952 PHYSIOLOGY 



The auriculo-ventricular bundle forms the only continuous muscular 

 tissue between the auricles and ventricles, and destruction of it causes com- 

 plete abolition of the normal sequence of beat between auricles and ventricles. 



FIG. 432. Left ventricle laid open to display the interventricular septum on 

 which the course of the left division of auriculo-ventricular bundle and its 

 ramifications are shown in black. (After TAWARA.) 



FIG. 433. Fibres of Purkinje, from the subendocardial network. (TAWARA.) 



By leading off different parts of the heart to the string galvanometer it is 

 possible to determine the time relations of the excitatory process. It is 

 then found that the mass of Purkinje tissue, known as the sino-auricular 

 node, is the starting-point of the excitatory process concerned in each heart- 

 beat. It is therefore spoken of as the ' pace-maker ' of the heart. At each 

 beat a contraction starts at the sino-auricular node, spreads a short way up 



