THE CAUSATION OF THE HEART BEAT 



997 



In view of the mechanism of the propagation of the excitatory wave 

 in the ventricle, we should not expect the cardiogram obtained in this 

 indirect fashion to be easy of interpretation, at any rate so far as regards 

 the course of the wave through the ventricular muscle. Such an electro- 



FIG. 443. Electrocardiogram of man, obtained by leading off from the two hands 

 to a string galvanometer. 



c is the carotid pulse tracing. The different parts of the curve are designated 

 by the letters P, Q, R, s, T, first applied to them by Einthoven. 



cardiogram however is of considerable use clinically, especially for the 

 determination of the relation between the auricular and the ventricular con- 

 tractions. The different points in a typical tracing, such as that contained 

 in the figure, are designated by the letters p, Q, R, s, T, which were first 

 applied to them by Einthoven and are retained because they do not involve 

 any theoretical interpretation of the curves. Of these p is certainly due to 



Rod. art. 



FIG. 444. Simultaneous tracings of the jugular venous pulse and the radial arterial 

 pulse, from a case in which the A.V. bundle was destroyed by disease. The 

 contractions of the auricles are marked by the a waves on the venous pulse. 

 They are more rapid than and quite independent of the ventricular contractions. 

 (MACKENZIE.) 



the auricular contraction and Q marks the beginning of the ventricular 

 contraction. The A.V. interval is given by the/distance between p and Q, 

 the total duration of the excitatory condition in the ventricle by the distance 

 between Q and T. 



The fibres of the auriculo- ventricular bundle may be destroyed by disease- In 

 such cases we get a series of phenomena known under the name of Stokes-Adams' 

 disease, the main characteristic of which is the slow contractions of the ventricle, accom- 

 panied by a rapid venous pulse at a rhythm entirely independent of the ventricular 

 pulse. The automatic activities of auricle and ventricle are in fact dissociated (Fig. 444) 



