THE HEART BEAT. 



543 



excitation toward the apex of the heart. The interpretation of Q 

 has not been made satisfactorily, but in accordance with the ana- 

 tomical arrangement of the auriculoventricular bundle referred to 

 in the last paragraph it is possible that it indicates that the initial 

 excitation in the ventricle starts in the fibers of the papillary mus- 

 culature near the apex. Q is immediately followed by the large 

 wave R, which indicates an excitation at the base of the ventricles, 

 followed by a rapid transition to the opposite phase S, as this 

 excitation passes to the apex of the ventricle. The wave T, 

 occurring at the end of the ventricular systole, has been difficult to 

 interpret. According to one view* it indicates that the wave 



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). 



of excitation passes last to the basal portion of the ventricle, and 

 consequently that the contraction of the ventricle terminates 

 at this point. This conception is in harmony with the embryo- 

 logical fact that the ventricle develops originally from a tube 

 having a venous and an arterial end, and that this tube becomes 

 bent upon itself so that these two ends, the ostium venosum and 

 the bulbus arteriosus, lie together at the base of the heart. As 

 expressed by Keith, the base of the ventricle consists in reality of 

 two parts an auricular base and an aortic base, the beginning and 

 the end of the ventricular tube, and the electric cardiogram traces 

 the wave of contraction from one to the other, R to T, by way of 

 * Gotch, "Heart," vol. i., p. 235, 1910. 



