1. American Heart Association (1938). Amer. Heart J., 15, 235.
2. Andrus, E. C. (1929). New York St. J. Med., 29, 661.
3. Essentials of Electrocardiography;Ashman, R.; Hull, E.;Clin. Sci,1937
4. Cohen, S. M., Edholm, 0. G., Howarth, S., McMichael, J., and Sharpey-Schafer, E. P. (1949). Clini. Sri., 7, 35.
5. Damashek, W. (1924). Boston med. surg. J., 190, 487.