Discovery of the sinus node by Keith and Flack: on the centennial of their 1907 publication.

In 1839, Jan Evangelista Purkinje discovered a net of gelatinous fibres in the subendocardium of the heart. Walter Gaskell in the 1880s observed that the impulse of the heart began in the sinus venosus, and that this region had the most rhythmic ability. A conducting bundle between the atrium and the ventricle was found by Wilhelm His, Jr in 1893. In 1906, Sunao Tawara found a "complex knoten" of tissue at the proximal end of the His bundle. He concluded that this was the inception of an electrical conducting system which continued from the AV node through the bundle of His, divided into the bundle branches, and terminated as the Purkinje fibres. The collaboration of Arthur Keith and Martin Flack led to discovery of the sinus node, finalising the discovery of the electrical system of the heart and providing an anatomical answer to the baffling mystery: "Why does the heart beat?"