THE HEART BEAT. 543 



heart, but there is little doubt that they constitute a conducting 

 system of modified muscular tissue through which the excitation is 

 conveyed from auricle to ventricle, or perhaps, more correctly 

 speaking, from the auriculoventricular node to the ventricular 

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

 mitted directly to the ventricle, but indirectly through the con- 

 ducting 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 firsi 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- 

 stitute, according to Keith and Flack, f a remnant of the original 



Fig. 226. 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. 



invagination of muscular tissue from the auricular ring (Fig. 224), 

 through which auricle and ventricle are connected in the lower 

 vertebrates. In addition to the auriculoventricular bundle Kent 

 describes two other connections between auricles and ventricles. 

 The more important of these lies on the right side of the heart, and 



* Eyster and Meek, "Heart," 15, 119, 914. 



t Keith and Flack, " Journal of Anat. and Physiology," 41, 172, 1906, and 

 43, p. 1. 



