1. Englemann, T. W. (1875). Arch. ges. Physiol., Bonn, Bd. 11, 465 (quoted in Schafer's Physiology, 1900 edition. Vol. II, p. 443).
2. Hill, A. (1939). Lancet, 2, 979.
3. Hunter, A., Papp, C., and Parkinson, J. (1940). Brit. Heart J., 2, 107.
4. Mechanism and Graphic Registration of the Heart Beat;Lewis, T.,1925
5. Clinical Electrocardiography, Shaw and Sons;London,1928