Control of Fibrillation in the Mammalian Heart. 305 



both prolonged ; the systolic power is relatively small. The individual beats 

 are quite discrete ; there is a very definite interval, varying in duration, of 



A. B. 



Fig. 2. — A shows normal curves, the upper one ventricular (systolic movement upward) 

 and the lower auricular (systolic movement downward). B shows two fibrillar beats 

 at a later phase of the experiment. Simultaneous points are marked by short 

 vertical lines at the first beat. The Au. and V. contract together ; the excitation 

 apparently originates in the A-V. junctional tissues. 



complete quiescence between them (fig. 2). The excitability of the cardiac 

 muscle is low when such separate beats are present ; the refractory period is 

 long. The occurrence of these fibrillar beats shows that the " fibrillar " 

 mode of contraction is not essentially dependent on or necessarily associated 

 with rapidity of succession at all, though the latter is a very strikii^ 

 feature of typical " fibrillation," giving complexity of movement, complete 

 in-coordination, and mechanical ineffectiveness as regards expulsive power. 



Continums Series of Fibrillar Beats as seen in a More Excitable Heart. 

 When the excitability is at a higher level, or when stimulation is applied 

 to make the fibrillar beats follow one another more quickly, a continuous 

 succession of contraction waves appears ; one fibrillar beat excites another, 

 and they are thus strung in a series, constituting a slow coarse fibrillation 

 (fig. 3). The rate depends on the excitability of the muscle, the degree of 



Fig. 3. — Continuous irregular series of fibrillar beats, each beat exciting a subse quent 

 one through the mechanism of circulating excitation. An overdose (intra-vaacula r) 

 of sodium carbonate induced this condition. 



