iqo THE CONTRACTION OF CARDIAC MUSCLE. 



choose to use the term " inner stimuli " to express the causation 

 of the internal changes which are going on in the automatic rhythmical 

 tissue of a normally nourished heart, then we can explain the normal 

 rhythm in the same way as the artificially produced rhythm, as a 

 necessary consequence of a constantly acting internal stimulus upon 

 a muscular tissue in which the refractory period is exceedingly 

 long. 



By the refractory period was originally meant by Budge and Marey, the 

 time during which the tissue was inexcitable to any stimulus of any strength. 

 This period of complete inexcitahility lasts during the time of systole ; 

 afterwards, during the diastole and pause, the excitability recovers itself, not 

 suddenly but gradually, so that, according to Engelmann's l latest researches, 

 the power of response to very weak stimuli is not regained until after the 

 completion of the diastole. By the refractory period, therefore, we mean 

 at the present day a sudden complete loss of excitability, caused by the 

 contraction, with a gradual slow recovery of excitability after the contraction 

 is over. It follows from Engelmann's measurements that the inner stimuli 

 upon which the normal rhythmical beat depends must be extremely weak. 



What is true of the rhythmical tissue in the sinus and large veins on 

 which the beat depends, is true also of the less rhythmical tissue 

 over which the contraction wave passes, such as the reticulated tissue of 

 auricle and ventricle ; here, too, each contraction is followed by a slow 

 recovery of conductivity, and the question whether the next contraction 

 wave, which reaches any particular spot, will be able to cause a further 

 travelling of the wave or not, depends upon the extent to which the 

 muscular tissue there has been able to recover its conductivity. 

 Anything, therefore, which diminishes the conductivity of any part 

 of the tissue, will tend to cause a block in the contraction wave at 

 that part ; and the question whether any block will be manifested 

 or not, will depend upon the rate of the heart-beat in relation to the 

 extent of the diminution of conductivity in the damaged part. A 

 very excellent example of the dependence of a blocking upon the rate of 

 beat can be shown in the following manner : — 



Slit up the auricle of the tortoise so that every contraction is just able to 

 pass the blocking point, then remove the sinus and apply electrodes, arranged 

 so as to give single induction shocks at regular intervals to the part of As 

 near the sinus. Owing to the removal of the sinus, tbe auricle and ventricle 

 remain still, and now by means of the electrodes on As, we can produce a 

 regular artificial rhythm, the rate of which can be varied at will ; we can, for 

 instance, send through single shocks every ten seconds, with the result that 

 every contraction of As causes a contraction of Av, and therefore of V. We 

 may then see that single induction shocks sent in to As every five seconds 

 will cause a regular series of contractions in As, but only every second 

 contraction can pass the blocking point and cause a contraction of Av, and 

 therefore of V. We see, in fact, a longer time after a contraction is required 

 for the recovery of the conductivity of the tissue at the blocking point than of 

 that of the rest of the auricle. 



This experiment illustrates many important facts concerning the 



heart : it shows that that part of the heart which possesses the quickest 



rhythm must of necessity be the leader ; under normal conditions, 



therefore, the great veins and sinus. It shows also, that in the beating 



1 Arch./, d. ges. Physiol., Bonn, 1894, Bd. lix. S. 309. 



