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



dissociation varies with the rate of succession — the faster the rate the 

 higher the grade of dissociation. In some cases the depression of conduction 

 may be of such a degree that a beat coming after a long interval may show 

 no distinct sign of dissociation by inspection or palpation, whereas, when a 

 quick series occurs, each beat is markedly dissociated, giving the charac- 

 teristic " wiry " feeling on palpation (fig. 4).' When the excitability of such a 











EflBl 









FiQ. 4. — The quick series of beats are fibrillar in character. The larger beats coming 

 after long intervals do not show evidence (on palpation) of that character. 



heart gradually rises, e.g., under the influence of massage, improved nutrition, 

 certain remedial drugs, removal of depressing influences, etc., the rate of 

 continuous movement may increase, with an accompanying increase in the 

 grade of dissociation. 



There is a very definite gradation from (a) the phase of discrete fibrillar 

 beats, through (b) slow and then quicker series of successive contraction waves, 

 up to (c) the rapid and mechanically ineffective oscillations of typical fibrilla- 

 tion. The increase in rate depends on the augmented responsiveness of the 

 more excitable muscle. The degree of asynchronism or dissociation increases 

 with the rise in the rate of succession, the partial blocking between the larger 

 fasciculi or bands and layers of fibres giving the lower grade of dissociation 

 seen in slow coarse fibrillation, while the higher grades of dissociation 

 between fasciculi are present in the condition of rapid fine fibrillation. 



Similarly with diminishing excitability and conductivity, a downward 

 gradation may be observed from typical fibrillation, through grades of slower 

 and coarser fibrillation, to the phase of individual fibrillar beats. 



Change in Mode of Conduction Due to Excessive Rapidity of Excitation. 



When the rate of beat is excessively accelerated by a series of induction 

 shocks of increasing rapidity, a gradation of changes is observable as the rate 

 of succession rises. The individual contractions become briefer and gradually 

 give smaller and smaller excursions of the recording lever. Inspection shows 

 evidence of dissociation becoming very pronounced at the higher rates, so as 

 to bear a close resemblance to the familiar appearance of the ventricular 

 surface in typical fibrillation. Palpation at the same time reveals increasing- 

 degrees of asynchronism as the rate rises, until the characteristic wiry 



