THE CIRCULATION OF THE BLOOD 



303 



between the auricular and ventricular contractions was due to an interfer- 

 ence with the passage of the contraction wave across the junction because 

 of the extreme scarcity of the muscle fibers in this region or to their 

 embryonic character. In recent years, however, this view has been 

 abandoned because the real bond of union between the auricular and 

 ventricular tissues, across which the excitation process passes, has been 

 found, as stated on page 271, in the system of muscle-fibers, described 

 in part by His, Retzer and Braunig, and Tawara and in part by Keith 

 and Flack and known as the conduction system of the hearty page 271. This 

 system it is believed constitutes the anatomic and physiologic path 

 across which the excitation process passes from auricles to ventricles. 

 The excitation process originating in the sino-auricular node passes 

 first to the auricular walls, exciting them to contraction and then into 

 and through the auriculo-ventricular bundle to the ventricular walls, 

 exciting them to contraction. The supposition that this was the case 

 has been demonstrated by Hering and others 

 who succeeded in dividing the muscle-bundle 

 in the excised hearts of rabbits and dogs, kept 

 actively beating by perfusion with Ringers' 

 solution. On division of the bundle both 

 auricles and ventricles continued to beat 

 though with different rates and independently 

 of each other. These and other experiments 

 of a similar character have demonstrated be- 

 yond question that the auriculo-ventricular 

 bundle with its widespread ramifications is 

 the true conducting system between auricles 

 and ventricles. In this system the sino-auric- 

 ular node is regarded as the primary domi- 

 nating "pace maker" of the rate and rhythm 



r \ , , J . 



of the heart. Inasmuch, however, as the 

 heart will continue to beat, after the destruc- 

 tion of the sino-auricular node it is evident 

 that it is not the only region that can initiate 

 the contraction. Whether the contraction under such circumstances 

 is due to an excitation arising in some other portion of the auricular wall 

 or in the subsidiary auriculo-ventricular node is a subject of discussion. 

 The cause assigned by Tawara, for the interval between the auricular 

 and ventricular contraction is not so much the embryonic character of 

 the fibers of the system, as it is the length of the system as a whole, 

 which he estimates at from 4 to 6 centimeters.. This time, estimated 

 from the beginning of the auricular systole to the beginning of the 

 ventricular systole amounts to from o.i to 0.2 second. The interval 

 between these two events, determined from the time between the oc 

 currence of the a and the c or s waves on the jugular pulse tracing is 

 known as the a-c interval, or the As-Vs interval. 



With the mammalian heart as with the frog heart it is possible to 

 increase the length of the interval between the auricular and the ven- 

 tricular contraction, the inter-systolic period, by compression of a por- 

 tion of the tissues between auricles and ventricles including presum- 



FIG. 134. The Erlanger 

 heart-block clamp compress- 

 ing the auriculo-ventricular 

 bundle (AVB). SM, Septum 

 membranaceum; MV, mitral 

 valve. (Hirschf elder .) 



