THE PHYSIOLOGY OF THE HEARTBEAT 189 



points, the amplitude of deflection of a galvanometer connected with the 

 pair of contacts will be most pronounced when these are radial to the 

 points of stimulation, for under these conditions it is evident that the 

 greatest possible difference will exist between the intervals required for 

 the w^ave to reach each contact. 



Bearing these principles in mind, we may now proceed to examine the 

 evidence pointing to the origin of the heartbeat at the sinoauricular node: 

 (1) When two electrodes are applied at different points of the au- 

 ricle, the amplitude of movement of the string of the galvanometer 

 produced by each heartbeat is greatest when the line joining the elec- 

 trodes converges on the sinoauricular node. To make this clear the 

 movement of the string must be photographed in the manner above 

 described, the resulting tracing being called an electrocardiogram. From 

 the experiments with the circular sheet of muscle alluded to it is evident 

 that the stimulus to produce this result must have arisen in the neigh- 

 borhood of the node. (2) If one electrode is placed on the sinoauricular 

 node and the other electrode is moved about from place to place on the 

 auricle, the deflection being noted at each new position, the electrode 

 on the node will always be found to be negative to the other electrode.* 

 No such consistency will be manifest if both electrodes are moved about 

 on other parts of the auricle. 



(3) As we shall see immediately, the current of action of the beating 

 heart may be recorded by connecting a galvanometer with various parts 

 of the body; for example with the right fore limb and the left hind 

 limb. On the electrocardiogram thus obtained are several waves, one 

 of which, called the P-wave, can easily be shown to correspond to the 

 contraction of the auricle (see Fig. 272). If now we compare such electro- 

 cardiograms with those obtained during contractions of the auricle 

 caused by applying electrical stimulation to various parts of it, it will be 

 found that the electrocardiogram of the artificial beat simulates the 

 normal curve only when the stimulated part is in the neighborhood of 

 the sinoauricular node. In other words, it is only when the stimulus is 

 applied to the sinoauricular node that a characteristic P-wave is obtained. 

 "When the appendix or the superior vena cava is stimulated, the P-wave is 

 distorted although the other waves of the electrocardiogram may be normal. 



(4) By taking electrocardiograms from various direct leads placed 

 on the auricle and comparing the records with that of a standard 

 limb lead taken simultaneously, we shall find by exact measurement that 

 the time of onset of the excitation wave of the auricle, as measured in 

 relationship to the P-wave on the standard electrocardiogram, is shortest 



*The connections between the electrodes and galvanometer are always arranged so that any 

 upward movement of the shadow of the string above the line of equal potential at the two electrodes 

 indicates electric negativity. 



