18 



SIR THOMAS LEWIS 



The position of the bulbus, relative to the rest of the ventricular muscle 

 with which it is in functional union 7 does not affect the direction of the 

 deflection. 



Consider now the initial deflection (7\?) or deflections (R and S). 

 The meaning of R according to my hypothesis is not obscure. It is 

 due to set of the current in the axis of the heart, i. c. from above down- 

 ward, over the greater part of that phase of the cycle during which the 

 ventricle is becoming activated. The ventricle is activated in the gross 

 from above downward. The excitation wave starts in the muscular 

 funnel and trabeculae within the heart (Fig. 6A) and is at first prop- 

 agated wholly in a downward direction. Actually the greater part 

 or entire upstroke of R is written before the excitation wave appears 

 at any point on the surface of the ventricle. To regard R as an evi- 

 dence of general basal activity is obviously inconsistent with this obser- 

 vation; R corresponds to the activity of muscle lying deep inside the 



Fig. 7. An electrocardiogram taken from a toad's heart ; leading from the 

 mouth and abdomen in the axis of the heart. The deflection produced by activity 

 of the bulbus arteriosus (5) is directed downward. 



central portions of the heart. The hypothesis of limited potential differ- 

 ences applied to the amphibian ventricles provides us with a clear con- 

 ception of the electrical events, a conception which is consistent with 

 the manner in which the excitation wave has been shown to spread. 

 During the early phases of R's inscription the muscle is becoming active 

 in the central trabeculae, this region is relatively negative, the relatively 

 positive region lies apexwards and in its immedate vicinity ( Fig 6B 

 1st phase). As the excitation wave proceeds downward, the advancing 

 border spreads laterally, but the balance of the potential differences 

 manifests itself still in the same direction (Fig. 6B midphase). In its 



7. The reply which has been made, that there is a natural line of block 

 between the two chambers, does not appear to me relevant. They are united func- 

 tionally by muscle fibers, the region of block being simply a region of slow propa- 

 gation; if it is held that this region of slow propagation constitutes a line of 

 complete separation from the standpoint under consideration, the burden of 

 proving this lies with those who hold the view. 



