THE CIRCULATION OF THE BLOOD 301 



pass over the auricles, thence to the ventricles. On the basis of this belief 

 it has been assumed that there is a specialized area in which the stimulus 

 arises and which determines the rate and rhythm of the entire heart. 

 At present it is believed that this area is identical with the region occu- 

 pied by the sino-auricular node, the lower portion of thesulcus tenninalis. 

 With the view of determining the truth of this assumption Flack per- 

 formed a number of experiments on the hearts of dogs, cats, and rabbits, 

 some of the results of which, abstracted from his paper, are as follows: 

 The application of cold either through metallic tubes or by means of an 

 ethyl chlorid spray, the remainder of the heart being protected, caused 

 slowing of both auricles and ventricles. Weak electric stimulation 

 caused marked inhibition of both auricles and ventricles; slightly 

 stronger stimulation caused a mixed effect of inhibition and acceleration, 

 the latter usually predominating; still stronger stimulation gave rise to 

 marked acceleration of the whole heart rhythm or an altered rhythm of 

 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 



^* a (the right duct of Cuvier); (2) the coronary sinus (the left duct of Cuvier); (3), a 

 stratum submerged beneath auricular tissue at the taenia terminalis; (4) the remnants of 

 the venous valves, i.e., the Thebesian and Eustachian valves (Flack). In addition there 

 is a remnant of primitive tissue at the sino-auricular junction, that is,- where the superior 

 vena cava joins the tsnia terminalis of the right auricle, and known as the sino-auncular 



joins 

 node. 



