On Electrical Stimulation of the Frog's Heart. 493- 



to it, while the auricle has continued to beat with its ordinary un- 

 altered rhythm undisturbed by the ventricular excitement. 



It is not however our purpose to do more in this paper than state 

 the results we have hitherto obtained, and we shall therefore reserve 

 to a future communication the consideration of this and some other 

 questions of importance closely allied to it. 



Another question is the nature of the inhibitory influence exerted 

 by one cavity of the heart upon another. Marey had shown that 

 stimulation of the ventricle during a great part of the refractory 

 period exercises an inhibitory instead of a motor action upon the 

 ventricle itself. It might be supposed then that a stimulus of either 

 kind, whether proceeding from the auricle in the form of a contractile 

 wave, or a nervous impulse, might produce inhibition of the ventricle, 

 provided the stimulus reached it during that part of the refractory 

 period in which stimulation usually causes' inhibition. From our 

 observations it seems that the inhibition of the ventricle which may 

 follow stimulation of the auricle is not due to the muscular wave 

 propagated from the auricle and striking the ventricle during 

 the refractory period. In fig. 6, we notice that the auricular 

 contraction succeeded by ventricular inhibition occurs after the 

 refractory period of the ventricle has passed ; we must, therefore, 

 look upon the inhibition as due to the propagation of a nervous 

 impulse from one cavity to another. In the auricle we find that 

 stimulation may produce inhibition of the auricular and ventricular 

 beats, or of the ventricular beats alone. "We may, therefore, suppose 

 that the stimulus applied to the auricle acts upon two different 

 nervous mechanisms ; seeing that it is enabled to inhibit the ventricular 

 beats without affecting the auricular ones, we are unable to say 

 precisely what the effect of a single stimulus applied to the venous 

 sinus is upon the sinus itself, but here we note that the same result 

 will follow stimulation of the sinus, as of the auricle, viz., inhibition 

 of the ventricular without inhibition of the auricular beat, or in- 

 hibition of both together. 



As has been already pointed out by Professor Marey, the refractory 

 period is increased when the heart is artificially cooled. We have- 

 also found that there is a prolongation of the time during which 

 stimulation causes an inhibition or omission of the following systole. 



It is very seldom that stimulation of the auricles or of the venous 

 sinus causes a ventricular contraction without auricular systole pre- 

 ceding it in the ordinary rhythm. In this respect the action of the 

 heart offers a contrast to the normal. Though the muscular wave 

 started in the auricle is usually succeeded by a ventricular contrac- 

 tion, it may occasionally be succeeded by a ventricular inhibition, or 

 auricular stimulation may be followed by inhibition of both auricle 

 and ventricle. 



