Anatomy of the Heart: Difference between revisions

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Occasional reference to this as the system of His-Tawara gives credit to two of the pioneering investigators in this field. The myocardial bridge that connects atrial myocardium to ventricular myocardium across the insulating fibro-fatty tissues of the atrioventricular junction was found by His in 1893 and given the appellation ‘penetrating bundle of His’.<cite>Tawara</cite> Tawara's monograph<cite>5</cite> accompanied by colour plates in 1906 gave a detailed description of the atrioventricular node and how it was a continuum with the bundle described by His and the ventricular fibres previously described by Purkinje.<cite>6</cite> This firmly estabIished the presence of an atrioventricular conduction system (Figure 10) and was subsequently confirmed by Keith and Flack in the same year.<cite>7</cite> Gross anatomical landmarks to the location of the atrioventricular system are invaluable guides to cardiac surgeons and interventionists who have to perform intracardiac procedures since trauma to any part of the system can produce dire complications. The atrioventricular node is located at the apex of an angle formed by the tendinous continuation of the Eustachian valve (tendon of Todaro) and the annular insertion of the septal leaflet of the tricuspid valve (Figure 12). The coronary sinus completes the base of the triangular shape which bears the name 'triangle of Koch' in recognition of Koch's elegant descriptions.<cite>8</cite> The tendon of Todaro inserts into the central fibrous body. In the adult the atrioventricular node measures about 4 mm in width and 8 mm in length. In histological sections the compact part of the node is easily recognisable being composed of interconnecting fascicles of small cells, closely adherent to the central fibrous body. In cross•section the node appears like a haIf-oval lying against the fibrous body (Figure 12D). A transitional zone of attenuated myocardial cells extends into the atrial myocardium. The node becomes the penetrating bundle as the conduction system passes through the central fibrous body (Figure 12C). The penetrating bundle veers to the left as it continues into the branching bundle to emerge in the left ventricle beneath the commissure that separates the right-coronary and non-coronary aortic valve leaflets. The bifurcation into left and right bundle branches marks the beginning of the branching bundle (Figure 12B).  The right bundle branch is cord-like and frequently is the continuation of the nodal-bundle axis. It turns downwards and passes intramyocardially into the substance of the septomarginal trabeculation directly beneath the medial papillary muscle complex. It then passes subendocardially towards the right ventricular apex and crosses the ventricular cavity within the moderator band before ramifying. The left bundle branch is morphologically different from the right bundle branch. It descends from the nodal-bundle axis as a sheet of cells within the subendocardial tissues of the aortic outflow tract. Tawara's original reconstructions show the bundle radiating in fan-like fashion into three major divisions which are interconnected distally by a subendocardial network that ramifies into the ventricular myocardium (Figure 13).<cite>5</cite> Later investigations using careful serial reconstructive techniques support the trifascicular concept seemingly in conflict with the 'hemiblock' theory which promotes a bifascicular morphology.<cite>9</cite>

#Lewis T. Oppenheimer BS, Oppenheimer A. Site of origin of the mammalian heart beat: the pacemaker in the dog. Heart 1910;2:147

#His W Jr. Die Thatigkeit des embryonalen Herzens und deren Bedeutung für die Lehre von Herzbewegung beim Erwachsenen. Ar Med Klin Leip 1893:14.

#Tawara pmid=16969729

#Purkinje JE. Mikroskopisch neurologische Beobachtungen. Archiv Anat Physiol u Wiss Med I845;12:28I.

#Keith pmid=15485521

#Koch W. Der funktionelle Bau des menschlichen Herzens. Berlin: Urban v Schwarzenburg,1922:92.