THE CAUSATION OF THE HEART-BEAT 



1073 



venous pulse at a rhythm entirely independent of the ventricular pulse. 

 The automatic activities of auricle and ventricle are in fact dissociated 

 (Fig. 437). At certain intervals, or at certain stages of the disease, 

 the fibres of the bundle may present only a partial block, so that the 

 ventricle responds once to every second contraction of the auricle. 

 The course of the fibres of the auriculo-ventricular bundle probably 

 throws light on the path taken by the excitatory process in its passage 

 through the heart. We may regard the contraction of the mammalian 

 heart as starting in the tissue between the superior cava and the coronary 

 sinus, the sino-auricular node, and as spreading from here to the 

 auricular septum and thence over both auricles. At the same time 

 it is spreading along the auriculo-ventricular bundle to the ventri- 



Rad.art. 



FIG. 437. Simultaneous tracings of the jugular venous pulse and the radial 

 arterial pulse, from a case in which the A.V. bundle was destroyed by 

 disease. The contractions of the auricles are marked by the a waves 

 on the venous pulse. They are more rapid than and quite independent 

 of the ventricular contractions. (MACKENZIE.) 



cular septum, to the papillary muscles, and to the rest of the heart, 

 reaching first the base and later spreading to the apex. Last of all 

 it reaches the neighbourhood of the pulmonary artery, that part, in 

 fact, which corresponds to the bulbus aortse. We can thus under- 

 stand why we get the polyphasic variation on leading off the mam- 

 malian heart in situ to the capillary electrometer, why, as shown by 

 Roy and Adami, the papillary muscles contract slightly before the 

 rest of the ventricular wall, and why under certain conditions, e.g. of 

 vagus stimulation, one may get a ventricular contraction at a normal 

 rhythm and strength, while the auricular contractions, as judged by 

 the movements of the auricular appendages, have been reduced to 

 disappearance. In the last case the rhythmic excitatory process 

 can still spread from its point of origin along the auriculo-ventricular 

 bundle to the ventricles. Numerous nerve fibres and ganglion-cells 

 are found to accompany the muscle fibres of the auriculo-ventricular 

 bundle. We have, however, no reasons for regarding the nervous 

 structures as concerned in the propagation of the excitatory wave. 



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