312 Mr. J. A. Mac William. The Mechanism and 



conditions of greatly depressed conduction power stimuli not faster than 

 rates comnaonly seen when the heart is beating in co-ordinated fashion may 

 cause fibrillation. The rate of oscillation when fibrillation is established in 

 such hearts is naturally a slow one, as the excitability is commonly reduced 

 as well as the conductivity. 



In such conditions of depressed ventricular conductivity, it is sometimes, 

 though rarely, possible to excite ventricular fibrillation by faradisation of the 

 auricles or of the sino-auricular junction in the region of the S.A. node. 

 Such a result has been quite definitely obtained in a very few cases. The 

 A-V. conducting mechanism was apparently able to transmit a series of 

 impulses to the ventricles sufficient to excite in the latter the relatively low 

 degree of acceleration necessary, in presence of their lowered conductivity, to 

 establish the circulating mechanism. 



Bates of Oscillation in Fibrillation. 



As has been stated, the rates of oscillation are usually high when fibrillation 

 is induced, and they remain high for some time ; if massage is employed, 

 quick oscillation may be maintained for an hour or more. But when, in the 

 absence of massage, etc., the excitability of the muscle becomes lowered, as 

 happens even with massage after a variable time under the usual experi- 

 mental conditions — the rate of oscillation falls markedly, the less excitable 

 muscle being unable to give such rapid responses to the circulating excita- 

 tions. And in conditions where the excitability is depressed when fibrillation 

 is induced, the rate of oscillation is, from the beginning, very much slower 

 than usual; such rates as about 280, 250, 240, 140, etc., being seen, i.e. 

 rates sometimes below the rhythm of a normally-beating heart when acting 

 rapidly. It must be noted that the graphic records of the oscillations have 

 to be interpreted with caution. For the oscillations caused by contraction 

 waves coursing along the interlaced fasciculi are very complex and irregular 

 and do not denote the succession of contractions in any one fasciculus. 

 Still, the rates observed are, within certain limits, quite definite and 

 significant, though on account of the irregularity precise figures may not he 

 obtainable. Such records must be controlled by the methods of inspection 

 and palpation, and, as a rule, yield results that are in accordance with the 

 evidence afforded by the latter methods. 



Injlmnce of Duration of Stimulation. 



When electrical stimulation, e.g., faradisation, is used to excite fibrillation, 

 its efficiency shows a marked relation to the duration of its application, as 

 well as to the strength of the current ; a longer application, e.g., 10 seconds. 



