The system responsible for initiation and propagation of the heartbeat became understood within the 20th century. Thus, it was Tawara (in 1906) who clarified the arrangement of the specialized muscular axis responsible for atrioventricular conduction, while Keith & Flack (in 1907) published the first account of the sinus node. Tawara's findings provide robust anatomic criteria for distinction of postnatal conduction pathways. Such pathways should be composed of cells which are histologically discrete, which can be traced from section to section in the histological series, and which are insulated by fibrous sheaths from the adjacent working myocardium. The cells making up the sinus and atrioventricular nodes fulfil the first two of these criteria, whilst those making up the ventricular conduction pathways fulfil all three criteria. The sinus node is found subepicardially within the terminal groove. The atrioventricular node, surrounded by its zones of transitional cells, is at the apex of the triangle of Koch. The penetrating bundle is sandwiched between the fibrous and muscular parts of the ventricular septum, with the bundle branches descending on either side of the septum. Other than nodal remnants found within the tricuspid vestibule, them are no other histologically discrete tracts to be found within the atrial myocardium.