280 



THE CIRCULATION OF THE BLOOD 



travels inwards to the Purkinje system; its subsequent spread occurs in 

 both directions along this pathway and subsequently through the ven- 

 tricular muscle from within outwards (see page 274). Since for the 

 instant the excitation wave is spreading in one ventricle alone the corn- 



Fig. 89. Paroxysmal tachycardia. Auricular origin. Note that the P deflection falls back on T. 



Rate 200 per minute. 



posite picture characteristic of the normal electrocardiogram is not seen, 

 but the effects of one or other ventricle predominate. Though it is not 

 always possible to accurately localize the site of impulse formation, it 



Fig. 90. Auricular fibrillation. Leads 1, 2, 3. Note the coarse fibrillation waves between the 

 R peaks, and the absence of any P deflections in relation to R. Also the unequal spacing of the R 

 deflections. 



may be taken as a general rule, that when the right ventricle is responsi- 

 ble, the record resembles the dextrocardiogram (see page 276) and when 

 the premature beat arises in the left ventricle the characteristics of the 



