150 THE CIRCULATION OF THE BLOOD AND LYMPH 



with this, it has been shown that the ventricles in the dog and cat 

 continue to beat in unison, after the attempt has been made to 

 sever any nerves connecting them by extensive zigzag incisions, so 

 long as they are united by a narrow bridge of muscle (Porter). 



In disease, interference with the conduction of the stimulus from 

 auricles to ventricles along the atrio-ventricular bundle is a not un- 

 common phenomenon. According to the degree of interference, the 

 ventricular contraction may be simply delayed, or only a certain pro- 

 portion of the auricular contractions (every second, every third, or 

 every fourth) may be conducted to the ventricle, or, finally, the block 

 may be complete, and the ventricle then contracts quite independently 

 of the auricle, the stimulus to contraction originating, perhaps, in the 

 uninjured portion of the bundle below the seat of the block. These 

 conditions are most easily recognized by comparing tracings simul- 

 taneously obtained from the jugular vein and the radial artery or apex- 

 beat (p. 90). When the block is complete the rate of the ventricle is 

 very slow (about 30 in the minute, or less), the time of the ventricular 

 beat is clearly unrelated to that of the auricular, and the stability of 

 the ventricular rhythm is abnormally great, such circumstances as 

 usually cause a marked increase in the pulse-rate mental excitement, 

 for instance affecting it little or not at all. This is the condition in 



Fig. 67. Polygraph Tracing fr ;m a Case of True Bradycardia (Carter). The iower 

 trace is the radial, the upper the jugular. Time-trace, fifths of a second. 



the so-called Stokes-Adams disease. In some of these cases pathological 

 (syphilitic) changes in the A-V bundle have actually been discovered 

 at necropsy. In others there is some reason to believe that abnormal 

 excitation of the cardio-inhibitory nerves may be responsible even for 

 long-continued block, especially when the conductivity of the bundle 

 has been already permanently diminished. 



Cases of slow heart are also known in which there is no block in 

 the conduction system, but the original rhythm of the auricle is slow 

 (so-called true bradycardia, Fig. 67). 



Kent has pointed out that the muscular connection between the 

 auricles and ventricles is not single and confined to the A-V bundle, 

 but multiple, and that the co-ordinated action of the chambers of the 

 heart is to some extent dependent upon the integrity of muscular 

 connections other than that which exists in the A-V bundle. One of 

 these he describes as the ' right lateral connection,' at the junction of 

 the right auricle, the right ventricle, and the tricuspid valve, at the 

 right -hand margin of the heart. The existence of this additional con- 

 nection, the importance of which relatively to that of the A-V bundle 

 need not be the same in every heart, may explain otherwise puzzling 



