ELECTROCARDIOGRAM 



273 



slight positive wave, Q; (2) the cause of the positive wave, S; (3) the 

 cause for the period of equal potential at the base and apex during ven- 

 tricular systole indicated by the portion of the curve between S and T ; 

 (4) the cause for the negative wave, T. To solve these problems it is 

 necessary to compare electrocardiograms taken from the surface of the 

 body with those from electrodes placed directly on the base or apex of 

 the ventricle of the exposed heart. 



Fig. 83. TClectrocardiogram (dog) taken simultaneously with curves from auricle and ven- 

 tricle. It will be observed that wave P slightly precedes auricular systole and that wave R occurs 

 just before the presphygmic period starts in the ventricle. (From Lev/is.) 



The Ventricular Complex 



In view of the nature of the electric change which occurs in a strip 

 of denervated muscle when a wave of contraction passes along it (page 

 188), the simplest interpretation of the ventricular part of the above 

 curve is that the contraction must pass into the ventricle at a little dis- 

 tance from the base, thus causing the latter, for a moment of time, to be 

 positive to the rest of the ventricle, and accounting for the slight down- 

 ward wave, Q. Immediately after this the base of the ventricle becomes 

 negative to the apex, giving us the marked upward wave, R, which 

 however lasts for but a short period of time, being folloAved by an inter- 

 val during which the base and apex are of the same electrical potential 

 (horizontal part of wave between R and T). Finally the base again be- 

 comes negative to the apex, thus accounting for the smaller upward 

 wave, T. The cause of the occasionally observed downward wave, S, 

 following R, is obscure. 



The most significant fact in the electrocardiogram is therefore that 



