20 SIR THOMAS LEWIS 



trical axis from instant to instant. To consider the constitution of this 

 electrocardiogram in full detail would detain us too long, it must suffice 

 if by a single example I illustrate again the general principle of inter- 

 pretation, and show that the hypothesis of limited potential differences 

 is applicable in this case also. Let us take the simpler case, the 

 excitation of the dog's left ventricle. 



The excitation wave is found by observation to start in the septum 

 of the ventricles ; it is traced down the septum to the apex and from the 

 latter up the lateral wall to the base. The times of arrival, relative to 

 each other, are indicated in Figure 8 in decimal points of a second. 

 But in moving along this semicircular path the wave does not proceed 

 in a direction parallel to the borders of the muscular track, it is pene- 

 trating the wall in successive segments of its course; it moves always 

 from within outward along centrifugal paths; each part of the wave 

 travels from the endocardial to the pericardial surface. Throughout its 

 passage, the electrical axis constantly changes so that, while in the 

 initial stages this axis sits from left to right in the body, it gradually 

 shifts to a base-apex direction ; from this it swings more and more to 

 the left until, eventually, and when the excitation wave is reach- 

 ing the base of the left ventricle, the electrical axis is directed 

 chiefly upward. This change in the direction of the axis is 

 responsible for the complexity of the corresponding electrocardio- 

 graphic curves. When the set of the axis is in the main down- 

 ward, a basal contact (right shoulder) is relatively negative to an 

 apical contact (left thigh) ; the corresponding upstroke in the electro- 

 cardiogram contributes to the formation of the well-known deflection 

 R. On the contrary, when the set of the axis is mainly upward, the 

 apical contact becomes relatively negative to the basal contact and the 

 corresponding downstroke in the electrocardiogram is the chief constit- 

 uent of the well known deflection 6". As in the amphibian heart, 5 does 

 not represent an apical activity, it represents a basal activity. Now this 

 example, though complex, is a striking one. There is, within the 

 reasonable bounds of error in observation, complete correspondence 

 between the readings which indicate the distribution of the advancing 

 excitation wave and the set of the electrical axis at corresponding 

 phases of the cycle, when the set of this axis is read in terms of my 

 hypothesis of limited potential differences. These observations are in 

 full accord with our knowledge of the anatomy of the ventricle, in 

 accord with a distribution of the excitation wave through the left divi- 

 sion of the auriculoventricular bundle and its arborization ; for the left 

 division enters the ventricle on its septal surface and distributes its 

 branches to the septum, to the apical parts of the heart and to the 

 lateral walls of the heart and to the lateral walls at the base, in this 

 order. Consider the ascertained order of spread and arrange theoreti- 



