The conduction system of the dog heart was studied morphologically and histochemically.
Seventy five fresh hearts of dogs of both sexes were examined partly in sections (hematoxlyin-eosin, azan, Masson-Goldner trichrome staining and periodic acid-Schiff reaction) and partly in macroscopic preparations treated with iodine reaction and modified PAS reaction (OTSUKA and HARA 1965). The following findings were obtained.
1. The sinus node is composed of an irregular network of muscle fibers finer than the ordinary auricular muscle. They are richly ramified and contain a few myofibrils and abundant sarcoplasma. Connective tissue fibers intervene among the nodal fibers. The sinus node has no clear-cut boundary. Each nodal fiber shows smooth transition to an ordinary muscle fiber at the periphery of the node. A specialized muscular pathway connecting the sinus node with the atrioventricular node is not recognized.
2. The atrioventricular node consists of the posterior and anterior portions. The former is close to coronary sinus-orifice and the latter continues to the atrioventricular bundle. Histologically, the former resembles the sinus node and the latter the atrioventricular bundle. Between the two, there is no boundary. The atrioventricular node is found in the connective tissue of the anulus fibrosus and separated from the ordinary heart muscle. In the posterior part of the atrioventricular node, there is a muscular pathway connecting the right auricle with the atrioventricular node. This pathway consists not of specialized, but of ordinary auricular muscle fibers. There is no other “bypass tracts” (JAMES 1963) between the atrium and the atrioventricular node.
3. The atrioventricular bundle is composed of thick fibers with few ramifications. They are rich in sarcoplasma and glycogen and poor in myofibrils when compared with ordinary cardiac muscle fibers. This specialized tissue resembles that of ungulates. These fibers first run forming a single bundle, then fork into two main bundles, right and left.
4. The right main bundle courses in the subendocardium as a single cord, drawing an arch (convex to pulmonary conus), without branching. Ariving at the anterior papillary muscle, it issues branches for 3 areas: for the conus pulmonalis, for the free wall, and for the septum.
5. The left main bundle becomes visible beneath the aortic valve as a wide bundle and spreads over the subendocardium as a fimbria, giving three groups of branches. Branches of the first group run toward the anterior papillry muscle via pseudotendons. Those of the second group reach the posterior papillary muscle via pseudotendons and give rise to a very thick distribution of Purkinje fibers to the left ventricular free wall. The third group of the peripheral branches spreads directly over the septum without passing pseudotendons.
6. The network of Purkinje fibers in the subendocardium is the thickest on the free wall of the both ventricles and the thinnest on the right side of the interventricular septum.
7. There is much glycogen throughout the conduction system, especially in the atrioventricular bundle and Purkinje fibers. The glycogen is less soluble in water compared with that of the ordinary heart muscle. Physiological meaning of this glycogen is not yet clear.