PROPAGATION OF STIMULI IN THE HEART. 619 



there is a time when stimulation applied to it has no apparent 

 action ; this time is, however, in many cases of very short dura- 

 tion and limited to the commencement of ventricular systole. At 

 the commencement of ventricular systole stimulation without 

 provoking contraction causes often a positive effect, namely, a 

 greatly prolonged diastolic pause, which we have been inclined 

 to regard as due to omission of a ventricular contraction. 



It seemed of interest to ascertain whether a similar condition 

 occurred in the other cavities of the frog's heart. We find that 

 in the auricular stimulation about or shortly after the period of 

 maximum contraction of the auricle may cause inhibition of 

 the next auricular beat. 



We have not yet succeeded in registering the contractions of 

 the venous sinus with sufficient accuracy to enable us positively 

 to determine the occurrence of a similar refractory period in the 

 venous sinus itself, but the results we have obtained lead us to 

 hope that we shall soon be able to do so. 



Another interesting consideration is, whether the stimulus 

 which each cavity of the heart transmits to the succeeding one, 

 consists in the propagation of an actual muscular wave, or in 

 the propagation of an impulse along the nerves. The observa- 

 tions of Gaskell have given very great importance to the 

 muscular wave occurring in each cavity of the heart of cold- 

 blooded animals as a stimulus to the contraction of the next 

 succeeding cavity. Our observations appear to us to show that 

 while this is an important factor, it is not the only one in the 

 transmission of stimuli. We have observed that stimulation of 

 the auricle rarely or never causes contraction of the ventricle 

 unless the auricle also contracts. When stimulation of the 

 auricle causes both itself and the ventricle to contract, the 

 auricular contraction precedes the ventricular one in such a 

 way that we might be justified in regarding the ventricular 

 contraction as due to the propaoation of the contractile wave 

 from the auricle to the ventricle. It would also appear 

 that a contractile wave may be propagated backwards, for on 

 stimulation of the ventricle we have observed the contraction 

 of the ventricle produced by stimulation has been succeeded by 

 an auricular contraction such as might be supposed to be due 



