1. 2 D. Slatter, Cornea and sclera. In: Fundamentals of Veterinary Ophthalmogy (ed. Slatter D). W.B. Saunders Co., Philadelphia, 1990 : 257, 303.
2. 3 GA. Severin Veterinary Ophthalmology Notes 2nd edn. (ed. College of Veterinary Medicine and Biomedical Sciences) Fort Collins, Colorado, 1976 : 67, 70 , 67, 70
3. 4 LC. Helper, The canine nictitating membrane and conjunctiva. In: Veterinary Ophthalmology. 2nd edn. (ed. Gelatt KN) Lea and Febiger, Philadelphia, 1991 : 338, 342.