294 TEXT-BOOK OF PHYSIOLOGY. 



with different rates and independently of each other. These and 

 other experiments of a similar character have demonstrated beyond 

 question that the auriculo-ventricular bundle with its widespread 

 ramifications is the true conducting system between auricles and 

 ventricles. In this system the sino-auricular node is regarded as the 

 primary dominating "pace maker" of the rate and rhythm 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 be- 

 tween these two events, determined from the 

 time between the occurrence of the a and 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 

 ventricular contraction, the inter-systolic period, 

 by compression of a portion of the tissues be- 

 tween auricles and ventricles including pre- 

 sumably the central part of the conducting sys- 

 tem, the muscle bundle of His. This has been 

 accomplished in the dog by Erlanger by means 

 of a specially devised hook clamp (Fig. 137), 



which consists of an L-shaped hook of steel wire the arm of which 

 can be made to approach a brass block by means of a bolt and 

 screw. The L-shaped hook is inserted into the right wall of the 

 aorta, then passed downward and backward into the left ventricle, 

 then pushed through the ventricular septum into the right ventricle. 

 In this position it lies under the auriculo-ventricular bundle. Com- 

 pression is now brought about by approximating the hook to the 

 brass block by means of the nut. When the compression is brought 

 about suddenly and completely the ventricles at once cease beating, 

 though the auricles continue to beat with their customary rate and 

 regularity. After a variable period of time, varying from a few 

 seconds to 70 seconds, during which the ventricles are relaxed and 

 gradually filling with blood from the auricles, the ventricular beat 

 returns, at first slowly but with a gradually increasing frequency until 

 a definite but a comparatively slow rate is attained. The rhythm thus 

 developed is termed the ideo-ventricular rhythm. 



FIG. 137. The Erlanger 

 heart-block clamp compress- 

 ing the auriculo-ventricular 

 bundle (AVB). SM, Septum 

 membranaceum; MV, mitral 

 valve. (Hirschf elder.} 



