ELECTRO-CARDIOGRAM 



837 



hands, or, better, with the right hand and the left foot. The two feet 

 are the most unfavourable combination. The reason is obvious from 

 the direction of the long axis of the heart, which determines the 

 direction of the lines of flow of currents due to differences of potential 

 between base and apex (Fig. 310). 



Relation of the Waves of the Electro-Cardiogram to the Mechanical 

 Events in the Cardiac Cycle. There is evidence that the excitation pro- 

 cess with its associated electrical change spreads over the auricle from the 

 sinus node, followed at each point by the mechanical change or contrac- 

 tion, in such a manner that the portions nearer the node begin to relax 

 before the more distal units have finished contracting. When a tracing 

 of the approximation of two distant points of the auricle is taken, 

 the total shortening represents the algebraic sum of the contractions 

 and relaxations of all the muscular units between the two points. 



Fig- 3I5- Upper curve, pressure in right auricle. Second curve from top, right 

 auricular myogram; the down-stroke corresponds to contraction. Third curve, 

 ventricular sounds. Bottom curve, electrocardiogram, lead II (left hind and 

 right fore leg) (Wiggers). 



Electrical effects obtained by leading off from the heart in any particu- 

 lar way must also be more or less complex resultants of the changes at 

 different points. Certain general relations, however, have been estab- 

 lished between the mechanical events and the electro-cardiogram. In 

 Fig. 315 are shown simultaneous records of the contraction of the 

 auricle (auricular myogram), the intra-auricular pressure, the heart 

 sounds and the electro-cardiogram (Wiggers). The first electrical 

 variation, commencing at i, is seen to precede the rise of pressure in 

 the auricle, 2, by a definite interval (about 0-02 sec.), and the onset of 

 the mechanical shortening ,3, by a somewhat greater interval (0-03 sec.). 

 The length of these intervals is, of course, not precisely the same in 

 different experiments. 



The relation of the beginning of the rise of intra-auricular pressure 

 and of the beginning of the mechanical systole of the auricles to the 



