1. Allison, P. R. (1959). Ann. roy. Coll. Surg. Engl., 25, 298.
2. Blackburn, C. R. B. (1956). Thorax, 11, 30.
3. Boerema, I. (1949). Arch. Chir. Neerland., 1, 253.
4. Clatworthy, H. W., and Boles, E. T: (1959). Ann. Surg., 150, 371.
5. Cooley, D. A., and DeBakey, M. E. (1954). A.M.A. Arch. Surg., 68, 854.