الفهرس 
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Abstract
This study was performed on the heart of twenty clinically healthy, mature, domestic dogs (Canis Familiaris). It aimed to study the morphological and histological structure of the four heart chambers and there. It also studied the histological structure of the cardiac conduction system. Moreover, it studied the fine structure of the contractile cardiomyocytes of both heart chambers; the atrium and ventricle, myoendocrine cells, telocyte cells, and the coronary arteries (muscular arteries). Furthermore, the localization of some antibodies in the cardiomyocytes by immunohistochemistry as cadherin, β-catenin, vimentin, desmin, c-kit were performed. The heart wall was composed of epicardium, myocardium, and endocardium from outside to inside. The subendocardial layer had Purkinje fibers in the heart ventricles that were paler and with fewer myofibrils when compared with the ordinary cardiomyocytes. The cells of SAN and AVN were different from that of the contractile cardiomyocytes. the contractile cardiomyocytes were enveloped by electron-dense sarcolemma. There were intercalated discs at the end of adjacent ones, which bound them side by side in the vicinity of the Z disc. This disc was composed mainly of desmosomes, adherent junctions, and gap junctions. Cross striations were visible on the myofibrils, which formed dark A band and light I bands.