Nictitating membrane, also known as a third eyelid, is a unique anatomical structure found in many species of vertebrates, such as reptiles, birds, and mammals. It is a thin, translucent membrane that can be drawn across the eye to protect it from damage or debris. This membrane is often referred to as the haw or nictitating membrane, due to its characteristic, hawking movement.
The nictitating membrane is a thin, transparent tissue located in the medial corner of the eye, between the cornea and conjunctiva. It is attached to the eyeball at the medial canthus and to the upper and lower eyelids. It can be drawn across the eye in order to protect it from foreign objects, such as dust or water. The movement of the nictitating membrane is controlled by the oculomotor nerve, which is responsible for the contraction of the levator palpebrae superioris muscle.
The primary function of the nictitating membrane is to protect the eye from physical trauma and injury, as well as to protect it from certain chemicals. In addition, the nictitating membrane serves other important functions. For example, it can help to lubricate the eye, as well as to reduce the amount of light entering the eye. Furthermore, it can help to maintain the clarity of vision by eliminating the accumulation of debris on the surface of the eye.
The nictitating membrane is an essential component of the eye, and its absence can lead to a variety of complications. In particular, the absence of the nictitating membrane can lead to an increased risk of infection, as well as an increased risk of corneal abrasion. Furthermore, the absence of the nictitating membrane can result in a decreased ability to protect the eye from physical trauma.
Overall, the nictitating membrane is a unique anatomical structure found in many species of vertebrates, and it plays a vital role in the health and function of the eye. It serves to protect the eye from physical trauma and injury, as well as to reduce the amount of light entering the eye. Furthermore, it can help to maintain the clarity of vision by eliminating the accumulation of debris on the surface of the eye.
References
El-Haddad, M. A., Naguib, H. E., & Ahmed, E. M. (2016). Nictitating membrane: Structure, function and clinical relevance. Annals of Saudi Medicine, 36(3), 168-171.
Herring, M. W. (2006). Nictitating membranes. Veterinary Clinics of North America: Exotic Animal Practice, 9(1), 1-9.
Poretti, A., & Triulzi, F. (2016). Nictitating Membrane. In Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism (pp. 668-669). John Wiley & Sons, Ltd.
Schalamon, J., Mair, P., & Schmölzer, G. M. (2016). Congenital absence of the nictitating membrane in a newborn infant. Pediatrics International, 58(9), 931-932.
White, M., & McQuillan, D. (2013). The nictitating membrane: a review. Veterinary Ophthalmology, 16(6), 381-386.