954 



PHYSIOLOGY 



hind limbs the other. In man, when the heart lies asymmetrically, it is 

 usual to lead off the right arm, and left arm, the right arm and left leg, or the 

 left arm and left leg, the hand or foot being immersed in salt water con- 

 nected to the galvanometer by a zinc electrode contained in a porous pot full 



FIG. 434. 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. 



of saturated zinc sulphate solution. By this means an electrocardiogram 

 is obtained similar to that shown in Fig. 434. 



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- 



Rad.art. 



FIG. 435. 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 con- 

 tractions. (MACKENZIE.) 



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 

 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, 



