1. S.W. Douglas, and H.D. Williamson(1970 ) Veterinary Radiological Interpretation , p.293 . Heinemann, London.
2. S.W. Douglas, M.E. Herrtage, and H.D. Williamson(1987 ) Principles of Veterinary Radiography , 4th edn , pp.256 -264 . Bailliere Tindall, London.