CHAPTER XXVI 

 THE ELECTROCARDIOGRAM 



" Providence . . . can make a harmony 

 In things that are most strange to human reason." 



MiDDLETON. 



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-positive 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 at the sino-atrial node. Therefore, the 

 node will become electro-positive (galvanometrically negative) to 

 the rest of the heart. The atria next contract as a whole, passing 

 on the excitation through a piece of primitive tissue (Bundle of 

 His) to the ventricles. Node, atria and ventricles, as they contract, 

 become electro-positive (zincative) to all other parts of the heart. 



(1) Rheoscopic Frog. The existence of this change in the sign 

 of the potential developed as the wave of contraction passes 

 over the various parts of the heart may be demonstrated, as it was 

 in muscle (p. 179), by the use of a fresh nerve- muscle preparation. 

 The nerve laid across the beating ventricle produces two muscle 

 twitches per beat. 



(2) Capillary Electrometer. Earlier experimenters used the 

 capillary electrometer (Fig. 12) as the instrument wherewith to 

 detect and measure these potential differences. They found, on 

 leading two electrodes from different points of the atrium, that the 

 amplitude of movement of the mercury produced at each heart- 

 beat is greatest when the line forming the shortest distance between 

 the electrodes would pass through the sino-atrial node. This is 

 interpreted as an indication that the electrical disturbance has its 

 origin at the node. 



Consider, for a moment, a large circular sheet of muscle, and 

 near the centre of the sheet are placed two electrodes {A and B) 



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