Scientific Proceedings. 



(221) 51 



satory pause following an extra stimulation of the ventricles of the 

 warm-blooded heart suffices to prove that the ventricles are beat- 

 ing independently of the auricles. 



The author's experiments on the dog's heart have shown that 

 in partial, as well as in complete heart -block, extra systoles of the 

 ventricles are not followed by compensatory pauses. This results 

 from the tendency for the same number of auricular beats to elapse 

 between the extra contraction and the next following natural con- 

 traction as intervene between two natural ventricular beats in any 

 stage of partial block. The following may be taken as an average 

 example: If the auriculoventricular rhythm is 3:1, a ventricular 

 extra cycle will last through any part of such auricular cycle as 

 may have been unfinished at the moment of stimulation, plus 

 two more auricular cycles if more than one half of the first auricu- 

 lar cycle was unfinished, or plus three or more auricular cycles if 

 less than half of the first auricular cycle was unfinished. 



In partial heart-block extra systoles of the auricules do not cause 

 contractions of the ventricles excepting, occasionally, when such 

 extra systoles fall close to the end of a ventricular cycle ; and 

 extra contractions of the ventricles never cause contractions of the 

 auricles. 



The irritability of the ventricles in partial and complete heart- 

 block is not reduced but rather it is increased over that which 

 obtains in the normal heart. Furthermore, in each ventricular 

 cycle of partial and complete heart-block the irritability of the ven- 

 tricles probably increases until they pass into the refractory state 

 which develops with their contraction. 



In order to determine the significance of these results, a strip of 

 terrapin ventricle was arranged so that rhythmic stimuli as well as 

 extra stimuli could be thrown into either end as desired. The 

 strip was suspended over a Gaskell clamp in such a way that the 

 impulses passing through the strip could be blocked either partially 

 or completely at its middle. In many such experiments it was 

 found that when the strip would beat normally, apparently, from 

 end to end in one direction, a partial or complete block would 

 sometimes be unmasked when the strip was made to beat in the 

 opposite direction. Such behavior is undoubtedly due to the fact 

 that the impulses generated in one end of the strip are more efficient 



