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



2. The relation of the vagus to fibrillation is quite different in auricles and 

 ventricles ; in the auricles the vagus favours fibrillation in the presence of 

 somQ irritation, e.g., electrical stimulation ; in the ventricles vagus influence 

 can often be clearly shown to retard or prevent fibrillation, while not able to 

 remove the latter once it has been established. The difference is due to the 

 stronger action of the vagus on conductivity than on excitability, as a rule, 

 in the auricles ; this naturally promotes fibrillation. In the ventricles, on 

 the other hand, in regard to these two properties, the main, if not the sole, 

 incidence of the vagus influence is on excitability ; this, of course, tends to 

 repress the development of fibrillation. Pilocarpine, in suitable doses, acts 

 similarly to the vagus, and its relation to fibrillation in auricles and ventricles 

 is to be explained on the same lines. 



3. Some drugs and toxic substances, etc., have a different incidence on the 

 auricles and ventricles respectively both in regard to promoting and retarding 

 fibrillation. 



Confirmation of Former Views. 



So long ago as 1887 the writer* put forward the view that the essential 

 mechanism of typical fibrillation is explicable not simply as an excessive 

 acceleration of rate per se or on the assumption of a mechanism of a different 

 nature, in the sense of muscular v. nervous, from that concerned in the normal 

 beat, but in a disturbance in the relation between the refractory period and 

 the conduction time in the cardiac musculature ; that when this relation is 

 upset by shortening of the refractory period or lengthening of the conduction 

 time or a combination of such changes, the excitation wave, in spreading over 

 the muscular systems, reaches fibres in which the refractory period has already 

 ended and further excitation occurs ; the co-ordinated beat is thus abolished 

 and replaced by a rapid and continued series of in-coordinated fibrillar con- 

 tractions. The alteration in conduction — the passage of the slowed contraction 

 waves in peristaltic fashion along the various complexly-arranged bundles of 

 the ventricular wall at different points of time was described — and also the 

 important fact that single beats may in certain circumstances be fibrillar in 

 character. 



Control of Ventricular Fibrillation. 

 The various actions of different agencies, in promoting or retarding the 

 development of fibrillation and of removing it after it has been established, 

 are to be explained by their incidence on the functions of conduction and 

 excitability and the effects which they bring about in the relations of these 

 functions in different conditions of the cardiac muscle (as a whole) and in the 

 different conditions that may obtain in the auricles and ventricles respectively. 

 * ' Journal of Physiology,' vol. 8, p. 296 (1887). 



