THEORIES OF THE HEART-BEAT IQ7 



from any other portion. With such a clamp the contraction waves can be 

 more or less completely interrupted in their passage from the sinus end of 

 the heart past the clamp toward the ventricular end. If the clamp is com- 

 plete, so as to interrupt the physiological continuity between the parts, then 

 any contractions in the apical portion \vill be entirely independent of those 

 in the sino-auricular portion. If the blocking is partial only, then the ven- 

 tricular end of the heart always contracts in unison with the sino-auricular, 

 although its rate may be as i to i, i to 2, i to 3, etc. In other words, only 

 every second or every third sino-auricular contraction will be able to pass 

 the block. 



The effects of blocking are due to the interruption of muscle continuity 

 rather than nerve continuity. This is beautifully demonstrated by an experi- 

 ment of zigzag cutting of the ventricle in the terrapin, since it cannot be 

 supposed that any nerves would pass through the ventricular mass by such 

 a zigzag course. In this experiment the contraction wave passes down 

 over the muscle and around the end of the cuts until it reaches the apex, 

 and the apex contracts in sequence with the auricle and base of the ventricle. 

 If the zigzag cuts are made almost complete so as to reduce to a minimum 

 the muscular tissue \vhich bridges from one cut to the next, then it happens 

 that occasional contractions will be unable to pass and the apex contracts 

 in the ratio of i to 2, or i to 3, etc., as described above. Thus, division of 

 the muscle has the same effect as partial clamping in the same position. 



It was thought for a long time that in the mammal there was no mus- 

 cular continuity between the auricles and ventricles to conduct the contrac- 

 tion wave and that this was an insurmountable difficulty in the way of accept- 

 ing the myogenic theory of the heart beat. In 1893 Kent described a bundle 

 of muscle fibers arising in the wall of the right auricle and near the septum 

 and running down into and forming a muscular connection with the ventri- 

 cles. This bundle was also independently described by His, Jr., and gen- 

 erally bears his name. This band is called the atrioventricular bundle. The 

 bundle consists of a somewhat less differentiated type of cardiac fibers 

 imbedded in a syncytium and not only connects the auricle and ventricle, but 

 forms a network of strands among the proper fibers of the auricles and 

 ventricles. Miss De Witt has recently (1909) made an excellent model of 

 this system which is reproduced in figure 176. 



The demonstration of the atrioventricular bundle has proven to be of 

 the strongest support to the myogenic theory. Erlanger has recently shown, 

 by an ingenious device for partially clamping this muscular band, that even 

 the mammalian ventricle exhibits the phenomenon of heart block. In his 

 experiments the ventricle contracts in unison with every auricular contrac- 

 tion, or only every second or every third, according to the degree of blocking. 



It was shown long ago (by Merunowicz in 1875) that the isolated apex 

 of the ventricle of the frog remains quiet when filled with blood, but readily 



