I9I4.] MECHANISM OF THE HEART-BEAT. 301 



remained at its original level, but became incorporated in the wall 

 of the right auricle as the sinus node ; the other portion, the left, 

 became dislocated and moved downward the distance of a whole 

 chamber, becoming, in the adult mammalian heart, the auriculo- 

 ventricular node which lies between the auricles and the ventricles. 

 The importance of these changes in position, from the point of view 

 of innervation, lies in the fact that the cardiac branches of the two 

 vagus nerves, which were distributed symmetrically in the primitive 

 heart, have followed the changed positions of the structures they in- 

 nervated originally and have become incorporated with them in their 

 new situations. Consequently, the nodes and their nerves have 

 assumed different functions. We were led to believe that this change 

 must have taken place, first by clinical observation, and we have tested 

 this hypothesis by experiment. As the result of experiments on 

 many dogs, we were able to decide that when the right vagus nerve 

 was electrically stimulated, the heart stopped beating. But when the 

 left vagus nerve was stimulated, the auricles did not stop beating, 

 but continued, though often at a slower rate. The striking thing 

 now observed was that these auricular beats failed, either entirely or 

 only occasionally, of being followed by a ventricular response 

 (Plate II). 



These were the facts. In the light of current teaching, the mo- 

 tions of the heart are initiated and rate is maintained by the sinus 

 node. Impulses so initiated are conducted from the auricles to the 

 ventricles over the narrow conduction path, with which we have be- 

 come familiar. It follows that, when we find the heart stop as the 

 result of a stimulus, we must assume the stimulus to have been dis- 

 tributed to that portion of the heart where the pace-making function 

 resides, that is to say, at the sinus node. In the same way, when a dis- 

 turbance in coordination between auricles and ventricles occurs, we 

 have sufficient evidence to indicate that this occurs as the result of an 

 eflfect produced at the junctional connecting tissues. We must, there- 

 fore, conclude that if stimulation of the left nerve brings about this 

 disturbance, it must necessarily be distributed to this portion of the 

 heart, that is to say, to the conducting system. Although we think 

 that these sites, the sino-auricular and the auriculo-ventricular nodes, 

 are the main terminals of these nerves, it is clear that other functions 



