1914.] MECHANISM OF THE HEART-BEAT. 291 



whole, its strength is uniform and changes so little within small 

 limits of time as to render the change negligible. There is conse- 

 quently no danger of confusing it with the rapid changes in electrical 

 potential which compose the electrocardiogram. In order to keep 

 the string in the optical field, from which the constant or skin current 

 tends to deflect it, a system of compensation has been found neces- 

 sary. This system comprises another source of current, a commu- 

 tator and a series of resistances which are introduced into the string- 

 heart circuit. The action current to be recorded, and with it the 

 constant current, are permitted by a shunt to pass through the string 

 in increasing amounts, so enabling one to allow a sufficient electro- 

 motive force of opposite polarity to the skin current to enter the 

 circuit. Compensation in routine examinations becomes a simple 

 procedure. To complete the records, a time curve and a millimeter 

 scale are photographed on the record. 



The most fruitful method of investigating the identity of the 

 parts of the electrocardiogram has probably been that of recording 

 synchronously on the same curve the electrocardiogram and mechan- 

 ical curves representing the motions of the heart. As the result of 

 these studies, it has been concluded that the contraction of the 

 auricles is represented electrically by the P wave, the first wave in 

 the cardiac cycle (Fig. 4), while the Waves Q, R, S, T, which follow 

 it form a group in the electrocardiogram which are associated with 

 ventricular activity. The term ventricular activity is purposely 

 chosen, for there is as yet no uniformity of opinion in respect to 

 designating which ventricular function it is which this complex of 

 waves represents. Opinion is divided as to whether the complex 

 represents the act of conduction, the state of muscular irritability, 

 or actual contraction. It is doubtless unprofitable to analyze this 

 discussion, and probably quite impossible to decide between these 

 interpretations now. Of this much one can be certain, that the 

 QRST complex does not occur unless the ventricles have been 

 seen or known to contract. The significance of the individual waves 

 of this group is also still a matter wrapped in doubt. Most writers 

 favor the view that the wave Q, w'hen present, signifies that the 

 earliest ventricular activity has taken place near the apex of the 

 heart, that the R wave represents the assumption of predominance 



