Control of Fibrillation in the Mammalian Heart. 315 



experiment, or at least since the heart was exposed. Such vagus control has 

 not appeared as a common cause of auricular fibrillation in these experiments, 

 but in some instances its influence has been unmistakable. 



The simplest and most easily available method of producing true auricular 

 fibrillation for a time is by a combination of electrical stimulation and vagus 

 stimulation. Eapid tachycardia set up by electrical stimulation is converted 

 by vagus influence into true fibrillation which persists as long as the vagus 

 influence is maintained in sufficient strength to provide the condition in the 

 auricular musculature necessary for the keeping up of circulating excitation ; 

 the fibrillation so excited goes on under vagus influence long after the 

 electrical stimulation has been discontinued ; the latter may indeed have been 

 applied only for a second or two. Under vagus influence the fibrillation 

 oscillations, though very rapid, become greatly weakened, the irregular 

 movements of the recording lever becoming minute. With pretty strong 

 vagus control this weakening may go on to invisibility, so that the auricles 

 look entirely quiescent, even when their surface is scrutinised with a lens. 

 As the vagus influence wears off during prolonged stimulation of the nerve, 

 very fine fibrillation oscillations again begin to become perceptible, and these 

 gradually gain in vigour and range until after a variable time the normal 

 type of beat replaces the fibrillation movement. 



A similar sequence of events, more quickly passed through, is evident when 

 vagus stimulation is diminished or discontinued instead of the influence of 

 the nerve being allowed to wear off duriag continued stimulation. What 

 evidently occurs in these cases when the auricles become motionless under 

 vagus influence, is that the mechanism of circulating excitation goes on 

 working in spite of the inhibitory influence which cuts down the mechanical 

 response to invisibility ; there is no true inhibition of the essential mechanism 

 of fibrillation. 



The experiment may be done in another way. Instead of first exciting the 

 tachycardia and then stimulating the vagus, the latter may be brought into 

 action first so as to reduce the auricles to complete quiescence ; during this 

 period an electrical current is applied briefly {e.g., for one or two seconds) to 

 the auricle ; a fine tremulous (fibrillation) movement of small range may at once 

 appear and continue until the vagus influence wanes or is discontinued. 



Mechanism of Circulating Excitations without Contractions. 



But if the vagus is strongly inhibiting the muscle when the electrical 

 current is briefly applied, there may be no visible effect at all ; the auricles 

 remain perfectly motionless until the vagus control has become weakened, 

 when the fine tremulous movement usually appears and gradually gains in 



