596 ELECTRICAL STIMULATION OF THE FROG'S HEART. 



The reduplication is at all times, except in the last stage of 

 systole, preceded by an auricular contraction. 



Fig. 18.^ 



Stimulation of Venous Sinua (maximal). 



The auricular induced contraction appears to follow stimula- 

 tion more rapidly than in the case of the normal heart. There- 

 fore the induced ventricular contraction (Fig. 185, ventricular 

 tracing alone given) which follows the auricular has a shorter 

 latency than is normally the case. The heating process having 

 been carried so far that a rapid cardiac rhythm with imperfect 

 systole has resulted, it is often found that there is an indifference 

 ^0 stimulation in the so-called refractory period, or even in all 

 phases of the cardiac cycle alternating with the usual sen- 

 sibility. 



On the Effect of Strychnia upon the Frog's Heart. 



The apparatus used in this series of experiments was identi- 

 cal with that employed in the investigation of stimuli applied 

 to the frog's heart. The frog was killed by the brain being 

 destroyed, and a small dose of strychnia was then introduced 

 into the dorsal lymph sac. As soon as the effect of the drug 

 upon the spinal cord was evidenced by distinct spasm, the 

 heart was rapidly exposed, placed on the cardiograph, and 

 stimulation applied. The same order will be observed as in the 

 description of the experiments on the normal heart. 



Stimulation of the Ventricle — Minimal. 



On applying a minimal stimulus to the strychnia heart (Fig. 

 186) we were struck, in the first instance, by the extreme length 

 of the refractory period. Stimulation has usually no effect, not 

 only when applied before the maximum of the systole as in the 



