Control of Fibrillation in the Mammalian Heart. 313 



may elicit persistent fibrillation when a shorter one, e.g., 3 seconds, only 

 causes a rapid tachycardia or pseudo-fibrillation. The greater effect of the 

 more prolonged application may be ascribed to at least two factors : — 



1. The time needed for the current to produce its full effect in the way of 

 acceleration of the succession of contractions. With suitable strengths of 

 current, the tracings clearly show an increasing acceleration for some little 

 time after the beginning of the application, the excursions become more 

 rapid and smaller until, when the circulating mechanism is established, 

 fibrillation supervenes with its very irregular oscillations. With strong 

 currents the characters of fibrillation may become manifest in the tracing 

 immediately or almost immediately. It is evident that, with relatively 

 weak currents, some time is needed to get up the full rate, with its 

 influence in promoting fibrillation by shortening the refractory period and 

 slowing and impairing the propagation of the excitations. 



2. A continuance for some time of the rapid succession of contractions may 

 be assumed to promote fatigue in the more vulnerable parts of the inter- 

 fascicular connections (in analogy to what is , known of fatigue of the 

 A-V. conducting mechanism) by an unduly early repetition of an impulse to 

 be conducted. Continuance of the stimulating current after the circulating 

 mechanism has been established seems to be of no importance. 



Parallelism between Auricles and Ventricles. 



There are close analogies between the behaviour of the auricular and the 

 ventricular muscular systems as regards (1) the occurrence of single con- 

 traction waves passing slowly through the muscle, constituting fibrillar beats 

 in the ventricles, and (2) the development of (a) regular tachycardias, 

 (b) irregular tachycardias, (c) pseudo-fibrillation, and (d) fibrillation, as 

 results of graduated artificial stimulation. 



The persistence or non-persistence of fibrillar movements is clearly 

 explicable on the same principles in both auricles and ventricles — by the 

 altered relation between conduction and refractory period — and the mode of 

 conduction in fibrillation is, as in the ventricles, a fascicular one, depending 

 on the presence of more or less extensive blocking in the inter-fascicular 

 connections. Slow coarse fibrillation may be seen in the auricles as in the 

 ventricles, and separate waves of contraction sweeping over the auricles in 

 irregular fashion, more or less resembling what have been described as 

 fibrillar beats in the ventricles, are often very striking in conditions of 

 depressed conductivity ; the progress of the greatly slowed wave can be 

 followed by the eye with the greatest ease. And, with some increase of 

 excitability, the wave of excitation may excite another, just as in the 



