568 



Mr. T. Lewis. 



completely involved, the right ventricle receives the wave through the 

 muscle which unites the two chambers ; thus, after section of the right 

 stem of the bundle the right ventricle is activated in a high abnormal 

 manner. 



Another experiment explores the network itself. The natural spread of 

 the excitation wave over the conus of the right ventricle is away from the 

 chamber of the ventricle and towards the outlet of the pulmonary artery. A 

 cut traversing the conus at right angles to the general direction of spread 

 and penetrating the muscle fibres for a considerable depth, fails to delay the 

 appearance of the excitation wave above the incision, while a shallow 

 incision or even a scratch in a similar plane, but applied to the inner or 

 endocardial surface, produces a profound delay. Normal conduction is 

 therefore through the branches of the auriculo-ventricular bundle and 

 ultimately through the arborisation and network. 



But this demonstration still leaves several questions unsolved. If the 

 impulse which descends the bundle from the auricle is distributed to the 

 networks lining the cavity of the heart, by what paths is it conveyed to 

 the surface of the heart ? It is conveyed, as the following experiment 

 clearly shows, by direct penetration of all the muscle layers along paths 

 radiating from the cavity. The muscle of the left ventricle is very thick, 

 and can be deeply incised with impunity ; if a point is chosen on the 

 surface, and the time at which the excitation wave arrives at it is deter- 

 mined, this time is uninfluenced by deep incisions which completely surround 

 the point in question. But if an incision is made which undercuts the 

 point examined, the wave is greatly delayed in its passage. This simple 

 experiment shows that the wave is carried from within outward from one 

 muscle band to the overlying one, and that it travels in a plane which is 

 at right angles to the direction of the fibres. In support of the same 

 conclusion are time readings taken from the cavity of the heart ; if corre- 

 sponding points on the inside and outside of the heart are tested, it is 

 found that the excitation wave always reaches the lining before it reaches 

 the surface of the heart. 



Now the facts which I have so far related point steadily to one conclusion, 

 namely, that the distribution of the excitation wave in the mammalian 

 heart is accomplished through the branches of Tawara's tree. Arriving 

 at the bundle the impulse travels through the main branches, through 

 the smaller branches and twigs, until the whole lining of the cavity becomes 

 implicated ; from the network which lines the cavities it spreads directly 

 into the ventricular wall, penetrating each layer of it in succession. This 

 general account appears a simple and satisfactory explanation of the facts so 



