540 



CIRCULATION OF BLOOD AND LYMPH. 



a negativity toward the apex is shown by a movement in the 

 opposite direction. Similar electrocardiograms, differing in certain 

 details, may be obtained by connecting the electrodes with other 

 parts of the body. In experimental as well as clinical work it has 

 become customary to use the three "leads" introduced bj^ Ein- 

 thoven — namely, lead I, the electrodes connected with the right and 

 left hands respectively; lead II, right hand and left foot; lead III, 

 left hand and left foot. The electrocardiogram taken from lead I 

 (Fig. 228) shows three positive waves, P, R, and T, and two nega- 



ojSee 



Fig 228. — Electrocardiogram obtained by photographing the movements of the 

 thread of a string-galvanometer. The upper figure shows the photographed curve, while 

 the lower one is a diagram constructed from the photograph to make clearer the electrical 

 changes in a single cardiac cycle. To obtain this record the electrodes were connected 

 with the right and left hands. Waves with the apex upward indicate that the base of the 

 heart (or tlie right ventricle) is negative to the apex (or left ventricle). Waves with the 

 apsx downward have the opposite significance. Wave P is due to the contraction of the 

 auricle. Waves Q, R, S, and T occur during the systole of the ventricle. {Einthoven), 



live waves, Q and S. The first positive wave, P, indicates an initial 

 development of negativity toward the base or auricular end of the 

 heart. All observers practically agree that this wave is due to the 

 contraction of the auricles. On the other hand, the waves Q, R, S, 

 and T occur during the systole of the ventricles. R and T are the 

 more prominent and constant of these waves. Q and S may be 

 absent or scarcely detectable in normal electrocardiograms. There 

 is much difference of opinion in regard to the exact significance of 

 these waves. As the galvanometer is arranged and connected with 

 the heart one can saj^ that the negative wave Q indicates the exist- 

 ence of a negative potential in the apical portion of the heart. It 

 would, therefore, suggest that the excitation wave reaches first 

 some region of the heart toward the apex. The succeeding positive 

 wave, R, indicates, on the contrary, negativity and, therefore, an 



