THE PHYSIOLOGY OF THE HEARTBEAT 197 



the varying conditions of excitability of the ventricular muscle, depend- 

 ing upon the existence of the refractory phase (page 178). 



In auricular flutter, when three or four hundred impulses per minute 

 are passing along the bundle to the ventricle, the contraction produced 

 by the first one will scarcely have started before the second and imme- 

 diately succeeding ones arrive, so that the ventricle will beat at a rate 

 that is much less than that of the auricle, and a condition simulating 

 heart-block will become established. The characteristic feature which 

 distinguishes this from true heart-block, however, is the fact that the 

 ventricular rate is above normal, whereas in true heart-block the rate 

 is much below normal. By means of the electrocardiogram or by 

 polysphygmographic tracings, it can also be shown that the auricle is 

 beating with perfect regularity although very rapidly. 



In auricular fibrillation the ventricles obviously will respond at a very 

 irregular rate to the impulses transmitted to them, and the auricular 

 contractions, if examined by the methods above described, will show no 

 regular sequence. Further details of the method of eliciting these signs 

 will be described later (page 285). 



