THE CAUSATION OF THE HEART-BEAT 951 



as the sino-auricular node and the auriculo-ventricular node. The sino- 

 auricular node (Fig. 431) lies in the groove between the superior vena cava 

 and the right auricle. The auriculo-ventricular node lies at the base of 

 the auricular septum on the right side, below and to the right of the opening 

 of the coronary sinus. From this point a bundle of muscular fibres (the 

 bundle of His or the auriculo-ventricular or A.V. bundle) runs along the top 

 of the interventricular septum just below its membranous part and then 



Superior Vena Cava. _/.... 



'Appendix. 



Foramen Ovate. 

 Auriculo - Vent 



Inferior 



renar-^ 

 Cava 



FIG. 431. 



divides into the right and left septal divisions, which pass down in each ven- 

 tricle on the interventricular septum into the papillary muscle arising from 

 the septum. Each half of the bundle gives off several branches which break 

 up more and more, finally forming a reticulated sheet of tissue over the 

 greater part of the interior of the ventricles just below the endocardium. 

 The fibres composing this tissue are more primitive in character than the 

 rest of the cardiac musculature and have long been distinguished as the 

 ' fibres of Purkinje.' In them the fibrillation is confined to the periphery of 

 the muscle cell. They are distinguished by a high glycogen content. They 

 may be regarded as a part of the muscular wall of the heart specially differen- 

 tiated for the rapid conduction of the excitatory process to all parts of the 

 ventricles (Figs. 432 and 433). 



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. 



