THE PHYSIOLOGY OF THE HEARTBEAT 193 



ond, third, or fourth auricular contraction. If after such a complete or 

 partial heart-block has been instituted, the clamp is removed, it will 

 usually be found that the heart-block disappears and the auricular and 

 ventricular contractions fall back into their usual sequence. The im- 

 portance of this discovery, apart from its physiological interest, rests in 

 the fact that it is exactly duplicated in clinical experience. If the pulse 

 tracing of the radial artery is compared with that of the jugular vein 

 in certain types of heart disease, it will be found that the auricle is beat- 

 ing two or three times more quickly than the ventricles. In many of 

 these cases it has been found on autopsy that definite lesions often syphi- 

 litic in nature involve the auriculoventricular bundle. In other cases, 

 however, such lesions have not been discovered. Sometimes the bundle 

 is so severely diseased that the block is complete, the ventricles con- 

 tracting quite independently of the auricle. In such cases it is assumed 

 that the beat originates in the uninjured part of the bundle below the 

 seat of the block. It should be pointed out here, however, that all cases of 

 slow pulse in the arteries are not necessarily dependent upon heart-block, 

 but may depend upon a slow beat of the auricle itself. This is called 



Sometimes after complete destruction of the auriculoventricular bun- 

 dle the beat continues to be transmitted to the ventricle, and conversely 

 this transmission has sometimes been observed to be upset by lesions not 

 affecting the bundle. The explanation of both of these exceptional re- 

 sults almost certainly is that the right lateral connection described above 

 (page 184) is serving as the main pathway of transmission for the beat. 



The facility of conduction through the auriculoventricular bundle is 

 subject to alteration by the impulses passing to it along the vagus nerve, 

 particularly the left vagus. It can also be altered by certain drugs, 

 especially digitalis and strophanthin. The clear demonstration that it is 

 along this bundle that the beat is transmitted is strong evidence in favor 

 of the myogenic hypothesis (page 171) concerning the transmission of 

 the heartbeat, but it does not necessarily disprove the neurogenic hypoth- 

 esis, for histological investigation has shown that the bundle is closely 

 surrounded by an intimate plexus of nerve fibers. 



Spread of the Beat in the Ventricle 



After the impulse has been transmitted by the bundle into the ven- 

 tricles, it spreads along the many branches into which, as we have seen, 

 the two main divisions of this bundle separate. The first part of the 

 ventricular musculature to contract is therefore located near the ter- 

 mination of these branches, at the papillary muscles. That these should 

 contract before the rest of the muscle of the ventricles, has an obvious 



