276 THE MECHANICS OF THE HEAET 



ferred to as the auriculoventricular node. Having pierced the fibrous 

 tissue of the groove, it passes along the interventricular septum immedi- 

 ately below the endocardium, and divides eventually into two branches. 

 This bifurcation takes place at about the point where the posterior 

 and median flaps of the aortic valve are joined. The main bundle of 

 the average human heart is about 18 mm. in length and 1.5 to 2.5 mm. 

 in width. One of its branches is distributed to the right, and the other 

 to the left ventricle, but before the distant musculature is reached, the 

 bundle spreads out fan-Hke and forms an intricate network of fibers. 

 This peripheral ramification was clearly recognized by Pur kin je, but 

 no particular attention was paid to it until Tawara^ proved that its 

 constituents are intimately connected with the bundle of His. 



Fig. 136. — Left Ventricle Laid Open to Display the Interventbicular Septum. 

 The Course of the Auriculoventricular Bundle and Its Ramifications are Shown 

 IN Black. {After Tawara.) 



It has previously been stated that in the lower animals the contrac- 

 tion wave originates in the sinus venosus, and eventually reaches the 

 apex of the ventricle by travelHng across bridges of muscle tissue. 

 The sinus, therefore, must give lodgment to a certain group of cells 

 in which the wave of excitation is generated. For this reason, this 

 particular area of the sinus has been designated as the pacemaker of 

 the heart. 



Very similar conditions are met with in the mammals. Thus, the 

 embryonic heart presents the sinus venosus as a separate cavity which 

 is bounded by the orifices of the venae cavse, the Eustachian valve and 

 the interauricular septum. The adult organ, on the other hand, does 

 not possess a distinct vestibular enlargement, because the sinus has 

 been incorporated in the main cavity of the auricle. The remnants 

 of the Eustachian and venous yalves, however, are still discernible 

 in conjunction with the taenia terminalis. Even a very casual observa- 

 1 Pfliiger's Archiv, cii, 1906, 300. 



