THE ELECTROCARDIOGRAM 



61 



It is conventional to take the records in such a manner 

 that negativity at the base is shown on the photographic 

 record by a deflection upwards. 



All electrocardiogram thus obtained is shown in Fig. 13. 

 It will be seen to differ considerably from the simple 

 diphasic variation of skeletal muscle. There are two 

 reasons for this discrepancy. First, the right hand and left 

 foot do not accurately represent the base and apex of the 

 heart respectively; secondly, the heart is far from being 

 a simple muscle. 



Fig. 13. — Human electrocardiogram (from Starling's Principles of 



Physiology). 



The record usually consists of five waves, to which are 

 given the conventional names, P, Q, R, S, and T. Of these 

 three, P, Q, and T indicate base-negative currents; the 

 remaining two, Q and S, base-positive. 



The interpretation of the electrocardiogram is a matter 

 of considerable difificulty. The wave P is admittedly of 

 auricular origin. During the iso-electric period following 

 it, neither auricle nor ventricle is contracting, the impulse 

 passing from the one to the other along the auriculo- 

 ventricular bundle. Q is of inconstant recurrence and 

 uncertain origin. R, which is always the most striking 

 feature of the electrocardiogram, indicates contraction at 



