THE CAUSE OF THE RHYTHMICAL ACTIVITY OF THE HEART 187 



The number of authors who have taken the side of this hypothesis has con- 

 stantly increased a fact not difficult to understand in view of the great logical 

 precision with which it has been developed. Nevertheless it appears that there 

 are still certain difficulties to be overcome. For example, the questions: how it 

 transpires that the vena3 cavas and the pulmonary veins, although separated by a 

 considerable distance, are roused to action simultaneously; and how the nor- 

 mal coordination of the heart muscle, as well as the disturbances of the same 

 by electrical stimulation, by anaamia and by mechanical abuse (cf. page 183), 

 are brought about have not yet been satisfactorily answered. 



Any explanation of the origin of the contraction must take account also 

 of the question as to how the excitation is propagated through the heart, 

 whether through the musculature itself or through a set of nerves; because, 

 if the muscular theory of the heart beat is correct, it follows almost of neces- 

 sity that the propagation of the stimulus is muscular, and vice versa. 



If the ventricle be artificially fed by the coronary arteries, and be divided 

 up into different parts connected together by the arterial branches, and joined 

 by thin muscular bridges, all the portions beat synchronously, no matter in 

 what direction the cuts are made. After section of the muscular bridge, the 

 synchronism stops and each part beats after its own rhythm, but does not 

 show any signs of fluttering (Porter). 



Neither these phenomena nor the corresponding observations made on the 

 frog's heart are, however, to be regarded as conclusive proof of the muscular 

 theory of propagation, inasmuch as the cardiac muscle fibers themselves are 

 surrounded by nerves which could only be excluded by complete division of the 

 last muscular bridges, and might therefore cause the synchronism. 



The passage of the excitation from the auricles to the ventricles once con- 

 stituted a serious difficulty in the way of a muscular theory. It was supposed 

 that the musculature of these two divisions were completely separated. It has 

 been shown, however, that direct muscular connections are indeed present 

 between the two (Kent, His, Jr.), and the excitation might therefore pass 

 from the auricle to the ventricle without any participation of nerves. 



As mentioned above, a certain time intervenes between the auricular and 

 the ventricular systoles. From the standpoint of the ganglion hypothesis this 

 delay would not be difficult to explain, since we know from many other ob- 

 servations that ganglia in general do delay the propagation of impulses. But 

 the muscular theory also has been able to offer an explanation by supposing 

 that the transmission of the motor stimulus takes place very quickly within 

 each separate division of the heart, while over the cells which form the con- 

 nections between the separate parts, the transmission is very slow, just as it 

 would be over smooth or embryonic muscles. 



What appears to disprove conclusively the hypothesis of nervous propagation, 

 and is therefore a very weighty support for the muscular hypothesis, is the fol- 

 lowing. If the auricle of a frog's heart be injured by a light pinch, the rhythmic 

 excitation travels just as before over the entire heart. But if the vagus be now 

 stimulated, as long as the inhibitory action lasts, the ordinary excitation passes 

 only up to the injured spot and stops there. If the propagation were through a 

 nervous mechanism, we should have to suppose that the conductivity is tem- 

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