THE CAUSATION OF THE HEART BEAT 



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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 

 the great veins, and along the auricular muscle in all directions. When it 



FIG 



. 441. Left ventricle, laid open to display the interventricular septum, on 

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

 its ramifications are shown in black. (After TAWARA.) 



FIG. 442. Fibres of Purkinje, from the subendocardial network. (TAWARA.) 



arrives at the auriculo-ventricular node, the impulse is carried on to the 

 ventricles along the auriculo-ventricular bundle, spreading along the branches 

 of this bundle to almost all parts of the ventricular muscle. Although 

 normally the sino-auricular node initiates each heart beat, this node can be 

 put out of action by injury or cooling without stopping the rhythmic 

 sequence of the heart beat, the office of pace-maker being now taken up by 



