35° HANDBOOK OF PHYSIOLOGY ^^ CIRCULATION I 



msec 

 50 



uo 



SVC 



60 70 80 30 



mm 



FIG. 31. Pathway of the atrial excitation waves. The atria 

 of a dog are seen from above, with the auricles. The figures in 

 circles indicate the latencies observed at the various points, 

 against the earliest visible excitation at SKn, the sinus node. 

 Insel: the latencies are plotted against the distances from the 

 sinus node. The propagation takes place radially on nearly 

 straight lines from the sinus. [From Brendel el al. (124).] 



the bundle of His are about 10 mm- maximally, 

 calculated from the data of Read et al. (379), and 

 scarcely more than i mm- according to the findings 

 of Glomset & Glomset (224). These cross sections, 

 therefore, are just at or below the borderline of a 

 recordable potential for extremity leads. In pre- 

 cordial leads it should perhaps be detectable, but no 

 special attempt has ever been made to discriminate 

 it from the large potentials of atria or ventricles. In 

 direct unipolar derivations, the action potential of 

 the His bundle can be detected only v^^hen there is 

 close contact between bundle and electrode (82). Any 

 clinically reliable information about the action 

 potential of the bundle therefore will scarcely be 

 gained by the conventional lead systems. As long as 

 the excitation travels in the bundle alone, the' EGG 

 is mute, if the atria are not still producing potentials 

 in their lateral parts. The details of this propagation 

 process have been described in the preceding chapter 

 (260, 296, 415). 



FIG. 32. Diagram showing distribution of specialized heart 

 muscle. RA and L.^, right and left atria; RV and LV, right 

 and left ventricles; IVS, interventricular septum; AO, as- 

 cending aorta; CS, coronary sinus; SVC, superior vena cava; 

 PV, pulmonary veins; SN, sinus node, AN, atrioventricular 

 node, CB, common bundle, RB and LB, right and left bundle 

 branches; A, the peripheral Purkinje net. [From Katz (39).] 



The essence of the spread of excitation in the 

 ventricles, which is by no means known in every 

 detail, lies in the following facts. The excitation wave 

 is first propagated through the more or less un- 

 branched bundles of His down to a midpoint region 

 on the septum between the ventricles, where the 

 first big branches leave the system and the excitation 

 enters the mass of the ventricular wall (fig. 32). 

 Propagation into the myocardium is effected thus: 

 the conduction fiber converts itself into an ordinary 

 myocardial fiber (360), which branches several 

 times, producing a cone-like system (387, 389). The 

 specific form and site of the conductive tissue exhibits 

 now a doublefold divergence. First, the excitation 

 front spreads from that midpoint region in all direc- 

 tions, so that at the surface of the heart the excitation 

 occurs earliest at a central point and spreads in the 

 form of concentric rings. Second, myocardial activa- 

 tion starts near the endocardial surface — how near, 

 is still a matter of controversy. But at the point at 

 which the conduction system branches, as experi- 

 ments to be described later have shown, a divergent 

 wave runs from the specific system in l)oth directions. 



