190 



THE CIRCULATION OF THE BLOOD 



when one electrode is over the upper end of the sinoauricular node, and 

 that in other regions of the auricle it always appears at a later interval. 

 Further details on this subject will be found in the papers by Eyster and 

 Meek 8 and in Lewis monographs. 



Frequently, in taking electrocardiograms from different parts of the auricle, it is 

 found that certain of the curves show small waves of positivity below the line of equal 

 potential preceding the main wave of negativity. These initial deflections are most 

 marked when both the electrodes are far removed from the sinoauricular node for ex- 

 ample, when they are placed on the auricular appendix ; but they are never present when 



Fig. 54. Simultaneous electrocardiograms to show the cause for extrinsic deflections. The 

 upper curves are from the appendix and the lower ones from lead II. The chief or intrinsic 

 deflection (Tn) is seen to disappear in the right-hand appendix electrocardiogram, because the 

 base of the appendix has been crushed. The extrinsic deflection (Ex) remains, as do the ven- 

 tricular deflections (V 1 F 2 ), (From Lewis.) 



one of the electrodes is placed on the sinoauricular node itself. In other words, curves 

 taken from leads at a distance from the sinoauricular node are more or less composite 

 in form, being made up partly of the main deflection due to the arrival of the excitation 

 and partly of the secondary deflections dependent upon extrinsic influences acting on 

 the electrodes; that is, the electrode picks up electric discharges from distant areas of 

 muscle while these are in a condition of contraction (Fig. 54). From these considera- 

 tions it follows that the intervals between the intrinsic and extrinsic deflections 

 should be longest in leads that are farthest from the node, and gradually become 

 less as one of the contacts approaches the node, until over this structure the ex- 

 trinsic deflection is no longer recorded. Such has been found to be the case. (Lewis.) 



