Upon the Motion of the Mammalian Heart. 569 



far as I have related them, but the problems which arose during the 

 researches were not so simple ; an observation, of the soundness of which we 

 were convinced, for a long while mystified us. Our surface maps showed 

 that the earliest parts of the surface to be activated are the extreme apex of 

 the left ventricle and an area of the right ventricle lying a little above its 

 apex. So far as the latter was concerned there was no great difficulty, for in 

 the dog, upon which our experiments were conducted, this region of the right 

 ventricle is directly supplied by large branches of the arborisation which pass 

 directly to it. ISTo such direct strands pass to the apex of the left ventricle ; 

 moreover the activation of the surface of the left ventricle at a little distance 

 from the apex, although this surface lies nearer to the trunk of the tree than 

 does the actual apex, is activated almost as late as any part of the heart's 

 surface. This discrepancy is one of many observed ; the surface distribution 

 is not to be explained solely by the length of conducting fibres which pass to 

 underlying points of the lining; there ^ is some other and chief factor 

 influencing the surface distribution. This factor was discovered when the 

 rates of conduction of the excitation wave propagated by electrical stimulation 

 were investigated. If two points, one of which lies on the surface and the 

 other of which lies on a corresponding part of the lining, are tested and 

 readings are obtained from them while the heart is beating naturally, the 

 point on the lining is the first to show activity ; this is natural, seeing that 

 the impulse has its starting point within. But if the same points are tested 

 when the excitation wave is artifically excited from a point on the surface at 

 some little distance from the tested points, the same ■phenomenon is vjitnessed. 

 The point on the lining becomes active first, and by the same time interval, 

 as it does when the heart is beating naturally. The excitation wave reaches 

 the point on the lining first, although that point is further away from the 

 point of stimulation than is the point on the heart's surface. It now began to 

 be evident that conduction along the lining is more rapid than is conduction 

 along the surface. That the rapidity of conduction in the lining is due to 

 the Purkinje network is shown by scratching the lining between the points 

 tested and the stimulating electrodes ; after this interference the point on the 

 lining is no longer activated first. On the other hand a deep incision into 

 the surface muscle, though almost penetrating to the cavity, does not affect 

 the time relations. Further investigations showed that the interval which 

 elapses between the arrival of the wave at the points tested varies within 

 certain limits according to their distance from the point of stimulation. If 

 this distance is reduced sufficiently, then the excitation wave arrives at both 

 points simultaneously. By suitably arranged experiments and measurements 

 of the network and thickness of the muscle walls it is possible to calculate 



