On Electrical Stimulation of the Frog's Heart. 469 



the ventricular reduplication is preceded by the auricular induced 

 contraction (fig. 14). Thus stimulation applied just before the ven- 

 tricular relaxation is completed, instead of having an instantaneous 

 ventricular and auricular response resulting from stimulation of 

 auricle or ventricle, has a long latency, wherein the auricle redupli- 

 cates. The further on in the systole the stimulation is applied, the 

 shorter is the latent period, and the more perfect the reduplicated 

 contraction. 



Stimulation of the Venous Sinus — Maximal. 



Omission may be caused by stimulation applied at the commence- 

 ment of systole, or reduplication may occur in all phases (fig. 15). 



Reduplication has the longest latency at the commencement of 

 systole, and there is true auricular precedence up to or beyond the 

 maximum in this phase. In the decline of systole, after the maximum 

 is passed, and the abscissa has been nearly reached, there is occasion- 

 ally reduplication with short latency, the auricular and ventricular 

 contractions being synchronous. 



Action op Heat on the Heart. 



In this series of experiments the pithed frog in which the brain and 

 spinal cord had been destroyed, was laid upon a metal plate, the tempe- 

 rature of which was gradually raised by means of a flame beneath it. 



Ventricular Stimulation — Minimal. 



The refractory phase is generally wanting in the ventricular systole, 

 but it may be present in exceptional cases, not unfrequently in the 

 same tracing in which stimulation most generally produces reduplica- 

 tion (fig. 16). 



It may be noted that irregularity of response to stimulation is one 

 of the characteristics of the heated condition. Stimulation usually 

 causes reduplication. Should stimulation fall at the commencement of 

 ventricular systole, no effect is produced till the whole cycle of the systole 

 has been passed through, when reduplication by a very perfect systole 

 occurs. Latencies diminish in proportion as the stimulation occurs 

 later in the systole of the heart. Reduplication occurring in response 

 to stimulation falling at the maximum is often demonstrated by a 

 beat originating when the relaxation after systole is completed, and 

 therefore distinct from the original beat : this is due to the fact that 

 the curve of the heated heart is much shorter in duration, and 

 therefore the reduplication falls outside the systole during which 

 stimulation occurs, the latency being actually shorter, however, than, 

 in the unheated heart. 



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