THE HEART 137 



the ' refractory period ' of the physiologists. Should 

 the auricular stimulus happen to arrive during this 

 period, the ventricle will fail to respond, with the result 

 that the heart ' misses a beat.' 



The arrival of the auricular stimulus at the wrong 

 moment may be due either to (1) undue excitability of 

 the auricles, so that they pass prematurely into con- 

 traction again before the ventricles have had time to 

 complete their systole.* This is the chief cause. 

 (2) To alterations in the conductivity of the heart 

 muscle, either in the direction of increase, so that the 

 auricular impulses reach the ventricles too soon, and 

 whilst they are still refractory ; or of diminution, so that 

 some of them never get there at all, and the ventricles 

 are not fired off.' (3) The excitability of the ven- 

 tricles themselves may be raised, so that they occasion- 

 ally go into contraction whilst waiting for the auricular 

 signal, with the result that when the latter arrives they 

 are incapable of responding. 



These ventricular 'extra- systoles' are commonly in- 

 efficient, and do not expel enough blood to reach the 

 wrist, so that we arrive at the apparent paradox that 

 intermittence of the pulse is sometimes due to extra 

 heart-beats, t Their occurrence, however, can be told by 



* Gaskell points out that the fact that the conductivity of the 

 heart muscle is diminished immediately after a contraction will 

 tend to counteract the effect of extra systoles of the auricle, for the 

 impulse so produced will be conducted slowly to the ventricles, and 

 its effect largely neutralized, the ventricular contractions being 

 more regular than those of the auricles (Schafer's 'Physiology,' 

 ii. 195). 



t Wenckebach, ' Zur Analyse des Unregelmassigen Pulses,' Zeit* 

 f. Klin. Mcd., 1899, xxxvi. 181. 



