THE AUTOMATICITY OF THE HEART 



205 



generated at this point and conduction proceeds in an orderly manner in all 

 directions not only toward the ventricle but out over the atria and on to the 

 veins themselves. These last points have been most carefully studied and 



FIG. 1760. Normal electrocardiogram of man, lead II. 



established by Lewis and a number of his associates. Tawara, Eyster and 



Meek, Wilson, Greene and Gilbert, and others have explained that under 



certain conditions rhythm may arise lower down in the conducting tissue, 



namely, at the auriculo-ventricular node or center or even as low as the 



bundle branch (Greene and Gilbert). In this case the conduction is from 



the point of rhythm production not 



only toward the ventricles which 



contract together in response to the 



stimulus reaching them from the 



auriculo-ventricular node, but con- 



duction is reversed toward the auri- 



cle leading to delay in the auricular 



contraction in comparsion with the 



normal. 



It is obvious that the tissue of 

 the bundle system is differentiated 

 highly in the direction of rhythm 

 production and of conductivity. 

 When once the rhythm arises in the 

 sino-auricular node the stimulus is 

 conducted five to ten times more 

 rapidly over the bundle system to 



FIG. 1766. The figure gives the times 

 the nroner auricular tissue and to of be g mnin g contraction of the respective 



> 

 the larger ventricular mass than 



points measured in fractions of a second 

 after the beginning of the R in lead II. The 



mass 



would be possible through the slower 



conducting cardiac muscle. This area and occurs latest near the base of the 

 insures the contraction of the entire aorta - ( Lewis -) 



ventricular walls at more nearly the same instant than would otherwise 

 be the case. In fact, the arrangement of muscle bands of the ventricle 



