THE CIRCULATION OF THE BLOOD. 293 



auricles and ventricles. When electric stimuli were applied to other 

 regions of the superior vena cava or sulcus no effects were noticeable. 



Mechanic stimulation as pinching the node with forceps called forth 

 similar results. Destruction of the node, however, had no effect on 

 the rhythm. The application of a weak solution of atropin abolishes 

 the customary effects of both vagus and sympathetic nerve stimulation. 

 From the foregoing facts it may be assumed that the usual seat of origin 

 of the stimulus to the cardiac contraction is the sino-auricular node, but 

 as the heart continues to contract after the node is destroyed, it is 

 evident that some other portion or portions of the auricular wall are 

 also capable of developing under the circumstances an adequate stimulus. 



A further proof that the sino-auricular node is the initiator of the car- 

 diac contraction is found in its change of electric potential. It has long 

 been established that when any portion of living material enters into a 

 state of activity it becomes electro-negative to all other portions which 

 are at the same instant electro-positive. Lewis with special electrodes 

 in connection with a string galvanometer found in a series of determina- 

 tions that with the beginning of a cardiac contraction, the sino-auricu- 

 lar node was the point of initial electro-negativity, a fact that is in accord 

 with the general truth that the region of greatest activity exhibits the 

 greatest degree of negativity. The sino-auricular node may therefore 

 be regarded as the primary seat of the stimulus or excitation process 

 and the initiator of the beat. 



From the sino-auricular node the excitation process is conducted to 

 the auricles and ventricles in quick succession, though between the end 

 of the auricular contraction and the beginning of the ventricular con- 

 traction there is also a perceptible interval similar to that observed in the 

 frog heart. For a long time it was assumed that the excitation process 

 and the contraction wave passed directly from auricles to ventricles 

 across the auriculo-ventricular junction as in the frog and that the interval 

 between the auricular and ventricular contractions was due to an interfer- 

 ence with the passage of the contraction wave across the junction because 

 of the extreme scarcity of the muscle fibers in this region or to their 

 embryonic character. In recent years, however, this view has been 

 abandoned because the real bond of union between the auricular and 

 ventricular tissues, across which the excitation process passes, has been 

 found, as stated on page 268, in the system of muscle-fibers, described 

 in part by His, Retzer and Braunig, and Tawara and in part by Keith 

 and Flack and known as the conduction system of the heart. This 

 system it is believed constitutes the anatomic and physiologic path 

 across which the excitation process passes from auricles to ventricles. 

 The excitation process originating in the sino-auricular node passes 

 first to the auricular walls, exciting them to contraction and then into 

 and through the auriculo-ventricular bundle to the ventricular walls, 

 exciting them to contraction. The supposition that this was the case 

 has been demonstrated by Hering and others who succeeded in dividing 

 the muscle-bundle in the excised hearts of rabbits and dogs, kept 

 actively beating by perfusion with Ringers' solution. On division 

 of the bundle both auricles and ventricles continued to beat though 



