ELECTROCARDIOGRAM 7 



uninjured muscle (Fig. 1 A) and now stimulate the muscle to contract 

 in the region of one contact, the galvanometer shows the development 

 of a difference in potential between the two contacts, that which lies 

 at the point stimulated (Z) becoming negative relative to its fellow (C). 

 The galvanometer shows us that an electromotive force is developed 

 between these two points ; current flows from one to the other through 

 the muscle and through the galvanometer. This experiment, easily 

 performed, has. been carried out by very numerous observers with 

 uniform results; the same event is witnessed whether voluntary or 

 cardiac muscle is used, but it is the last only which I shall ask you to 

 hold in mind during this lecture, for it is on cardiac muscle alone that I 

 feel competent to speak, and in respect of which alone I shall draw my 

 conclusions. It is on this experiment that the conclusion, which we 

 have all accepted, is based, namely, that cardiac muscle entering the 

 active state shows relative negativity to muscle which is inactive. But 

 although this observation is easy to confirm, and although the con- 

 clusion derived from it does not seem open to question, I believe that it 

 often gives rise to misconception when the meaning of the electro- 

 cardiogram is considered in the light of it. 



The way in which the chief misconceptions arise I will endeavor 

 to explain ultimately. Meanwhile, it should be remarked that in the 

 simple experiment described the electrodes are placed against very small 

 areas of the muscle strip; this method of leading chiefly signals the 

 events which occur at the ends of the muscle, rather than the events in 

 its whole length. Further, it is to be noted that when a difference of 

 potential arises between the two ends as a result of activity of one end, 

 the current flow is necessarily confined to the line of the muscle; for 

 the muscle is isolated. The line of the muscle represents the line of 

 the electrical axis, or the line of maximal potential difference. 



Let us now take a second example (Fig. IB), one which, in the 

 simple form illustrated, is theoretical, but one which would be more 

 comparable to the experiment of electrocardiography. Imagine a strip 

 of cardiac muscle completely embedded in a moist conducting sub- 

 stance, and that our contacts are placed, not immediately on the ends 

 of the strip, but on the substance in which the muscle lies embedded. 

 So they lie in human electrocardiography. If now the muscle end 1 

 becomes active, the galvanometer will again record current flow. The 

 muscle end 1 we know becomes relatively negative to the muscle end 3 ; 

 the contacts Z and C will no doubt reflect this change. If you wish 

 to do so you may regard the contacts as prolonged through the medium 

 in which the muscle is embedded, to the nearest points of the muscle, 

 i. e., to the original ends 1 and 3. There is this contact, but it is not 

 the sole contact, for our electrodes are now connected, not only to 

 the ends of the muscle, but to its whole surface. This difference 



