77 



contraction wave had already passed. Hence, if the fibres are 

 sufficiently excitable and ready to respond to contraction waves 

 reaching them, there would evidently be a more or less rapid series 

 of contractions in each muscular bundle in consequence of the 

 successive contraction waves reaching that bundle from different 

 directions along its fibres of anastomosis with other bundles. Hence 

 the movement would tend to go on until the excitability of the 

 muscular tissue had been lowered so that it failed to respond with a 

 rapid series of contractions. Then there might be some isolated 

 peristaltic contractions, such as I have often seen after the cessation 

 of the fibrillar movement." 



Those conclusions were confirmed and extended in a paper 

 in 1918,* while in the interval similar views had been 

 advanced and supported by experimental evidence — by 

 Mines' (1913) in the case of the frog heart and by Garrey^" 

 (1914) in the mammalian heart. Suggestive experiments 

 had been made by Mayer^^ (1908) on medusa, etc. 



lielations of Refractory Period and Conduction Time. 



If the fundamental importance of the relation between the 

 duration of the refractory period and the conduction time 

 of the muscle is kept in view it is easy to understand how the 

 mechanism of circulating excitation may come into operation 

 under very diverse conditions affecting the ventricles. Any 

 influence cutting down the refractory period or lengthening 

 the conduction time disproportionately must naturally tend 

 to favour the process of re-excitation ; a combination of such 

 changes is, of course, still more effective. Hence the develop- 

 ment of fibrillation is witnessed at one time as an apparently 

 " spontaneous " event in a vigorous heart manifesting signs 

 of extreme irritability (for example, from chloroform, 

 digitalis, etc.) or in a normal heart subjected to stimulation 

 — excessive rapidity of excitation playing an essential part 

 by shortening the refractory period and slowing the con- 

 duction time. At another time fibrillation appears in a 

 heart that presents features of grave depression, diminished 

 contraction force, loss of tone, lessened excitability, and — 

 what is the determining factor in this case — pronounced 

 slowing of the propagation of the excitatory wave, relatively 

 to the duration of the refractory period; such may be seen 

 in poisoning by potassium salts, extreme cooling, etc. ; thus 

 it is often a terminal, or approximately terminal, phase in 

 the dying heart. In some depressed hearts there is a decided 

 liability to fibrillation from mechanical disturbance, external 

 pressure on the ventricles, incising the pericardium so as to 

 remove its support from a relaxed ventricular wall, etc. In 

 either case, whether fibrillation is a manifestation of per- 

 verted irritability or of abnormal depression, the same 

 explanation of disturbed relationship between the processes 

 named holds good. 



In accordance with this conception it is readily intelligible 

 that the absolute values of refractory period and conduction 

 time may undergo extensive variation without fibrillation 

 being set up; if both of these factors vary proportionately 



