536 CIRCULATION OF BLOOD AND LYMPH. 



traction wave of the auricle is not transmitted directly to the 

 ventricle, but indirectly through the conducting system, the A-V 

 bundle. Some authors* believe, in fact, that the excitation proc- 

 ess which originates in the sino-auricular node spreads indepen- 

 dently to the auricular muscle, on the one hand, and to the ven- 

 tricular muscle on the other. The path in the latter case is first 

 to the auriculoventricular node, and thence through the auriculo- 

 ventricular 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 constitute, according to Keith and 

 Flack, t a remnant of the original invagination of muscular tissue 



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. 



from the auricular ring (Fig. 224), through which auricle and 

 ventricle are connected in the lower vertebrates. 



The Sequence of the Heart Beat. — In the rhythmic heart beat 

 it is believed that an excitation or impulse arises first in the sino- 

 auricular node and thence spreads over the heart in the form of a 

 wave or pulse. The wave of excitation is followed by a wave of 

 contraction. That the excitation starts in the sino-auricular node 

 is indicated by the fact that the first appearance of negativity, 

 as detected by the galvanometer (see following paragraph), occurs 



*Eyster and Meek, "Heart," 15, 119, 19U 



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

 43, p. 1. 



