1. N. J. Eastman, and L. M. Hellman (1961 ), Williams' Obstetrics , 12th edn. Appleton, N.Y., p.689 .
2. J. P. Greenhill (1955 ), Obstetrics , 11th edn. Saunders, Philadelphia, p.586 .
3. I. Jackson, and E. Holland (1959 ), British Obstetric Practice , 2nd edn. Heinemann, London, p.775 .
4. J. C. Moir (1956 ), Munro Kerr's Operative Obstetrics , 6th edn. Bailliere, Tindall & Cox, London, p.250 .