Control of Fibrillation in the Mammalian Heart. 307 



wriggling feeling, practically indistinguishable from that of true fibrillation, 

 becomes very marked, instead of the solid push normally given to the 

 palpating finger. These phenomena are obviously due to the rapid series of 

 short contraction waves traversing, at relatively slowed rates, the various 

 layers, bands or fasciculi of the ventricular musculature according to the lower 

 or higher grades of inter-fascicular blocking and dissociation that are present, 

 thus giving asynchronous contractions at different parts of the thickness of 

 the muscular walls. These changes in their various grades are attended by 

 related degrees of lowering of the arterial pressure, and by auricular accelera- 

 tion and irregularity. At high rates the force and range of the contractions 

 become small, the output from the ventricles is cut down and a great fall of 

 arterial pressure results. 



When the rapidly stimulated ventricles have been brought into the condition 

 above described — presenting many features of resemblance to true fibrillation 

 but not identical in mechanism as will be explained later — diminishing rates 

 of excitation are attended by graded changes of converse order — slower 

 succession of contractions, less dissociation, quicker conduction, apparent 

 coarsening of the oscillations and a gradual return, as the rate falls, to the 

 characters of normal beats. 



Pseudo-fibrillation and Fibrillation. 



The above-described condition into which the ventricles may be brought by 

 rapidity of excitation (graduated series of shocks or faradic currents of suitable 

 strength) short of the rate necessary to induce true fibrillation, may for 

 convenience be termed pseudo-fibrillation (figs. 5 and 6). As regards the 

 evidence afforded by inspection, palpation, tracings of the oscillations, fall of 

 blood-pressure, etc., the two conditions may be difficult or impossible of 

 distinction, but they differ strikingly as regards persistence ; pseudo-fibrilla- 

 tion ceases immediately or at varying short periods after the cessation of the 

 stimulation, while true fibrillation in ordinary circumstances, in the absence of 

 remedial measures, goes on as a rule to the death of the heart. (The duration of 

 pseudo- fibrillation after cessation of the stimulation varies according to the 

 excitability of the stimulated area, the strength and duration of the stimulating 

 current, etc.) The difierence depends on the fact that in true fibrillation a 

 mechanism of circulating excitation has been established, whereas in pseudo- 

 fibrillation this is not so. The latter condition depends on the emanation of 

 an excessively rapid series of excitation waves from the area of stimulation ; 

 these short waves travelling at reduced speed over the interlaced fasciculi give 

 rise to the condition described. But as soon as the issue of excitations from the 

 stimulated area ceases, the disturbance ceases and the conditions revert to the 



