[mines] CIRCULATING EXCITATIONS IN HEART MUSCLES 51 



fluence the rhythm. The next stimulus, coming a little later in the 

 cycle, set up fibrillation which persists for a certain length of time and 

 then suddenly ceases. In Fig. 4 it is seen that stimuli coming later 

 than the critical instant for the production of fibrillation merely 

 induce an extra-systole, while a comparison of the position of the last 

 stimulus shown with the first, indicates that the critical instant for 

 the production of fibrillation is immediately after the close of the 

 refractory phase. In figure 5 are given some examples of electrograms 

 taken before and during fibrillation set up in the manner described. 

 The appearances of the electrograms are compatible with the idea of 

 a rapid periodically repeated change under the electrode on the ven- 

 tricle. 



In the production of fibrillation in the manner just described, the 

 stimulus apparently arrives at some part of the ventricular muscle 

 just at the end of the refractory phase and probably before the re- 

 fractory phase has ended in some other regions of the muscle. If this 

 is so, we have again a difference in condition of different regions of the 

 muscle as a basis for the inauguration of the state of fibrillation. It 

 seems possible that circulating excitations may play an important 

 part in the maintenance of fibrillation, but it seems to me that there is 

 another possibility which deserves careful testing. Suppose that 

 A and B are two regions of heart muscle close together. If the region A, 

 is thrown into the excited state, in the ordinary course of events the 

 region B immediately after enters the excited state itself; this is the 

 ordinary conduction process. Now suppose that at the time when 

 excitation is set up in A, B is in the refractory state. It cannot then 

 be excited by A. But the excited state set up in A will persist for a 

 considerable time, and the refractory state will disappear from B 

 before the excited state has ceased in A. The question is: Is it ever 

 possible that under these circumstances A will excite B ? 



Conceivably such a state of affairs may arise only under partic- 

 ular conditions of the heart muscle. If we can, we have in such 

 residual excitations a basis for several varieties of anomalous cardiac 

 activities of which fibrillation is one. 



DESCRIPTION OF FIGURES. 



Fig. 1. — Ring cut from auricle of Acanthias vulgaries, in which 

 a circulating excitation was produced. From a cinematograph film. 

 About three quarters natural size. 



Fig. 2. — Ring cut from right ventricle of dog. A centimetre 

 scale photograph beside the muscle. Circulating excitations were 

 set going in this preparation. 



