1 8 2 THE CONTRA CTION OF CARD I A C MUSCLE. 



easy to obtain a partial block, so that every second contraction passes, 

 which may last a long while unaltered. Finally, if the cut is more 

 severe still, then the block becomes absolute, and no contractions are able 

 to pass ; consequently Av and the ventricle remain absolutely still until 

 they commence to beat with their own independent rhythm. Even 

 when the block is apparently absolute, as long as a very small bridge of 

 intact tissue remains, it is as a rule possible by vagus stimulation and 

 other methods to overcome the block to a greater or less extent, with 

 the result that as soon as a contraction passes, the quiescent ventricle 

 immediately responds with a contraction. 



A precisely similar experiment can be performed on the frog by 

 means of the screw-clamp in the auriculo-ventricular groove. 1 Accord- 

 ing to the tightness of the clamp, the ventricle can be made to beat 

 synchronously with the auricles, to respond to every second contraction 

 of the auricles, to respond to every third, fourth, or other contraction, 

 or to remain quiescent. 



In fact, we see that we can artificially reproduce, by means of 

 blocking to a greater or less extent the passage of the contraction wave 

 from one portion of auricular muscle to the other, all the phenomena 

 which are characteristic of the normal blocking at the auriculo-ventricular 

 junction between the contractions of the auricle and ventricle. The 

 muscular tissue of all parts of the heart can be blocked in this way — 

 ventricle or auricle, it is all the same ; by means of the clamp the apex 

 of the ventricle can be made to respond to only every second contraction 

 of the base, just as the whole ventricle to every second contraction of 

 the auricle ; it matters not the least whether nerve trunks and ganglion 

 cells are present, all that is necessary is a continuity of muscular 

 substance along which a contraction can travel. 



Clearly, then, the natural pause between the contractions of auricle 

 and ventricle, and the sequence of ventricular upon auricular beat, can 

 be explained without the intervention of any special nervous mechanism, 

 if the contraction of the heart signifies the passage of a contraction 

 wave from one end of the heart to the other, over muscular tissue 

 of varying power of conductivity. If part of the muscular tissue has 

 been modified, so as to contract with greater rapidity, the contraction 

 wave will necessarily pass more quickly over that part than over the 

 more primitive, less modified tissue of other parts ; in other words, the 

 contraction wave will pass more rapidly over the reticulated tissue of 

 auricle and ventricle than over the unaltered canalis auricularis, i.e. 

 auriculo-ventricular muscular ring, or the eonus arteriosus. 



Precisely as in the question of the rhythm, so also of the sequence, 

 the more rhythmical tissue of the auriculo-ventricular ring, and the 

 comes arteriosus, conducts the contraction wave more slowly than the 

 less rhythmical, more rapidly contracting tissue of the ventricle and 

 auricle proper. 



In strict accordance with this explanation of the meaning of the sequence, 

 are the observations of Fano - on the rate of the contraction wave in the heart 

 of the embryo. He found that the rate of travel of the wave in the heart of a 

 chick on the third day of incubation, at 38° C, varied from 11*5 to 3"6 mm. per 

 second, very much less than the rate observed by Engelmann, 3 and by 



1 Phil. Trans., London, 1882, pp. 999 and 1031. 



2 Arch, per le sc. med., Torino, 1890, vol. xiv. 



s Arch./, d. ges. Physio/., Bonn, Bd. xi. S. 480. 



