578 ELECTRICAL STIMULATION OF THE FROG'S HEART. 



The apparatus for stimulation consisted of a bicromate battery 

 with two zinc (3 J inches by 2 inches) and three carbon plates, 

 the size of these being 8 inches by 2 inches. This was con- 

 nected with a coil, and a key was interposed by which the 

 primary circuit could be made and broken at pleasure. The 

 moments of opening and closing the circuit were registered upon 

 the same revolving cylinder as that upon which the cardiac 

 pulsations were noted, by means of an electro-magnet, the 

 marker of which was placed immediately under the pens of the 

 cardiac levers. In all the tracings the upper curved line shows 

 the ventricular contractions, the lower curved line the auricular 

 contractions, and the broken straight line the moment of excita- 

 tion. The descent of the line indicates the opening, the ascent 

 the closing of the current. 



In the secondary circuit were placed the electrodes for stimu- 

 lating the various parts of the frog's heart, and this circuit also 

 could be broken or changed at pleasure by means of an inter- 

 posed double key. 



The heart was stimulated by a single induction shock. In 

 minimal stimulation only the breaking shock was effective, in 

 maximal stimulation both making and breaking shocks. The 

 apparatus, which is described in a separate note, admitted of the 

 venous sinus, auricle, or ventricle being stimulated at will. 



When recording the effects of stimulation of the venous sinus 

 we speak only of changes in rhythm of auricle and ventricle. 



We shall examine seriatim the results of irritation of each of 

 these. The temperature of the room in which the experiments 

 were conducted was 67° to 70° F. The frog employed was, on 

 all occasions, the Rana tempm^aria. 



Stimulation of the Ventricle — Minimal, 



On stimulating the ventricle with a single induction shock of 

 minimal potency we find — 



(1) That between the commencement of the ventricular 

 systole up to or nearly up to its maximum there is a refractory 

 period (Fig. 165, a and l) during which stimulation applied to the 

 ventricle has no effect whatever on that beat of the heart, or 

 the one succeeding it, nor is the auricle in anywise affected. 



