The process of septal activation in the dog's heart was studied by means of unipolar leads, distant bipolar leads, and bipolar leads using contiguous electrodes. The electrodes used in this study permitted the registration of the electrical phenomenon in any part of the septal surfaces and even in the interior of the septal muscle. The exact location of the site at which the lead was taken was determined precisely by post-mortem studies of the animal. The following findings were demonstrated: 1. 1. The mean process of septal activation is developed from below upward. 2. 2. Extrasystoles produced in the epicardial surface of the apex of the ventricles cause a process of activation of the septum which proceeds from below upward, similar to normal activation. Extrasystoles provoked in the base of the ventricles produce a septal activation which proceeds from above downward, contrary to the normal process of septal activation. 3. 3. The first region to be activated in the right surface of the septum is that portion in contact with the anterior papillary muscle. Those portions activated later are the part of the septum corresponding to the apex, the middle portions between the apex and the base septum, and, still later, those regions near the insertion of the atrioventricular valves and the pulmonary valves. These data are in accord with the distribution of the right branch of the bundle of His, since this branch descends enveloped in a sheath of connective tissue, and its first ramifications appear at the level of the base of the anterior papillary muscle. The remainder of the ramifications invade the septum from below upward. 4. 4. The differences found between the times of arrival of the wave of activation to the first and last portions of the right surface of the septum fluctuate between 0.02 and 0.03 second. 5. 5. In the left surface of the septum, the differences pointed out in 4 are much less and generally do not exceed 0.01 second. In the majority of the experiments, the first regions to be activated on the left side were those between the apex and the base. This is, in general, in accord with the appearance of the first ramifications of the left branch, which begins to subdivide at levels higher than the right branch. 6. 6. The greater part of the muscular tissue which makes up the septum behaves electrically as the left ventricle. For instance, when right bundle branch block is produced, the time of arrival of the wave of excitations is not altered, whereas with the production of left bundle branch block, there is a considerable retardation of the arrival of the wave of excitation. The muscular tissue, which behaves electrically as the left ventricle, approaches very closely the right surface of the septum, and there are even parts of the right surface that behave as left ventricle. The wave of activation reaches these portions rapidly (about 0.01 second). This makes it impossible to accept the concept that in bundle branch block the wave of excitation crosses the septum slowly (it was supposed in 0.04 to 0.05 second). All the evidence indicates that the retardation of the wave of excitation in bundle branch block takes place in the septum, but in a very small region, probably 1 to 2 mm., and very near the right surface. 7. 7. In one case limited portions of the left septal surface were found which behaved electrically as the right ventricle. That is to say, there was no retardation of the arrival of the wave of excitation when left bundle branch block was provoked but a marked retardation when right bundle branch block was produced. 8. 8. Anatomical cuts of the interventricular septum indicate that the distribution of the muscular fibers of the two ventricles is not incompatible with the findings related in paragraphs 6 and 7. 9. 9. In two experiments we were able to show the existence of a small zone in the superior portion of the septum and in its muscular substance, whose activation was not delayed with either right or left bundle branch block. This zone corresponds approximately to the site where Mahaim has described the accessory paraspecific bundle. Nevertheless, in this respect, our experiments are few, and for the present we do not feel that we should draw any definite conclusions.
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