On Electrical Stimulation of the Frog's Heart. 459 



Stimulation applied to the ventricle during the first period has no 

 effect whatever either in accelerating the occurrence of the second 

 beat, or altering the length of the subsequent pauses. This consti- 

 tutes the refractory period. 



Stimulation applied during the second period causes reduplication of 

 the systole, the next systole succeeding with a constantly diminishing 

 latency up to the end of the period. When the stimulation is applied 

 in this period, the two systoles being more or less united, there is no 

 distinct pause between them, but the diastolic pause succeeding the 

 second occupies very nearly the interval of time corresponding to two 

 normal diastolic pauses. In this second period the heart is more 

 sensitive to the action of minimal stimuli than in the first period. In 

 the third period, that of acceleration, stimulus applied to the ventricle 

 hastens the advent of the succeeding systole, and the latent period is 

 very short, being nearly equal throughout its whole extent to the 

 latency at the end of the second period. The sensibility of the heart 

 to stimuli is scarcely so great in this period as in the second. 



The length of the diastolic pause succeeding the accelerated systole 

 is longer than normal, the increase in length being nearly equal to the 

 amount of acceleration. 



Stimulation of the Ventricle — Maximal. 



When stimuli of maximal potency are applied to the ventricle 

 between the maximum auricular systole and the commencement of 

 ventricular systole, the ventricular systole immediately following the 

 stimulus is rarely slightly higher than normal, and the diastolic pause 

 succeeding it is excessively long — so long, indeed, as to be nearly, if 

 not quite, equal to the time which would, as a rule, be occupied by 

 two diastoles, so that the time occupied by the systole and diastole 

 after stimulation applied at this period of the heart's cycle, is equal 

 to the time usually occupied by one systole and two diastolic pauses. 



In most cases this systole was apparently no higher than normal, 

 and consequently we cannot with plausibility regard it as a case of 

 superposition of two systoles. 



In some cases the time within which this pause may be produced is 

 strictly limited to the point indicated ; in others, however, it may 

 extend some little distance towards the maximum of systole, though 

 it never reaches this. In other words, it may encroach upon the 

 refractory period which we have mentioned when speaking of 

 minimal stimuli, although it never extends through the whole of it. 



This phase may occasionally, though rarely, be absent. Its place is 

 then taken by reduplication, or very rarely by insensibility to stimula- 

 tion, as in the refractory period. 



Reduplication with maximal stimuli occurs during all times of the 

 cycle, except at the very commencement of the systole. 



