1. Bremer, J.L., Textbook of Histology 3rd Ed. ( Philadelphia, P. Blakiston's Son & Co. 1927) p. 445.
2. Broomell, I.N., AND Fischelis, P., Anatomy and Histology of the Mouth and Teeth. 6th Ed. (Philadelphia, P. Blakiston's Son & Co. 1923) p. 329-330.