384 



THE ELECTROCARDIOGRAM 



of the atrial electrode. This constitutes the second part of the 

 atrial diphasic response. Similarly a large upward excursion of 

 the mercury followed by a smaller downward movement demon- 

 strated a similar but greater ventricular diphasic response. 



It is not necessary to expose the heart and lay non-polarisable 

 electrodes on it in order to see this diphasic response by the electro- 

 meter. The right arm may be considered as electrically con- 

 tinuous with the base and the left leg (or arm) with the apex of 

 the heart. 



Using these leads, one may easily identify on the record the 

 P wave (Fig. 90) which may readily be shown to correspond to the 



I'lG. 91. — Diagram of tlie essential parts of the string galvanometer. N and .S' are the poles 

 of a powerful electromagnet, between which is stretched the fibre C. 



contraction of the atrium. As potential differences radiate from 

 the heart to the surface of the body, and may even be com- 

 municated to the air and detected at a reasonable distance from 

 the body (Potter), leads might be taken from any tAvo points on or 

 near the surface of the body. Certain leads, however, give better 

 results than others, due to the fact that the heart lies obliquely, 

 and, therefore, produces an asymmetrical distribution of lines of 

 equal potential. For this reason and for convenience three pairs 

 of leads have been adopted for standard practice, namel}' : — 



Lead I. — Right arm and left arm. 



Lead 11. — Right arm and left leg. 



Lead III. — Left leg and left arm. 



