1. American Rheumatism Association (1959). Ann. rheuni. Dis., 18, 49.
2. Ball, J. (1955). Ibid., 14, 159.
3. Ball, J. and Lawrence, J. S. (1961). Ibid., 20, 235.
4. Caplan, A. (1953). Thorax, 8, 29.
5. Personal communication;Caplan, A.; Gough, J.,1962