302 TEXT-BOOK OF PHYSIOLOGY 



pass over the auricles and 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 occupied by the sino-auricular node, the lower portion of the 

 . sulcus terminalis. With the view of determining the truth of this as- 

 sumption Flack performed 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 ven- 

 tricles. 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 vagus and perhaps 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 normally the initiator 

 of the cardiac contraction is found in its change of electric potential. 

 It has long been established that when any portion of living materials 

 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 determinations that with the beginning of a cardiac contrac- 

 tion, the sino-auricular 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 



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 taenia terminalis of the right auricle, and known as the sino-auricular 

 node. 



