ELECTROCARDIOGRAMS 265 



undoubtedly in connection with, the determination of the rate of trans- 

 mission of the excitation wave from auricle to ventricle ; thus, the P-R 

 interval, as it is called, indicates the time taken for the impulse to 

 travel from the sinoauricular to the auriculoventricular node and bundle. 

 In delayed transmission this interval becomes abnormally long. Obvi- 

 ously also conditions of heart-block, of auricular fibrillation, or of auric- 

 ular flutter will be immediately revealed by the electrocardiogram. The 

 interpretation of abnormalities in the contour of the ventricular portion 

 of the curve is, however, not so easy a matter, and should never be 

 undertaken unless curves from the three leads. have been secured, for it 

 will be found that the corresponding electrocardiograms differ from 

 one another in detail; for example, the R-wave is usually most prominent 

 in lead 2, although sometimes it is more prominent in lead 3. T is always 

 upright in normal individuals in curves taken from lead 2, but it is not 

 infrequently inverted in those of lead 3, and may show partial inversion 

 in those from lead 1. The Q-R-S group is often of peculiar contour in 

 curves from lead 3. These variations are possibly dependent upon the 

 relative preponderance of the musculature in the left and right ven- 

 tricles, for it is evident that the amount of muscle included in the path- 

 way between the two leads will vary. 



