60 THE HEART 



auriculo-ventricular node after a time takes up the function of 

 initiating the heart rhythm. Examples of this " nodal rhythm " 

 have been recorded clinically by Mackenzie. 



Coming now to the evidence of muscular conduction, it will 

 be remembered that there is no histological reason against it, 

 since the heart muscle is now regarded as a continuous network. 

 The earliest experiment on cold-blooded animals was the well- 

 known zig-zag experiment of Engelmann, in which the ventricle 

 of the frog is so cut that it is claimed that all conducting nerves 

 must be cut and yet the impulse still passes. Gaskell also showed 

 in the tortoise that section of the well-marked coronary nerve had 

 no effect on the passage of the impulse, whereas the clamping 

 of the muscular tissue induced varying degrees of allorhythmia 

 (2 A : IV, 3 A : IV, 4 A : IV), according to the tightness of the clamp. 

 Gaskell also showed that section of the part of A-V grooves 

 containing the most nerves had no effect upon the ventricular 

 rhythm. 



MacWilliam came to the conclusion that in the eel the con- 

 duction of the impulse was by muscle. He also drew attention to 

 the sino-ventricular rhythm, whereby the ventricle can follow the 

 rhythm of the sinus without the auricle being influenced. In this 

 case the impulse arising in the sinus passes down the basal wall 

 (Fig. 1) and thence to the ventricle. MacWilliam thought there 

 was evidence of such a rhythm in the mammalian heart. We 

 may here point out that it is indeed quite possible, for the exci- 

 tatory waves arising in the great veins may conceivably, under 

 certain conditions, pass down the interauricular septum which 

 corresponds to the basal wall, and thence to the ventricle, without 

 affecting the other parts of the auricle during its passage. 



As regards the evidence of muscular conduction in the 

 mammalian heart, Fredericq has brought good evidence to show 

 that such is the case in the auricles. He found that the two 

 auricles remained co-ordinate so long as he left a thin strip of 

 muscle connecting them. It did not matter where this strip was, 

 whether in neighbourhood of superior vena cava or of inferior 

 vena cava or coronary sinus. When, however, he cut this strip, 

 then he found that the two auricles became inco-ordinate. It is 

 interesting to note, however, that the ventricle continued to beat 

 with the same rhythm as the right auricle. He argued that con- 

 duction in this case was muscular and not nervous, since the bridge 



