1. P.A.M. Dirac, An extensible model of the electron, Proc. Roy. Soc. Lond.
A 268 (1962) 57 [INSPIRE].
2. A. Chodos, R.L. Jaffe, K. Johnson, C.B. Thorn and V.F. Weisskopf, New extended model of hadrons, Phys. Rev.
D 9 (1974) 3471 [INSPIRE].
3. W.A. Bardeen, M.S. Chanowitz, S.D. Drell, M. Weinstein and T.-M. Yan, Heavy quarks and strong binding: a field theory of hadron structure, Phys. Rev.
D 11 (1975) 1094 [INSPIRE].
4. P. Hasenfratz and J. Kuti, The quark bag model, Phys. Rept.
40 (1978) 75 [INSPIRE].
5. J. Hoppe, Quantum theory of a massless relativistic surface and a two-dimensional bound state problem, Ph.D. thesis, Massachusetts Institute of Technology, Cambridge U.S.A. (1982).