48 THE ROYAL SOCIETY OF CANADA 



any movement of the ventricle, yet the ventricle ready to respond 

 to direct stimulation and to call forth an extra beat of the auricles. 



The cause of uni -directional block may very likely be expressed 

 thus in terms of Adrian's work. The region of blocks is a region of 

 decrement, situated between two normal regions. If the decrement 

 is uniform, then the system is symmetrical and the blocking should be 

 equal in the two directions. But if the decrement is greater at one 

 end of the "depressed" region than at the other, we have the pos- 

 sibility that transmission in one direction may be easier than in the 

 other. 



It is evident that the uni-directional block need not necessarily 

 persist for more than a very short while in order to start a circulating 

 excitation. 



3. Tachycardia and circulating excitations. 



It is a fact familiar to the physician and to the physiologist that 

 a heart which has been beating regularly and in normal sequence may 

 sometimes suddenly exhibit a new rhythm of a totally different char- 

 acter. While in some cases the new rhythm is related to the original 

 rhythm in some simple ratio, and is explicable on the hypothesis of 

 partial heart blocks between the region originating rhythm and the 

 rest of the heart or between auricles and ventricles, there are other 

 instances where there is no such relation to be made out between the 

 normal and the abnormal rhythms. 



Last year I showed that certain instances of tachycardia ob- 

 served during experiments on cold-blooded hearts, where the excita- 

 tions of the auricle and ventricle were proved to be mutually de- 

 pendent, might best be explained as circulating excitations, the im- 

 pulse passing from auricle to ventricle by one path and returning 

 from ventricle to auricle by another path. 1 



I ventured then to suggest that some instances of paroxysmal 

 tachycardia observed in man where auricles and ventricles beat with 

 the same frequency might conceivably be explained along somewhat 

 similar lines. I now repeat this suggestion in the light of the new 

 histological demonstration by Stanley Kent 2 that the muscular con- 

 nexion between auricles and ventricles in the human heart is multiple. 



Stanley Kent shows that an extensive muscular connexion is to 

 be found at the right-hand margin of the heart at the junction of the 

 right auricle and right ventricle. 



Supposing that for some reason an impulse from the auricle 

 reached the main A-V bundle but fai led to reach this "right lateral" 

 ^ines, loc. cit. 

 2 Stanley Kent, Quart. Journ. Exper. Physiol. VII, p. 193. 1913. 



