THE HEART BEAT. 539 



over the inner surface of the ventricles, constituting; what were 

 formerly designated as Purkinje fibers. The auriculoventricular 

 node in the interauricular septum is connected with the muscula- 

 ture of the auricles, and through muscle bundles in the septum with 

 the remnant of sinus tissue (sino-auricular node) at the mouth of 

 the superior vena cava. The main bundle and the larger branches 

 of this system are surrounded by fibrous tissue, and it is uncertain 

 whether or not it actually contracts during the beat of the heart, 

 but there is little doubt that it constitutes a conducting system of 

 modified muscular tissue through which the excitation is conveyed 



Fig. 228. 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 divi- 

 sion 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 ven- 

 tricle is incomplete in the model, as the outer wall of this ventricle had been removed in the 

 dissection. 



from right auricle to the ventricles, or perhaps, more correctly 

 speaking, from the auriculoventricular node to the ventricular 

 musculature. The contraction wave of the auricle is not trans- 

 mitted, therefore, directly to the ventricle, but indirectly through 

 the conducting system, the A-V bundle. Some authors* believe, 

 in fact, that the excitation process which originates in the sino- 

 auricular node spreads independently to the auricular muscle, 

 on the one hand, and to the ventricular muscle on the other. The 

 path in the latter case is first to the auriculoventricular node, and 

 thence through the auriculoventricular bundle to the interior of 

 the ventricles (papillary muscles). The A-V node and the main 

 bundle in the human heart are small in size about 18 mm. long, 

 and from 1.5 to 2.5 mm. wide, and they and their dependent 

 system of fibers or strands in the interior of the ventricles con- 

 * Eyster and Meek, "Heart," 5, 119, 1914. 



