Fig. 47.—Electrocardiogram showing delayed conduction (lengthening of P-R interval). These P-R intervals are quite regular. When irregular there is apt to be extrasystole of ventricle or occasional blocking of impulse going to ventricle. (Courtesy of Dr. G. C. Robinson.)
Fig. 48.—Electrocardiogram showing partial heart-block in the three leads. Note the variability of P-R interval calculated in seconds in Lead II. (Courtesy of Dr. G. C. Robinson.)
Fig. 49.—Complete heart block. (Courtesy of Dr. G. C. Robinson.)
The bundle of muscles described by His in 1905, connecting the auricles and ventricles, has been definitely shown to be the path through which impulses having their origin in the orifices of the great veins pass to the ventricles. The situation and size of this bundle has been thus described in man by Retzer:
"When viewed from the left side, the bundle lies just above the muscular septum of the ventricles and below the membranous septum. In some hearts the muscular septum is so well developed that it envelops the bundle. It is then difficult to find, but occasionally it can be seen directly by means of transmitted light. From the left side the bundle can be followed no farther posteriorly than the right fibrous trigone, for here the connective tissue becomes so dense that it is difficult to dissect it away. The impression is, therefore, received that this mass of connective tissue forms the insertion of the bundle. The bundle may be followed anteriorly until it becomes intimately mixed with the musculature of the ventricles.