation involves the whole of the auricular muscle, apart from the isolated 

 area. It is plain that the mechanism of excitation necessary for fibrillation 

 has been established in the mass of the auricular muscle, and that it is 

 independent of a continued emission . of impulses from the stimulated area — 

 now isolated. In these experiments the isolation was effected {a) by 

 clamping off or (6) by section, after a weak clip or a ligature not too tightly 

 drawn had been applied along the base of the appendix to prevent haemor- 

 rhage. In some cases rapid cooling of the stimulated area was employed 

 instead of isolation. Control experiments were made to determine that the 

 methods used do not themselves cause fibrillation in the conditions present, 

 under vagus influence, etc.* The vagus evidently can act more strongly on 

 auricular contraction force, if not also on conductivity, than on excitability, 

 for the latter property must remain functional (though depressed) in auricles 

 that respond by subsequently manifested fibrillation movements to an 

 electrical stimulus applied during the period of mechanical quiescence of the 

 muscle. 



As ia rule, as stated above, the auricular muscle is not sufficiently depressed 

 by vagus influence to prevent excitation occurring in response to adequate 

 stimulation, or to stop the circulation of excitations once this mechanism has 

 been established, though the normally-associated mechanical response may 

 be cut down to the point of invisibility. But in some instances the vagus 

 seems to be able to act so strongly on excitability that after electrical stimu- 

 lation during the vagus period, fibrillation does not gradually appear in the 

 usual way as the vagus control is passing off, but visible action recommences 



* Under certain conditions it is clear that mechanical stimulation may sometimes 

 excite auricular fibrillation. 



( 316 ) 



