THE VASCULAR MECHANISM. 



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effects which follow either disease of the bundle in man, or division of 

 the bundle in animals. In man the continuity of the bundle may be 

 partially or completely destroyed by disease, the result being known as 

 partial or complete " heart block." In partial heart block one out of 

 every two or three auricular beats is conducted to the ventricle, the latter 

 beating at half or a third the rate of the auricles (2 : 1 or 3 : 1 rhythm). 

 In complete heart block the rhythm of the auricles is unaffected, 



FIG. 79. Diagram to show the distribution of the bundle of His (in red) in the 

 wall of the left ventricle. (After Tawara.) 



whereas the ventricles beat at a rate varying from 20 to 40 per minute ; 

 the patient usually exhibits characteristic symptoms (Stokes-Adams 

 disease), and a simultaneous record of the venous and radial pulse, 

 taken with the polygraph, shows that the rhythm of the ventricles is 

 quite independent of that of the auricles. 



When the bundle is divided in an animal, the rhythm of the 

 auricles remains unaltered, whereas the ventricles immediately begin to 

 beat at a slow rate having no relation to that of the auricles. Partial 

 heart block is sometimes seen in asphyxial conditions, even when the 

 bundle is intact. Partial or complete heart block can also be induced 

 in the frog's heart by compressing the tissue uniting the auricles and 



