44 THE ROYAL SOCIETY OF CANADA 



at the point first stimulated, this second wave presents two important 

 points of difference from the first wave. The second wave is pro- 

 pagated more slowly, and it lasts a shorter time at any point in the 

 tissue. 



Since the second wave lasts a shorter time than the first, a third 

 wave can be started sooner after the beginning of the second wave 

 than could the second after the beginning of the first. This third 

 wave is again slower in its rate and shorter in its duration at any one 

 point. By several repetitions it is possible thus to get a succession 

 of waves each of which is actually shorter than the strip of tissue. 



Thus by the time a wave has reached the far end of the strip it 

 has left the end to which the stimulus was applied. 



A series of similar waves can be kept going by a series of evenly 

 spaced stimuli at this rapid frequency; but if a few members of the 

 series of stimuli be cut out and then the series resumed, the responses 

 will be of a different character and the heart muscle will respond only 

 to every second stimulus. The slow propagation and short duration 

 of the waves of excitation are characteristic of the tissue when it is 

 thrown into activity with but very brief periods of rest. 



Let us suppose we have the strip now being stimulated at such 

 a rhythm that the waves of excitation are shorter than the strip. So 

 far as we can make out, the conduction of the excited state is fairly 

 expressed thus: — when one region of the muscle becomes excited and 

 is in physiological connexion with a neighbouring region which is 

 excitable but not excited, the excited state is induced in that neigh- 

 bouring region, and so on. 



Be it noted that our statement implies the assumption that the 

 "excited state" the rate of travel and the duration of which we measure 

 by the electrical disturbance and the refractory phase, is as it were a 

 self- propagating affair. The only justification for such an assumption 

 is that we can demonstrate no change antecedent to these signs of the 

 excited state. The possibility that there is some antecedent change 

 which is the thing really propagated and that the excited state is 

 called forth at successive points in the muscle by the passage of a 

 preliminary wave, must not be lost sight of. 



An observer of the mechanical response in the muscle, seeing 

 the wave of contraction start at the point stimulated, and course over 

 the muscle at a rapid rate, might well imagine that conduction meant 

 that when one region contracted it caused the neighbouring region 

 to contract and so forth. But a study of other changes in the muscle 

 shows very definitely that the mechanical contraction is a change 

 called forth at each point in the muscle, by an antecedent change in 

 the muscle. The wave of contraction reflects precisely enough the 



