THE CARDIAC CYCLE (REVOLUTIO CORDIS) 277 



tion of the beating mammalian heart must show that the contractions 

 begin in the tissue situated at the junction of the superior vena cava 

 with the right auricle. This region which corresponds to the sinus 

 reuniens of the embryonic organ, constitutes the pacemaker of the 

 higher type of hearts. One of these veno-auricular accumulations 

 of tissue has been adequately described by Wenkelbach. In confirma- 

 tion of this work, Flack and Keith 1 have applied to this area the name 

 of sino-auricular node, the further assertion being made by these in- 

 vestigators that it is intimately connected with the bundle of His. 

 It must be concluded, therefore, that the stimulus to contract 

 arises in the specialized tissue forming the sino-auricular node. When 



FIG. 137. THE AURICULOVENTRICULAR BUNDLE AND ITS TERMINAL RAMIFICATIONS 

 IN THE INTERIOR OF THE VENTRICLES (FROM MODEL CONSTRUCTED BY Miss DE WITT 

 ON BASIS OF DISSECTIONS). 



The division of the bundle into right and left branches is shown, and the ramifications 

 of each of these branches in the interior of the right and left ventricles. The branching 

 system in the left ventricle is incomplete in the model, as the outer wall of this ventricle 

 had been removed in the dissection. (Howell.} 



this area is warmed or cooled, the frequency of the heart as a whole is 

 either increased or decreased; and this effect cannot be produced if 

 other regions of this organ are subjected to changes in temperature. 2 

 Furthermore, it has been found by Gaskell 3 that the rhythmic power 

 of the muscle tissue of the venous vestibule is greater than that of the 

 ventricular musculature. 



The wave of excitation is propagated from the sino-auricular node 

 to the different segments of the auricles as well as to the auriculoven- 

 tricular node. Although the statement is generally made that the 



1 Jour, of Anat. and Physiol., xli, 1906, 172, and M. Flack, Jour, of Physiol., 

 xli, 1910, 64. 



2 Erlanger and Blackman, Am. Jour, of Physiol., xix, 1907, 125; also see: 

 Schlomovitz and Chase, Am. Jour, of Physiol., xli, 1916, 112. 



3 Schafer's Textbook of Physiol., 1900. 



