CHAPTER XXV 

 THE ELECTROCARDIOGRAM 



" Providence . . . can make a harmony 

 In things that are most strange to human reason." 



MIDDL.ETON. 



THE electrical changes that occur during each cardiac cycle have, 

 of late, become rather important to the clinician, as a rapid and 

 reliable indication of the state of the heart. Cardiac muscle, 

 just like any other muscle, or, in fact, like any other living tissue, 

 is the seat of electrical differences in potential. Ordinary skeletal 

 muscle on contracting develops potential in such a way that the 

 contracting part becomes electro-negative or zincative to the rest. 

 This causes a current to pass through the external or galvano- 

 metric circuit to the contracting part, from the rest of the muscle. 

 Heart muscle acts in a similar way. It has been found that the 

 wave of contraction starts in the auricles. Therefore the auricles 

 will become electro-negative to the rest of the heart. The auricles 

 contract as a whole, passing on the excitation through a piece of 

 primitive tissue (Bundle of His) to the ventricle. This will then 

 become negative to the rest of the heart and so on. 



(1) The existence of this change in the sign of the potential 

 developed may be demonstrated, as it was in muscle, by the use 

 of a fresh nerve-muscle preparation. The nerve is laid across the 

 beating ventricle and produces two muscle twitches per beat. 



(2) Earlier experimenters used the capillary electrometer 

 (p. 404) as the instrument wherewith to measure the potential 

 differences. They found (Fig. 63), on leading one electrode from 

 the auricular sinus and one from the ventricular apex, that on the 

 initiation of the contraction of the auricle, the mercury^an up the 

 capillary away from the tip, indicating the presence of a negative 

 difference of potential. This was followed immediately by a tiny 

 positive movement constituting the second part of the auricular 

 diphasic response. Similarly, a large upward excursion of the 



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