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



ventricles, and so set up a continuous slow series — slow here also because of 

 obviously depressed excitability, as shown by diminished readiness to respond 

 to stimuli of definite strengths. 



Pseudo-Fih'illation and, Fibrillation in the Auricles. 



Under gradually increasing electrical stimulation, the auricles, like the 

 ventricles, show higher and higher grades of disturbance : (1) extra-systoles, 

 (2) regular tachycardia, (3) irregular tachycardia, (4) pseudo-fibrillation, and, 

 at least in certain conditions of the auricular muscle, (5) fibrillation. The 

 gradually increasing rate of auricular response rises through the grades of 

 tachycardia or flutter, with diminishing range of lever excursions, up to a 

 condition of rapid tremulous movement (pseudo-fibrillation), with irregular 

 succession and range of oscillations more or less closely approximating to 

 the characters of true fibrillation and often hard to distinguish with 

 certainty from the latter, either by inspection of the auricles or in the 

 tracings, though in pseudo-fibrillation the oscillations are commonly larger 

 and of a less high grade of irregularity than in fibrillation. The movement 

 may last for variable periods after the stimulation has been discontinued. 



A ready method of discriminating between the two conditions is afforded 

 by the experiment of isolating the stimulated area (by clamping, etc.). 

 Tachycardia or pseudo-fibrillation is at once arrested, while true fibrillation is 

 not affected. 



In the majority of the animals examined special conditions are necessary 

 in the auricular muscle for the production of true fibrillation with its essential 

 mechanism by faradisation, etc., the stimulation per se is not, as a rule, 

 sufficient in the easier conditions of quick conduction normally present in the 

 auricles. Contractions in very rapid sequence, e.ff., 500-600 or more per 

 minute, may be excited without establishing the mechanism of persistent 

 fibrillation. Certain conditions involving an alteration of conductivity 

 without a great lowering of excitability, are often effective in determining the 

 occurrence of fibrillation, e.g., vagus influence, defective blood supply, certain 

 phases in the action of some drugs, such as chloroform, paraldehyde, 

 pilocarpine, etc. 



" Spontaneous " fibrillation, i.e. when the precise exciting cause cannot be 

 defined, depends no doubt on the presence of irritation plus an altered state 

 of conductivity. The latter is sometimes supplied, under experimental 

 conditions, by the tonic influence of the vagus centre exercised through either 

 the right or the left vagus, as can be seen when only one nerve is intact ; 

 section of the nerve in such cases is speedily followed by recovery from 

 fibrillation which may have persisted during the whole preceding part of the 



