10 SIR THOMAS LEWIS 



not consistent with all. For the sake of brevity we may term the first 

 hypothesis that of "distributed potential differences," the second that of 

 "limited potential differences." 



At this point it may, perhaps, be appropriate to allude to my previous 

 review. You will perceive that, if the hypothesis of limited potential 

 differences is correct, the direction in which the current sets in the 

 muscle will be the direction in which the excitation wave is at that 

 moment traveling, for it will always move from the point which is 

 becoming active to inactive muscle in the immediate vicinity of the 

 latter. This association, which I believe to be an absolutely constant 

 one, 2 is one on which I have laid stress ; in doing so I have tended to 

 imply that the first is dependent on the second. Such has not been my 

 meaning though it has been inferred from what has been, perhaps, an 

 unfortunate phraseology, and this inference is perhaps chiefly respon- 

 sible for misconception of my hypothesis. To state that the direction 

 which the excitation wave takes in traveling governs the form of the 

 corresponding curve, 3 cannot be considered fundamentally sound. 



A more correct expression would be that both are governed by one 

 series of events in the muscle, and in consequence are definitely asso- 

 ciated. It has been a matter of descriptive convenience to associate these 

 two phenomena, direction of movement and direction of current flow. 

 But because the meaning may be inferred that one is responsible for 

 the other, I shall endeavor to avoid this method of description in my 

 subsequent remarks. 



Returning to the question at issue, namely, the influence of outlying 

 muscle on the electrical reaction, let us consider the case of two entirely 

 separate masses of muscle (Fig. IE). Under this arrangement, if 1 

 becomes active, the contacts at C will become negative to the contact Z. 

 Here no one argues that the presence of the inactive muscle strip 2 to 

 3 will influence the result, it becomes part of the moist substance in 

 which the strip 1 and 2 is embedded. To illustrate this statement, take 

 the example of complete heart block. When the auriculoventricular 

 bundle is divided experimentally or by disease, the electrocardiogram 

 records the independent activities of auricle and ventricle. Each cham- 

 ber gives its separate curve, but the curves of one and the other fall in 



2. I would here express the view that, unless the conditions are very simple, 

 the direction in which the current is setting in a muscle strip as a whole is not 

 to be ascertained with certainty from an examination of two small points of 

 contact on its surface (as in Fig. 1A) ; but in my view it can be ascertained by 

 the method of leading which Fig. IB illustrates. When I say that the asso- 

 ciation between the set of current and direction of travel is constant, I wish 

 to refer to direction of current as ascertained by the method of indirect leading 

 (method of Fig. IB) and by this method only. 



3. The phrase actually used in "The Mechanism and Graphic Registration 

 of the Heart Beat," London, 1920. . 



