THE HEART BEAT 



547 



ventricle should be in a condition of equal potential during systole, 

 and this appears indeed to be the case in the period between the S 

 and T waves. But the rapid oscillations in potential at the begin- 

 ning of systole indicated by the Q, R, and S waves have not been 

 explained satisfactorily with reference to the course of the waves of 

 excitation and contraction. The T wave occurs at the end of sys- 

 tole. According to Einthoven this wave may be positive or nega- 

 tive or be absent altogether. He interprets it to mean that the 

 condition of contraction does not cease simultaneously in all parts 

 of the ventricle. When, as is usually the case, it is positive it in- 

 dicates a longer continuance of excitation and contraction in the 



oiSee. 



Fig. 227. 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 the right ventricle) is negative to the apex (or left ventricle). Waves with the 

 apex 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). 



basal portion of the ventricle.* While the precise significance of 

 the ventricular waves of the electrocardiogram is not yet de- 

 termined, it has been found by observation that variations in the 

 character or sign of these waves under pathological conditions can 

 be used in diagnosis, particularly in the diagnosis of disturbances of 

 the normal rhythm. For details in regard to this point, reference 

 must be made to clinical works, f 



Change in Form of the Ventricle During Systole. Much 

 attention has been paid to the external change of form of the 



* For discussion and literature see Kahn, "Ergebnisse d. PhysioL," 14, 

 1914; Eyster and Meek, "Archives of Int. Medicine," 11, 204, 1913, and Fahr 

 and Weber, "Deutsch Archiv f. Klin. Med.," 117, 361, 1915. 



t Barker, "The Clinical Diagnosis of Int. Diseases," vol. i., 787, 1916. 



