CH. XXI.] HEART-BLOCK 257 



this normal propagation of stimuli. It must not, however, be thought 

 that the wave of contraction is incapable of passing over the heart in 

 any other direction than from the sinus onwards; for it has been 

 shown that by the application of appropriate stimuli at appropriate 

 instants, the natural sequence of beats may be reversed, and the con- 

 traction starting at the arterial part of the ventricle may pass to the 

 auricles and then to the sinus. If clamps or ligatures are not applied 

 sufficiently tightly one often sees partial blocking ; a few waves get 

 through but not all; or if the ventricular wall is left connected 

 with other parts of the heart by only a small portion of undivided 

 muscular tissue, the effect is much the same, the wave is only able 

 to pass the block every second or third beat. 



Heart-Block. The phenomena of blocking just described were 

 made out by Gaskell many years ago in his study of the hearts of 

 cold-blooded animals. But the same sort of thing occurs also in the 

 mammalian heart. The starting-point in our knowledge of this 

 branch of the subject was the discovery by Stanley Kent of bands 

 of muscular tissue passing across from auricles to ventricles, and 

 the principal one was subsequently and independently re-discovered 

 by His, and is known as Kent's bridge or the Bundle of His, or 

 better as the auricula-ventricular bundle. It arises in close con- 

 nection with the fibres of the interauricular septum, and with the 

 tissue of the sino-auricular node at the junction of the superior 

 vena cava and right auricle. It comes also into relationship with 

 another similar mass or foetal residue known as the auriculo- 

 ventricular node which lies at the base of the auricular septum on 

 the right side below the coronary sinus. From here the bundle 

 runs along the top of the intraventricular septum and divides into 

 right and left sub-divisions which run down to their respective 

 ventricles along the septum which separates them, and into the 

 papillary muscles arising from the septum. From the papillary 

 muscles fine strands run to other parts of the ventricle immediately 

 under the endocardium, and finally reach the apex. All these 

 structures are made of the peculiar muscular fibres known as 

 Purkinje's fibres (see p. 71). There is no doubt that this is the 

 main conducting path from auricles to ventricles, and although there 

 are nerve-fibres mingled with the muscular fibres of Purkinje there 

 is no reason for modifying the view already expressed that con- 

 duction is myodromic. 



The conclusion that the auriculo- ventricular bundle is the 

 important link which propagates the rhythmic wave has been 

 reached first by experiments on animals, and secondly, by observa- 

 tions in disease in man. In animals, cutting through the bundle 

 abolishes the ordinary sequence of cardiac events. The auricles go 

 on beating, but the ventricles stop altogether, and later when they 



K 



